11institutetext: Centro de Astrobiología, CSIC-INTA, ESAC campus, 28692
Villanueva de la Cañada, Madrid, Spain
11email<EMAIL_ADDRESS>22institutetext: Instituto de Astrofísica e
Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762
Porto, Portugal 33institutetext: Departamento de Fisica e Astronomia,
Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
44institutetext: Observatoire de l’Université de Genève, 51 chemin Pegasi,
1290 Sauverny, Switzerland 55institutetext: Max Planck Institute for
Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany 66institutetext: Aix
Marseille Univ, CNRS, CNES, LAM, 38 rue Frédéric Joliot-Curie, 13388
Marseille, France; Division Technique INSU, CS20330, 83507 La Seyne sur Mer
cedex, France 77institutetext: Department of Astronomy and Astrophysics,
University of California, Santa Cruz, CA, USA 88institutetext: Centro de
Astrobiología, CSIC-INTA, Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz,
Madrid, Spain 99institutetext: Physikalisches Institut, University of Bern,
Gesellsschaftstrasse 6, 3012 Bern, Switzerland 1010institutetext: Center for
Space and Habitability, University of Bern, Gesellsschaftstrasse 6, 3012 Bern,
Switzerland 1111institutetext: INAF – Osservatorio Astronomico di Brera, Via
E. Bianchi 46, 23807 Merate (LC), Italy 1212institutetext: Department of
Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
1313institutetext: Instituto de Astrofísica de Canarias, E-38205 La Laguna,
Tenerife, Spain 1414institutetext: Universidad de La Laguna, Dept.
Astrofísica, E-38206 La Laguna, Tenerife, Spain 1515institutetext: NASA Ames
Research Center, Moffett Field, CA 94035, USA 1616institutetext: INAF -
Osservatorio Astrofisico di Torin, Via Osservatorio 20, I-10025 Pino Torinese,
Italy 1717institutetext: Departamento de Matemática y Física Aplicadas,
Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850,
Concepción, Chile 1818institutetext: Univ. Grenoble Alpes, CNRS, IPAG, F-38000
Grenoble, France 1919institutetext: INAF – Osservatorio Astronomico di
Trieste, via G. B. Tiepolo 11, I-34143, Trieste, Italy 2020institutetext:
European Southern Observatory, Av. Alonso de Cordova 3107, Casilla 19001,
Santiago de Chile, Chile 2121institutetext: Centro de Astrofísica da
Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
2222institutetext: Instituto de Astrofísica e Ciências do Espaço, Faculdade de
Ciências da Universidade de Lisboa, Campo Grande, PT1749-016 Lisboa, Portugal
2323institutetext: Département d’Astronomie, Université de Genève, Ch. des
Maillettes 51, 1290 Versoix, Switzerland 2424institutetext: Department of
Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts
Institute of Technology, Cambridge, MA 02139, USA 2525institutetext:
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts
Institute of Technology, Cambridge, MA 02139, USA 2626institutetext:
Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue,
Cambridge, MA 02139, USA

# An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).

A. Castro-González An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). O. D. S. Demangeon An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). J. Lillo-Box An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). C.
Lovis An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). B. Lavie An unusually low-density super-Earth transiting
the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed
Time Observations collected at the European Southern Observatory (ESO) under
ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to
A.5 are only available in electronic form at the Centre de Données
astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). V. Adibekyan An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). L.
Acuña An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). M. Deleuil An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). A. Aguichine An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). M.
R. Zapatero Osorio An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). H. M. Tabernero An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). J. Davoult An unusually low-density
super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). Y.
Alibert An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). N. Santos An unusually low-density super-Earth transiting
the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed
Time Observations collected at the European Southern Observatory (ESO) under
ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to
A.5 are only available in electronic form at the Centre de Données
astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). S. G. Sousa An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). A.
Antoniadis-Karnavas An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). F. Borsa An unusually low-density super-Earth transiting
the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed
Time Observations collected at the European Southern Observatory (ESO) under
ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to
A.5 are only available in electronic form at the Centre de Données
astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). J. N. Winn An unusually low-density
super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). C.
Allende Prieto An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). P. Figueira An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). J. M. Jenkins An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). A.
Sozzetti An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). M. Damasso An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). A. M. Silva An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). N.
Astudillo-Defru An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). S. C. C. Barros An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). X. Bonfils An unusually low-density
super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). S.
Cristiani An unusually low-density super-Earth transiting the bright early-
type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). P. Di Marcantonio An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). J. I. González Hernández An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). G.
Lo Curto An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). C. J. A. P. Martins An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). N. J. Nunes An unusually low-
density super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). E.
Palle An unusually low-density super-Earth transiting the bright early-type
M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time Observations
collected at the European Southern Observatory (ESO) under ESO programs
108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only
available in electronic form at the Centre de Données astronomiques de
Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS). F. Pepe An unusually low-density super-Earth transiting
the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed
Time Observations collected at the European Southern Observatory (ESO) under
ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to
A.5 are only available in electronic form at the Centre de Données
astronomiques de Strasbourg (CDS).An unusually low-density super-Earth
transiting the bright early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on
Guaranteed Time Observations collected at the European Southern Observatory
(ESO) under ESO programs 108.2254.002, 108.2254.005, and 108.2254.006. Full
Tables A.1 to A.5 are only available in electronic form at the Centre de
Données astronomiques de Strasbourg (CDS). S. Seager An unusually low-density
super-Earth transiting the bright early-type M-dwarf GJ 1018
(TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS).An
unusually low-density super-Earth transiting the bright early-type M-dwarf GJ
1018 (TOI-244)††thanks: Based on Guaranteed Time Observations collected at the
European Southern Observatory (ESO) under ESO programs 108.2254.002,
108.2254.005, and 108.2254.006. Full Tables A.1 to A.5 are only available in
electronic form at the Centre de Données astronomiques de Strasbourg (CDS). A.
Suárez Mascareño An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).An unusually low-density super-Earth transiting the bright
early-type M-dwarf GJ 1018 (TOI-244)††thanks: Based on Guaranteed Time
Observations collected at the European Southern Observatory (ESO) under ESO
programs 108.2254.002, 108.2254.005, and 108.2254.006. Full Tables A.1 to A.5
are only available in electronic form at the Centre de Données astronomiques
de Strasbourg (CDS).

(Received 31 March 2023 / Accepted 2 May 2023)

###### Abstract

Context. Small planets located at the lower mode of the bimodal radius
distribution are generally assumed to be composed of iron and silicates in a
proportion similar to that of the Earth. However, recent discoveries are
revealing a new group of low-density planets that are inconsistent with that
description.

Aims. We intend to confirm and characterize the TESS planet candidate
TOI-244.01, which orbits the bright ($K$ = 7.97 mag), nearby ($d$ = 22 pc),
and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4
days.

Methods. We used Markov Chain Monte Carlo methods to model 57 precise radial
velocity measurements acquired by the ESPRESSO spectrograph together with TESS
photometry and complementary HARPS data. Our model includes a planetary
component and Gaussian processes aimed at modeling the correlated stellar and
instrumental noise.

Results. We find TOI-244 b to be a super-Earth with a radius of $R_{\rm p}$ =
1.52 $\pm$ 0.12 $\rm R_{\oplus}$ and a mass of $M_{\rm p}$ = 2.68 $\pm$ 0.30
$\rm M_{\oplus}$. These values correspond to a density of $\rho$ = 4.2 $\pm$
1.1 $\rm g\cdot cm^{-3}$, which is below what would be expected for an Earth-
like composition. We find that atmospheric loss processes may have been
efficient to remove a potential primordial hydrogen envelope, but high mean
molecular weight volatiles such as water could have been retained. Our
internal structure modeling suggests that TOI-244 b has a $479^{+128}_{-96}$
km thick hydrosphere over a 1.17 $\pm$ 0.09 $\rm R_{\oplus}$ solid structure
composed of a Fe-rich core and a silicate-dominated mantle compatible with
that of the Earth. On a population level, we find two tentative trends in the
density-metallicity and density-insolation parameter space for the low-density
super-Earths, which may hint at their composition.

Conclusions. With a 8$\%$ precision in radius and 12$\%$ precision in mass,
TOI-244 b is among the most precisely characterized super-Earths, which,
together with the likely presence of an extended hydrosphere, makes it a key
target for atmospheric observations.

###### Key Words.:

Planets and satellites: individual: TOI-244 b, detection, composition – Stars:
individual: GJ 1018 (TIC 118327550) – Techniques: radial velocities,
photometric

##  1 Introduction

Over the last decades, exoplanet detection has been very successful. Since the
first discoveries (Wolszczan & Frail 1992; Mayor & Queloz 1995), the number of
known planets has increased year by year to the 5347 we know today111According
to the NASA Exoplanet Archive (Akeson et al. 2013).. This success has been
achieved to a great extent thanks to the space-based Kepler mission (Borucki
et al. 2010, 2009-2014), which, jointly with its extended K2 mission (Howell
et al. 2014, 2014-2018), detected 62$\%$ of the known planets by means of the
transit method (e.g., Lillo-Box et al. 2014; Crossfield et al. 2016; Morton et
al. 2016; Mayo et al. 2018; Livingston et al. 2018a, b; Castro González et al.
2020; de Leon et al. 2021; Christiansen et al. 2022).

This plethora of planets has allowed us to perform statistical studies of many
regimes of the parameter space, revealing different exoplanet populations. The
Kepler and K2 results definitively confirmed what Mayor et al. (2011) hinted:
small planets (i.e., $R_{\rm p}$ ¡ 4 $\rm R_{\oplus}$) are the most common
ones within $\sim$1 AU of Sun-like stars (Howard et al. 2012; Batalha et al.
2013; Petigura et al. 2013; Burke et al. 2015). Also, Kepler data revealed the
existence of a bimodal radius distribution in the small planets sample (Fulton
et al. 2017), which matches the radius gap previously predicted by
photoevaporation numerical analysis (Owen & Wu 2013; Jin et al. 2014; Lopez &
Fortney 2014; Chen & Rogers 2016). However, the physical cause of the
distribution as well as the internal composition of the higher mode planets
(2-4 $\rm R_{\oplus}$; gas dwarfs or water worlds) is still under debate
(e.g., Ginzburg et al. 2018; Gupta & Schlichting 2019; Zeng et al. 2019;
Venturini et al. 2020; Luque & Pallé 2022; Rogers et al. 2023a). In contrast,
there is not such a debate as to the internal structure of the lower mode
planets (1-2 $\rm R_{\oplus}$), whose densities point toward rocky-dominated
compositions with a proportion of silicates and iron similar to that of the
Earth (33$\%$ Fe, and 67$\%$ $\rm MgSiO_{3}$ in mass; e.g., Zeng et al. 2019).

Recent discoveries have revealed the existence of rocky planets inconsistent
with Earth-like compositions. For example, planets K2-229 b (Santerne et al.
2018), Kepler-107 c (Bonomo et al. 2019), K2-233 c (Lillo-Box et al. 2020b), L
168-9 b (Astudillo-Defru et al. 2020), and K2-38 b (Toledo-Padrón et al. 2020)
have been found to have unusually high densities, similar to that of Mercury.
These densities are thought to be caused by the presence of elevated iron
content in the planetary cores, which is usually explained through the
existence of high iron abundances in the initial protoplanetary disks (e.g.,
Scora et al. 2020; Aguichine et al. 2020; Johansen & Dorn 2022; Bogdan et al.
2023), or through external factors such as mantle stripping caused by
collisions during planetary formation (e.g., Marcus et al. 2010). On the other
hand, planets TOI-561 b (Lacedelli et al. 2021; Weiss et al. 2021; Brinkman et
al. 2023), L 98-59 c and d (Demangeon et al. 2021), HD 260655 c (Luque et al.
2022), and TOI-4481 b (Palle et al. 2023) have been found to have lower
densities than expected for an Earth-like composition. These densities could
be explained by a scarcity or total absence of iron in the planet structure,
by the presence of a significant amount of volatile elements, or by a mixture
of both. Interestingly, all these light planets are found to orbit metal-poor
stars (i.e., [Fe/H] between -0.20 and -0.45 dex). This emerging density-
metallicity correlation could be explained by the fact that the building
blocks in the original protoplanetary disks of metal-poor stars have lower
iron and higher water mass fractions than those expected for stars with solar
metallicities, and hence, they are expected to form iron-poor and water-rich
planets (Santos et al. 2017; Adibekyan et al. 2021). Currently, interior
structure models can shed some light on the composition of these low-density
planets. However, they are limited by degeneracies that prevent us from
determining their bulk compositions from mass and radius information alone.
Future atmospheric observations through transmission spectroscopy will allow
us to determine the volatile content of these planets, which will help us to
uncover their nature.

The identification of rocky planets inconsistent with Earth-like compositions
has only been possible recently thanks to the measuring of very precise planet
masses. Today, only 24$\%$ of exoplanets have a true dynamical mass measured,
and the percentage is reduced down to 9$\%$ for small planets (i.e., $R_{\rm
p}$ ¡ 4 $\rm\leavevmode\nobreak\ R_{\oplus}$). Measuring an accurate mass for
transiting exoplanets is of crucial importance in order to perform internal
structure analysis (e.g., Lillo-Box et al. 2020a; Delrez et al. 2021) as well
as atmospheric characterization through transmission spectroscopy. Recently,
Batalha et al. (2019) conducted retrievals on simulated transmission spectra
from the James Webb Space Telescope, concluding that a 20$\%$ mass precision
or better is required so that inferences of atmospheric properties are not
limited by the mass precision of the planet. However, the percentage of small
planets meeting this threshold is lower than 5$\%$.

In 2018, the launch of the TESS space telescope (Ricker et al. 2015), together
with the start of operations of the ESPRESSO high-resolution echelle
spectrograph (Pepe et al. 2021), opened a new window to exoplanet
characterization. TESS, by continuously monitoring a field of view (i.e.,
sector) of 24 x 96 degrees that changes every $\sim$27 days, is performing
photometric observations of almost the entire sky, focusing on stars
significantly brighter than those surveyed by the Kepler mission (e.g.,
Vanderburg et al. 2019; Espinoza et al. 2020). ESPRESSO, with a spectral
resolving power of 140 000 over the 380-788 nm wavelength range, and mounted
on the Very Large Telescope (VLT), is able to achieve an unprecedented
instrumental radial velocity precision of 10 cm$\cdot\rm s^{-1}$. The powerful
combination of TESS and ESPRESSO observations has allowed several exquisite
planet characterizations (e.g., Damasso et al. 2020; Lillo-Box et al. 2020a;
Demangeon et al. 2021; Sozzetti et al. 2021; Barros et al. 2022; Lavie et al.
2022).

In this work, we confirm and characterize the small ($R_{p}$ = 1.5 $\rm
R_{\oplus}$) and close-in ($P$ = 7.49 d) transiting planet TOI-244 b (recently
validated in Oddo et al. 2023) orbiting the bright ($K$ = 7.97 mag) and nearby
($d$ = 22 pc) M 2.5V star GJ 1018. Based on the transit signal detected from
TESS data, we carried out an intensive radial velocity campaign with ESPRESSO
in order to confirm its planetary nature, obtain a precise mass measurement,
as well as to search for additional planets. In addition to TESS and ESPRESSO
data, we used complementary spectroscopic and photometric data sets from HARPS
and ASAS-SN in order to maximize the information for this system.

In Sect. 2, we describe the TESS, ESPRESSO, HARPS, and ASAS-SN observations.
In Sect. 3, we present our stellar characterization based on precise
photometry and the ESPRESSO spectra. In Sect. 4, we describe our analysis of
photometric and spectroscopic data and present the derived planetary
parameters. In Sect. 5, we discuss the results, and we conclude in Sect. 6.

##  2 Observations

###  2.1 TESS photometry

Figure 1: TESS target pixel file of GJ 1018. The orange grid is the selected
aperture and the red circles correspond to the nearby Gaia DR3 sources. Symbol
sizes for Gaia sources scale to their $G$ magnitudes. This plot has been
prepared through tpfplotter (Aller et al. 2020).

Figure 2: Left: TLS periodogram of the complete and flattened TESS light curve
of GJ 1018, where the highest peak (highlighted with the green broad line)
corresponds to the TOI-244.01 transit signal. The green thin dashed line
corresponds to the second harmonic. Right: TLS periodogram of the TESS light
curve after masking the TOI-244.01 signal.

The star GJ 1018 (TOI-244, TIC 118327550) was observed by TESS (camera $\\#$2,
CCD $\\#$3) at a 2-min cadence in sector 2 (S2) from 22 August 2018 to 20
September 2018, and in S29 from 26 August 2020 to 22 September 2020, resulting
in a total of 36 229 target pixel files (TPFs) spanning a temporal baseline of
two years. The central time of each sector shows a short gap ($\sim$1-2 days)
due to the satellite repointing toward the Earth to downlink the data,
resulting in a total duty cycle of 50 days.

The observations were processed by the Science Processing Operation Center
(SPOC) pipeline (Jenkins et al. 2016) and are publicly available in the TESS
archive of the Mikulski Archive for Space Telescopes
(MAST)222https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html. The
TESS SPOC data products include simple aperture photometry (SAP) and presearch
data-conditioned simple aperture photometry (PDCSAP), being the latter the SAP
processed by the PDC algorithm, which corrects the photometry of instrumental
systematics that are common to all stars in the same CCD (Smith et al. 2012;
Stumpe et al. 2012, 2014). The photometric aperture was automatically selected
by the SPOC module Create Optimal Apertures (COA), which maximizes the signal-
to-noise ratio of the flux measurement (Bryson et al. 2010; Smith et al.
2020). Besides, COA estimates the fraction of flux inside the photometric
aperture that comes from the target star, and uses it to correct for
contamination within the PDCSAP light curve. This fraction (also called
CROWDSAP) is 0.9996 for GJ 1018. In Fig. 1, we plot the selected aperture over
a TPF of GJ 1018, together with all the nearby stars detected in the Gaia Data
Release 3 (DR3; Gaia Collaboration et al. 2022). There are no additional
sources within the aperture, and the nearby sources surrounding the aperture
have a magnitude difference $\Delta G$ ¿ 6 mag with GJ 1018 in the Gaia
passband. These large contrasts ensure negligible contamination (e.g.,
Livingston et al. 2018a; Castro-González et al. 2022).

In October 2018, the SPOC pipeline identified a periodic flux decrease (known
as threshold crossing event; TCE) of 7.4 days through the Transiting Planet
Search (TPS) algorithm. The algorithm first characterizes the power spectral
density of the observation noise, and then estimates the likelihood of the
existence of a transit-like signal over a wide range of trial transit
durations and orbital periods (Jenkins 2002; Jenkins et al. 2010, 2020). A
transit model fit was performed (Li et al. 2019) and a suite of diagnostic
tests were carried out to help making or breaking confidence in the planetary
hypothesis (Twicken et al. 2018), all of which the candidate signal passed.
Finally, the TESS Science Office examined the light curve and additional
information to designate TIC 118327550.01 as a TESS Object of Interest
(TOI-244.01) that would benefit from follow-up observations (Guerrero et al.
2021).

We downloaded from MAST the PDCSAP light curves and removed all data points
with a quality flag different from zero. As a result, we removed one data
point in S2 (1359.6474 TJD) due to an argabrightening event (bit 5, value 16).
In sector S29, we removed 3005 data points (located between 2098.7624 TJD and
2101.3791 TJD, and between 2112.6041 TJD and 2114.4374 TJD) due to scattered
light caused by the Earth or Moon (bit 13, value 4096), and we also removed
four data points cataloged as impulsive outliers (bit 10, value 512) which
have fluxes deviated 5.2$\sigma$ (2097.5194 TJD), 6.5$\sigma$ (2103.2500 TJD),
7.4$\sigma$ (2103.3000 TJD), and 7.1$\sigma$ (2105.0583 TJD) from the PDCSAP
light curve. The final TESS light curves are presented in Table 7.

We computed the generalized Lomb-Scargle periodogram (GLS, Zechmeister &
Kürster 2009) of the TESS light curve (for S2 and S29 separately and jointly)
and found no significant periodicities. We also computed the transit least
squares periodogram (TLS, Hippke & Heller 2019) in order to unveil the
significance of the TOI-244.01 signal and to determine whether there are
additional transit signals. To do so, we first produced a flattened version of
the PDCSAP light curve in order to mitigate possible trends caused by the
stellar activity or uncorrected systematics. For that matter, we filtered the
photometric data of each sector separately using the robust time-windowed
biweight method implemented within the wotan package (Hippke et al. 2019) with
a 0.5-day window length. We detrended the PDCSAP and joined the two flattened
light curves to create a long time series. We present the detrended light
curve in Table 7. In Fig. 2 (left panel), we plot the TLS periodogram of the
flattened light curve, which shows a strong peak at 7.4 days with a signal
detection efficiency (SDE) of 36.6. In the right panel, we plot the
periodogram over the same light curve with the TOI-244.01 transits masked.
This periodogram shows no prominent peaks. However, the six highest peaks have
SDEs between 6 and 10, which are slightly above some empirical thresholds for
transit detection333Empirical thresholds for transit detection range from SDE
¿ 6 (Dressing & Charbonneau 2015), SDE ¿ 6.5 (Livingston et al. 2018b), SDE ¿
7 (Siverd et al. 2012), to SDE ¿ 10 (Wells et al. 2018).. To check the
reliability of those peaks, we folded the flattened light curve to the
corresponding periodicities and inspected a binned version of the folded data.
We found no hints of any transit signal, so we conclude those peaks are
spurious.

###  2.2 ESPRESSO spectroscopy

Figure 3: Left panel: Time series of the ESPRESSO RVs and activity indicators.
Center panel: GLS periodograms of the corresponding time series. The red
circles highlight the maximum power frequencies. The green dotted vertical
lines indicate location of the orbital period of TOI-244 b ($P_{\rm orb}$ =
7.4 days). The magenta dotted vertical lines indicate the rotation period of
the star identified in the activity indicators ($P_{\rm rot}$ $\sim$ 56 days)
and its second and third harmonic. The gray vertical bands indicate the
periods by which the time series in the right panel are folded. The horizontal
dotted lines correspond to the 10$\%$ (orange), 1$\%$ (blue), and 0.1$\%$
(green) FAP levels. Right Panel: ESPRESSO time series folded to the gray bands
periods. The triangle markers within the left and right panels indicate the
location of data points outside the boundaries of the plot.

We observed GJ 1018 with the ESPRESSO high resolution echelle spectrograph
(Pepe et al. 2021), which is mounted on the Very Large Telescope (VLT) located
at ESO’s Paranal Observatory (Chile) and has been operational since 2018. The
observations were performed in the course of the ESPRESSO Guaranteed Time
Observations (GTOs) under the programs IDs 108.2254.002, 108.2254.005, and
108.2254.006, whose main aim is to determine precise mass measurements of
transiting planet candidates. We obtained a total of 57 spectra between 9
October 2021 and 2 October 2022 with a typical cadence of 2-3 days and a
typical exposure time of 1200 s, resulting in a mean signal-to-noise ratio
(S/N) of 57 at 650 nm. All observations were made in the slow-readout single
UT high resolution mode (HR21; 2$\,\times\,$1 binning), which has a spectral
resolution power of 140 000 and embraces a wavelength range from 380 nm to 788
nm. For each exposure, we simultaneously illuminated the second fiber with the
Fabry-Pérot interferometer, which allows the calibration of the instrumental
drift with a precision better than 10 cm$\cdot\rm s^{-1}$ (Wildi et al. 2010).

We reduced the data through the ESPRESSO Data Reduction Software (DRS)
pipeline version 3.0.0444The ESPRESSO Data Reduction Software is publicly
available at https://www.eso.org/sci/software/pipelines/index.html. (Pepe et
al. 2021). The DRS also extracts the radial velocities (RVs) based on a
modified implementation of the original cross-correlation technique presented
by Baranne et al. (1996), in which the different spectral lines of a numerical
mask are weighed as a function of their RV information (Pepe et al. 2002). In
particular, we used an M3 mask to obtain the cross-correlation function (CCF)
of each observation, which was later fitted to a Gaussian profile in order to
compute the RVs (center of the Gaussian). Finally, all the RVs were corrected
from secular acceleration. The standard deviation of the RV observations is
2.7 m/s and the median uncertainty per data point is 0.7 m/s. The DRS also
computes activity indicators such as the full width at half maximum (FWHM) and
the amplitude or contrast of the CCF, the bisector span (BIS), the $\rm
H_{\alpha}$ and Sodium doublet (NaD) line depths, and the S-index. We present
the complete ESPRESSO data set in Table 8.

In Fig. 3, we show the GLS periodogram of the ESPRESSO RVs and activity
indicators time series. The FWHM, S-index, Contrast of the CCF, and $\rm
H_{\alpha}$ show maximum power periods of 56.0, 55.9, 56.8, and 60.6 days,
with false alarm probabilities (FAPs)555All the FAPs in this work have been
computed analytically following Eq. 24 of Zechmeister & Kürster (2009). of
$5\times 10^{-6}\leavevmode\nobreak\ \%$, $1.8\times
10^{-3}\leavevmode\nobreak\ \%$, $1.2\times 10^{-1}\leavevmode\nobreak\ \%$,
and 1.1$\leavevmode\nobreak\ \%$ respectively, thus unveiling the presence of
a significant activity-related signal that most likely corresponds to the
rotation period of the star. The remaining two indicators BIS and NaD show no
significant peaks within the periodogram. Their maximum power periods
correspond to 2.9 days (39.0$\%$ FAP), and 3.1 days (30.4$\%$ FAP)
respectively, being most likely related to the mean observing cadence of
$\sim$2-3 days. It is remarkable that, when folded to the $\sim$56-day
periodicity found in most indicators, the NaD time series shows moderate signs
of a sinusoidal behavior, suggesting that the activity signal might be
manifested in this indicator as well. The RV periodogram shows a peak
coinciding with the orbital period of TOI-244.01 that cannot be seen in any of
the indicators, which suggests a planetary origin for the signal. However, the
signal is not significant. The maximum power period of the RV time series is
17.8 days ($2.9\times 10^{-1}\%$ FAP), which is compatible with the third
harmonic of the $\sim$56-day signal present in most activity indicators. Also,
in this region of the periodogram, there is another peak at 28.3 days with a
9.5$\%$ FAP that is compatible with the second harmonic of the $\sim$56-day
signal. The presence of these two prominent peaks indicates that the RV time
series is significantly affected by the stellar activity. Finally, there is
another relevant peak at 2.5 days (2.2$\%$ FAP). This periodicity is
compatible with the mean observing cadence, but we also explore the
possibility of it being an additional planetary signal in Sect. 4.2.

###  2.3 HARPS spectroscopy

GJ 1018 was observed by the High Accuracy Radial velocity Planet Searcher
(HARPS), which is mounted on the 3.6 m telescope located at ESO’s La Silla
Observatory (Chile) and has been operational since 2003. HARPS is a fiber-fed
cross dispersed echelle espectrograph located in a vacuum vessel that protects
the instrument from temperature and refractive index variations. It has a
spectral resolution power of 115 000 and covers a wavelength range between 378
and 691 nm. A total of 15 spectra were acquired between 15 December 2018 and 8
January 2019 under the program 1102.C-0339 (PI X. Bonfils). We downloaded the
reduced spectra (DRS version 3.8), which are publicly available in the ESO’s
Science Portal666http://archive.eso.org/scienceportal/home. Eleven spectra
were acquired with 1800 s exposure time, resulting in a median S/N of 14.6 per
resolution element at 650 nm, and the remaining four spectra were acquired
with 1500 s exposure time, resulting in a median S/N of 11.9. We present the
complete HARPS data set in Table 9. The standard deviation of the RV
observations is 4.3 m/s and the median uncertainty per data point is 2.1 m/s.
The S/N values obtained with relatively long exposures are remarkably low,
leading to a photon-noise limited RV precision (i.e., three times worse than
ESPRESSO).

###  2.4 ASAS-SN photometry

Table 1: Summary of the ASAS-SN observations of GJ 1018. Start date | End date | Camera | Band | Station | Location |  $\\#$Obs
---|---|---|---|---|---|---
11 May 2014 | 15 Sep 2018 | bf | V | Cassius | Cerro Tololo International Observatory (Chile) | 263
18 Sep 2017 | Operational | bj | g | Henrietta Leavitt | McDonald Observatory (USA) | 385
10 Nov 2017 | Operational | bn | g | Cecilia Payne | South African Astrophysical Observatory | 196
7 Oct 2018 | Operational | bF | g | Cassius | Cerro Tololo International Observatory (Chile) | 266

Figure 4: Top panel: ASAS-SN photometric time series of GJ 1018. The magenta,
gray, and blue vertical lines correspond to the TESS, HARPS, and ESPRESSO
observing windows, respectively. Left panels: GLS periodograms of the ASAS-SN
photometry corrected for the long-term trend. The vertical blue solid line
indicates the median rotation modulation period obtained by the GLS, and the
vertical blue dashed line indicate its second harmonic. The horizontal dotted
lines indicate the 10$\%$ (orange), 1$\%$ (blue), and 0.1$\%$ (green) FAP
levels. Right panels: Phase-folded light curves to the maximum power period
obtained by the GLS periodogram. The gray data points correspond to a 5-day
binning.

The sky region encompassing GJ 1018 is being observed by three different
stations of the All-Sky Survey for Supernovae (ASAS-SN, Shappee et al. 2014).
In Table 1, we include the observing periods, the number of observations, the
observing bands, and the location of the different stations. Each station
consists of four Nikon telephoto lenses of 14 cm aperture equipped with a 2048
$\times$ 2048 pixels CCD camera. The pixel scale is 8.0 arc seconds, which
corresponds to a 4.5 $\rm deg^{2}$ field of view per camera. Every night, the
cameras take three consecutive exposures of 90 s exposure time each, which are
combined afterward to increase the signal-to-noise of the light curves by a
factor of $\sqrt{3}$. Images obtained in poor weather conditions, that are out
of focus (FWHM ¿ 2.5 pixels), or where the studied source is near the detector
edge (at a distance ¡ 0.2 deg) are discarded by the survey pipeline. The
pipeline performs aperture photometry at a selected location through IRAF
(Tody 1986) considering a 2 pixel radius aperture and 7-10 pixel radius
annulus for the target star and for the reference stars, which are selected
from the Photometric All-Sky Survey (APASS, Henden et al. 2012; Henden 2019)
following the procedure from Kochanek et al. (2017).

We computed the light curves through the ASAS-SN Sky Patrol web
interface777https://asas-sn.osu.edu/. Given that GJ 1018 is a high proper
motion star ($\mu_{\alpha}$ = $-154.88\pm 0.02$ $\rm mas/yr$, $\mu_{\delta}$ =
$45.63\pm 0.03$ $\rm mas/yr$) and the typical FWHMs are comparable to the
radius of the aperture ($\sim$ 2 pixels), we shifted the aperture location
over time in order to ensure the target centering and thus avoid flux loses.
GJ 1018 is visible from the three observatories from mid-May to mid-February
of the following year, so we computed the photometry on a year-by-year basis,
introducing the coordinates corrected for proper motion corresponding to the
central time of each observing window (1 October). After the light curves
computation, we discarded those epochs in which the flux is below the
estimated 5-$\sigma$ detection limit for the target location, as well as those
data points with a deviation greater than 5-$\sigma$ of a flattened version of
the photometric time series. In Table 10, we present the complete photometric
data set.

In Fig. 4, we show the ASAS-SN photometric time series of GJ 1018. The
complete data set shows a long-term trend that could be caused by instrumental
systematics, or more likely by the magnetic cycle of the star, given the
similarity of the trends in each individual camera. We also show the GLS
periodograms of the photometry corrected for the abovementioned trends; that
is, we divided the time series of each camera by a degree three polynomial
that was previously fit to the data. For cameras bf, bj, and bF, we obtain
maximum power periods of 58.3, 60.9, and 60.0 days, with FAPs of 0.011$\%$,
0.0019$\%$, and 2.79$\%$ respectively. These periodicities are consistent with
the ones observed in most of the ESPRESSO activity indicators (Sect. 2.2).
Hence, they most likely reflect the rotation period of GJ 1018 (see Sect. 4.3
for an accurate determination). The maximum power period for camera bn is 30.7
days (FAP = 0.70$\%$), which coincides with the second harmonic of the
aforementioned rotation period.

##  3 Stellar characterization

###  3.1 General description of GJ 1018

Table 2: Stellar properties of GJ 1018.

Parameter | Value | Reference
---|---|---
Identifiers
TOI, TIC | 244, 118327550 | [1], [2]
GJ | 1018 | [3]
LP | 937-95 | [4]
2MASS | J00421695-3643053 | [5]
Gaia DR3 | 5001098681543159040 | [6]
Coordintates, parallax and kinematics
RA, DEC | 00:42:16.74, -36:43:04.71 | [6]
$\rm\mu_{\alpha}$ (mas/yr) | -154.880 $\pm$ 0.020 | [6]
$\rm\mu_{\delta}$ (mas/yr) | 45.632 $\pm$ 0.029 | [6]
Parallax (mas) | 45.300 $\pm$ 0.027 | [6]
Distance (pc) | 22.075 $\pm$ 0.013 | [6]
RV (km/s) | 15.1410 $\pm$ 0.013 | [7], Sect. 4.3
U (km/s) | -23.30 $\pm$ 0.02 | [7], Sect. 3.5
V (km/s) | 17.09 $\pm$ 0.02 | [7], Sect. 3.5
W (km/s) | -8.62 $\pm$ 0.03 | [7], Sect. 3.5
Gal. population | Thin disk | [7], Sect. 3.5
Atmospheric parameters and spectral type
$T_{\rm eff}$ (K) | 3433 $\pm$ 100 | [7], Sect. 3.2
log $g$ (dex) | 4.66 $\pm$ 0.07 | [7], Sect. 3.2
$\rm[Fe/H]$ (dex) (1) | -0.39 $\pm$ 0.07 | [7], Sect. 3.2
$\rm[Fe/H]$ (dex) (2) | -0.03 $\pm$ 0.11 | [7], Sect. 3.2
SpT | M2.5 V | [7], Sect. 3.2
Physicall parameters |  |
$\rm R_{\star}$ ($\rm R_{\odot}$) | 0.428 $\pm$ 0.025 | [7], Sect. 3.4
$\rm M_{\star}$ ($\rm M_{\odot}$) | 0.427 $\pm$ 0.029 | [7], Sect. 3.4
L ($\rm L_{\odot}\times 10^{-2}$) | $2.277\pm 0.042$ | [7], Sect. 3.3
$\rm P_{rot}$ (days) | $53.3^{+1.2}_{-1.1}$ | [7], Sect. 4.3
Age (Gyr) | 7 $\pm$ 4 | [8]
Chemical abundances |  |
$\rm[Mg/H]$ (dex) (1) | -0.28 $\pm$ 0.07 | [7], Sect. 3.5
$\rm[Si/H]$ (dex) (1) | -0.33 $\pm$ 0.07 | [7], Sect. 3.5
$\rm[Mg/H]$ (dex) (2) | 0.00 $\pm$ 0.10 | [7], Sect. 3.5
$\rm[Si/H]$ (dex) (2) | -0.01 $\pm$ 0.10 | [7], Sect. 3.5
Magnitudes |  |
$\rm M_{bol}$ (mag) | 8.847 $\pm$ 0.020 | [7], Sect. 3.3
TESS (mag) | 10.3475 $\pm$ 0.0073 | [2]
G (mag) | 11.553 $\pm$ 0.001 | [6]
B (mag) | 14.199 $\pm$ 0.033 | [9]
V (mag) | 12.684 $\pm$ 0.047 | [9]
g’ (mag) | 13.426 $\pm$ 0.063 | [9]
r’ (mag) | 12.097 $\pm$ 0.045 | [9]
i’ (mag) | 10.831 $\pm$ 0.062 | [9]
J (mag) | 8.827 $\pm$ 0.023 | [5]
H (mag) | 8.251 $\pm$ 0.033 | [5]
$\rm K_{s}$ (mag) | 7.970 $\pm$ 0.029 | [5]
888Based on (1) SteParSyn (2) ODUSSEAS analysis. References. [1] Guerrero et
al. (2021); [2] Stassun et al. (2019); [3] Gliese & Jahreiß (1991); [4] Luyten
(1979); [5] Skrutskie et al. (2006); [6] Gaia Collaboration et al. (2022); [7]
This work; [8] Angus et al. (2019); [9] Henden (2019).

GJ 1018 is a bright ($K$ = 7.97 mag) early-type M-dwarf star located in the
solar neighborhood. The Gaia DR3 provides a parallax of $\pi$ = 45.300 $\pm$
0.027 mas, which corresponds to a distance of $d$ = 22.075 $\pm$ 0.013 pc. The
photometry-based TESS Input Catalog (TIC v8.0.1, Stassun et al. 2019)
estimates an effective temperature of $T_{\rm eff}$ = 3407 $\pm$ 157 K,
surface gravity of log $g$ = 4.820 $\pm$ 0.004 dex, stellar radius of $R$ =
0.41 $\pm$ 0.01 $\rm R_{\odot}$, and stellar mass of $M$ = 0.40 $\pm$ 0.02
$\rm M_{\odot}$. In the next sections, we describe our stellar
characterization based on precise photometry and a high-resolution, high S/N
spectrum obtained from the combination of all ESPRESSO spectra. In Table 2, we
summarize the general properties of GJ 1018 as well as all our derived
parameters.

###  3.2 Stellar atmospheric parameters

We computed the stellar atmospheric parameters of GJ 1018 by means of the
SteParSyn code999https://github.com/hmtabernero/SteParSyn/ (Tabernero et al.
2022). The code implements the spectral synthesis method with an MCMC sampler
to retrieve the stellar atmospheric parameters. We employed a grid of
synthetic spectra computed with the Turbospectrum code (Plez 2012) alongside
BT-Settl stellar atmospheric models (Allard et al. 2012) and atomic and
molecular data of the Vienna atomic line database (VALD3; Ryabchikova et al.
2015). We considered a selection of Fe i, Ti i lines, and TiO molecular bands
that are well-suited to analyze M-dwarf stars (Marfil et al. 2021a). In all,
SteParSyn has allowed us to compute the following stellar parameters: $T_{\rm
eff}$ $=$ 3433 $\pm$ 10 K, $\log{g}$ $=$ 4.66 $\pm$ 0.07 dex, and [Fe/H] $=$
$-0.39$ $\pm$ 0.07 dex. In order to account for systematics associated with
$T_{\rm eff}$ when it is determined by spectral fitting techniques (e.g.,
Marfil et al. 2021b found a dispersion between 40 and 100 K among different
model atmospheres valid for M-dwarfs), we increased the $T_{\rm eff}$
uncertainty up to 100 K.

We performed an independent computation of the atmospheric parameters through
the newly machine learning tool
ODUSSEAS101010https://github.com/AlexandrosAntoniadis/ODUSSEAS/ (Antoniadis-
Karnavas et al. 2020), which has been designed to compute the $T_{\rm eff}$
and [Fe/H] of M-dwarfs. The method is based on measuring the pseudo Equivalent
Widths (pEWs) of specific absorption and blended lines in the wavelength range
5300 Å-6900 Å (Neves et al. 2014). Briefly, ODUSSEAS compares the measured EWs
to the machine learning models generated from reference HARPS spectra,
convolved to the resolution of the observed spectrum. Although ODUSSEAS was
initially developed for spectra with resolution powers from 48 000 to 115 000,
it has been applied successfully to ESPRESSO spectra with a resolution power
of 140 000. For the very high resolutions of ESPRESSO and HARPS, the results
derived by ODUSSEAS are essentially the same either by using directly the
original highest-resolution spectra of ESPRESSO, or by convolving first the
ESPRESSO spectra to the lower, but still high, resolution of HARPS. We ran
ODUSSEAS by considering a reference data set composed of 47 stars with
interferometry-based $T_{\mathrm{eff}}$ (Khata et al. 2021; Rabus et al.
2019), and [Fe/H] derived through the photometric calibration by Neves et al.
(2012) using the Gaia DR3 parallaxes. We obtain $T_{\mathrm{eff}}$ = 3419
$\pm$ 92 K and [Fe/H] = -0.03 $\pm$ 0.11 dex.

The ODUSSEAS effective temperature is in very good agreement with the value
computed by SteParSyn. However, there exist a strong discrepancy between the
metallicities. Obtaining accurate metallicities for M-dwarfs is a complicated
task, given the strong blending of spectral lines and molecular bands. For
example, Passegger et al. (2022) found mean deviations of around 0.1-0.3 dex
between different techniques, showing that any uncertainty below those values
is underestimated. Overall, we adopt the $T_{\rm eff}$ and $\log{g}$
estimations from the spectral syntesis method. However, given the strong
discrepancy in [Fe/H], we decide not to adopt either estimate, and instead
discuss both results independently in the subsequent sections.

###  3.3 Stellar bolometric luminosity

To determine the bolometric luminosity of GJ 1018, we first built the
photometric spectral energy distribution (SED) of the star using broadband and
narrowband photometry from the literature. The stellar SED is shown in Fig. 5,
which includes the Galaxy Evolution Explorer (GALEX) near-ultraviolet
photometry (Bianchi et al. 2017), the Johnson $BVRI$ photometry (Ducati 2002),
the $BV$ data from the AAVSO Photometric All-sky Survey (APASS, Henden 2019),
the $griz$ data from the Sloan Digital Sky Survey (York et al. 2000), the Gaia
Early Data Release 3 photometry (Gaia Collaboration et al. 2021), the $y$ data
from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS,
Flewelling et al. 2020), the Two Micron All Sky Survey (2MASS) near-infrared
$JHK_{s}$ photometry (Skrutskie et al. 2006), the Wide-field Infrared Survey
Explorer (WISE) $W1$, $W2$, $W3$, and $W4$ data (Wright et al. 2010), and the
optical multiphotometry of the Observatorio Astrofísico de Javalambre (OAJ)
Physics of the Accelerating Universe Astrophysical Survey (JPAS) and
Photometric Local Universe Survey (JPLUS) catalogs accessible through the
Spanish Virtual Observatory (Bayo et al. 2008). In total, there are 89
photometric data points defining the SED of GJ 1018 between 0.23 and $\sim$25
$\mu$m. The OAJ/JPAS data cover very nicely the optical region in the interval
0.40–0.95 $\mu$m with a cadence of one measurement per 0.01 $\mu$m. We used
the Gaia trigonometric parallax to convert all observed photometry and fluxes
into absolute fluxes, which we present in Table 11. The SED of GJ 1018 clearly
indicates its photospheric origin for wavelengths longer than 0.4 $\mu$m;
there are no mid-infrared flux excesses up to 25 $\mu$m.

Figure 5: Photometric spectral energy distribution of GJ 1018 from 0.23 to
$\sim$25 $\mu$m. The PHOENIX model corresponding to 3300 K, solar metallicity
and high gravity is added (Allard et al. 2003) in order to show that most
fluxes are photospheric in origin and that there is no infrared flux excesses
at long wavelengths. Vertical error bars denote flux uncertainties and
horizontal error bars account for the width of the passbands. The fluxes,
effective wavelengths and widths of all passbands were taken from the Virtual
Observatory SED Analyzer database.

We integrated the SED over wavelength to obtain the absolute bolometric flux
($F_{\rm bol}$) using the trapezoidal rule. We did not include the $\sl Gaia$
$G$-band flux in the computations because the filter large passband width
encompasses various redder and bluer filters. We then applied $M_{\rm
bol}=-2.5\leavevmode\nobreak\ {\rm log}\,F_{\rm bol}-18.988$ (Cushing et al.
2005), where $F_{\rm bol}$ is in units of W$\cdot$m-2, to derive the absolute
bolometric magnitude $M_{\rm bol}$ = 8.8465 ${}^{+0.0203}_{-0.0200}$ mag for
GJ 1018, from which we obtain a bolometric luminosity of $L$ =
$2.277\,\pm\,0.042\times 10^{-2}$ L⊙. The quoted error bar accounts for the
photometric uncertainties in all observed bands and the trigonometric distance
error.

###  3.4 Stellar radius and mass

We determined the radius of GJ 1018 from the well-known Stefan-Boltzmann law.
Based on the effective temperature from Sect. 3.2 and the bolometric
luminosity from Sect. 3.3, we obtained a radius of $R_{\star}=0.428\pm 0.025$
R⊙. This radius determination is independent of any evolutionary model and
depends only on distance (well known from ${\sl Gaia}$), bolometric luminosity
(well determined from the SED, see Sect. 3.3), and the model atmospheres used
to fit the observed spectra.

The mass of GJ 1018 can be derived following different approaches. One is
given by the empirical mass-luminosity relationship of Mann et al. (2019),
which is derived from 62 nearby, late-type binaries with known orbits. The
authors calibrated the stellar mass as a function of the absolute $K$-band
magnitude of the stars, finding that metallicity has little impact on the mass
of the stars for approximately solar composition. According to the Mann et al.
(2019) relationship, GJ 1018 has a mass of $M_{\star}$ = 0.400 $\pm$ 0.025 M⊙,
where the error budget includes the photometric error and the dispersion of
the mass-luminosity relation. Another widely used relation valid for M-dwarfs
is the mass-radius equation given in Schweitzer et al. (2019), obtained from
55 detached, double-lined, doubled-eclipsing, and main-sequence M-dwarf
binaries from the literature. Following this relation, we obtain a mass of
$M_{\star}$ = $0.427\pm 0.029$ $\rm M_{\odot}$. Both mass determinations are
compatible at the 1$\sigma$ level. Given that the relation by Schweitzer et
al. (2019) makes use of the Stefan-Boltzmann law, its mass derivation is
consistent with our method for obtaining the stellar radius, thus we adopted
$R_{\star}=0.428\pm 0.025$ R⊙ and $M_{\star}=0.427\pm 0.029$ M⊙.

###  3.5 Galactic membership and Mg and Si abundances

Because of heavy line blending, the determination of the individual elemental
abundances of M-dwarfs from the visible spectra is very hard (e.g., Maldonado
et al. 2020). In this work, we estimated the abundance of Mg and Si closely
following the procedure presented in Demangeon et al. (2021). In brief, we
used the systemic RV, parallax, ra/dec coordinates and proper motions from
Gaia DR3 to derive the Galactic space velocity UVW of GJ 1018. We obtain U =
-23.30 $\pm$ 0.02 km/s, V = 17.09 $\pm$ 0.02 km/s, and W = -8.62 $\pm$ 0.03
km/s with respect to the local standard of rest (LSR) adopting the solar
peculiar motion from Robin et al. (2003). Based on these velocities, we used
the widely-known kinematic approach from Bensby et al. (2003) and the
kinematic characteristics for stellar components in the Solar neighborhood
from Robin et al. (2003) to estimate the probability that GJ 1018 belongs to
the thin disk (D), the thick disk (TD), and the halo (H), obtaining 99.11$\%$,
0.889$\%$, and 0.001$\%$ respectively. Hence, it is very likely that GJ 1018
is a member of the Galactic thin-disk population. Then, from the APOGEE DR17
(Abdurro’uf et al. 2022) we selected cool stars with metallicities similar to
that of GJ 1018 and belonging to the chemically defined Galactic thin disk. We
obtained a sample of several thousand stars for which we calculated the mean
abundances of Mg and Si and their standard deviation (star-to-star scatter).
Considering the SteParSyn metallicity, we obtain [Mg/H] = -0.28 $\pm$ 0.07 dex
and [Si/H] = -0.33 $\pm$ 0.07 dex. Considering the ODUSSEAS metallicity, we
obtain [Mg/H] = 0.00 $\pm$ 0.10 dex and [Si/H] = -0.01 $\pm$ 0.10 dex.

##  4 Analysis and results

###  4.1 TESS photometry analysis

We first analyzed the TESS PDCSAP photometry described in Sect. 2.1 through a
model that consists of two components: a transit model, and a Gaussian process
(GP) that models the correlated photometric noise (Rasmussen & Williams 2006;
Roberts et al. 2012).

We implemented the Mandel & Agol (2002) quadratic limb darkened transit model
through batman (Kreidberg 2015). The model is defined by the orbital period of
the planet ($P_{\rm orb}$), the time of inferior conjunction ($T_{\rm 0}$),
the orbital inclination ($i$), the quadratic limb darkening (LD) coefficients
$u_{\rm 1}$ and $u_{\rm 2}$, the planet-to-star radius ratio ($R_{\rm
p}/R_{\star}$), and the semimajor axis scaled to the stellar radius, which we
parametrized through $P_{\rm orb}$ and the stellar mass ($M_{\rm\star}$) and
radius ($R_{\rm\star}$) following the Kepler’s Third Law. We modeled the TESS
correlated noise through a GP with an approximate Matérn-3/2 kernel (Foreman-
Mackey et al. 2017; Foreman-Mackey 2018), which has been successfully used to
model the unknown mixture of stellar variability and residual systematics of
TESS SPOC photometry (e.g., Morello et al. 2022; Mori et al. 2022; Murgas et
al. 2023). This kernel is especially suitable to model TESS light curves where
the stellar rotation modulation is not observable (as is the case of GJ 1018,
see Sect. 2.1), and instead, residual systematics are a significant component
of the photometric variability, given that it has covariance properties that
are especially well matched to short-term instrumental red-noise structures
(Pepper et al. 2017; Stefánsson et al. 2020). The approximate Matérn-3/2
kernel can be written in terms of the temporal separation between two data
points $\tau=t_{i}-t_{j}$ as

$K_{3/2}=\eta_{\sigma}^{2}\left[\left(1+\frac{1}{\epsilon}\right)e^{-(1-\epsilon)\sqrt{3}\tau/\eta_{\rho}}\cdot\left(1-\frac{1}{\epsilon}\right)e^{-(1+\epsilon)\sqrt{3}\tau/\eta_{\rho}}\right],$
(1)

where the hyperparameters $\eta_{\sigma}$ and $\eta_{\rho}$ are the
characteristic amplitude and timescale of the correlated variations,
respectively, and $\epsilon$ controls the approximation to the exact
Matérn-3/2 kernel. Given that the amplitudes and timescales of the TESS
systematics can vary from one sector to another, we fit those parameters
independently ($\eta_{\sigma_{i}}$ and $\eta_{\rho_{i}}$, where i denotes the
sector), while $\epsilon$ was fixed to its default value of $10^{-2}$
(Foreman-Mackey et al. 2017). We also included a jitter term for each sector,
which we added quadratically to the TESS flux uncertainties in order to model
the uncorrelated noise not taken into account in our model.

We used a Markov Chain Monte Carlo (MCMC) affine-invariant ensemble sampler
(Goodman & Weare 2010) as implemented in emcee (Foreman-Mackey et al. 2013) in
order to sample the posterior probability density function of the different
parameters involved in our model. To do so, we used four times as many walkers
as the number of parameters, and performed two consecutive runs. The first run
(or burn-in) consisted of 200 000 iterations. After this run, we reset the
sampler and initialized the second run (or production) with 100 000 iterations
while considering the initial values from the last iteration of the burn-in
phase. To ensure the convergence of the chains, we estimated the
autocorrelation time for each parameter and checked that it is at least 30
times smaller than the chain length.

We ran an MCMC fit starting from wide uniform priors for all the parameters
involved in the model except for those for which we have prior information,
which we constrained through Gaussian priors. These parameters are the $T_{0}$
and $P$ of TOI-244.01 (from the TLS periodogram, Sect. 2.1), the stellar
radius and mass (from our spectroscopic analysis, Sect. 3.4), and the
quadratic LD coefficients, which we computed from the ldtk package (Parviainen
& Aigrain 2015). The package infers the coefficients of a given LD law relying
on the Husser et al. (2013) synthetic spectra library, spectroscopic $T_{\rm
eff}$, log $g$, and [Fe/H], and the instrument transmission curve. We used the
ldtk uncertainties as the widths of the Gaussian distributions. In order to
account for possible systematics in the estimated LD coefficients (e.g., Patel
& Espinoza 2022), we also ran the MCMC fit by considering conservative widths
of 0.2, obtaining identical results for the planetary parameters. We show the
prior distributions for this analysis and subsequent ones in Table 3.

In Table 12, we include the median and 1$\sigma$ (68.3$\%$ credible intervals)
of the posterior distributions of the fit parameters. In Fig. 6, we show the
complete TESS light curve together with the global model (transit + GP)
evaluated on the fit parameters.

Figure 6: TESS light curve of GJ 1018 with the median posterior global model
(transit + GP) superimposed, in which we can see the different locations of
the TOI-244.01 transit events over the smooth photometric modulation. The gray
data points correspond to the SPOC 2-minute cadence PDCSAP photometry, and the
blue and orange data correspond to 50-minute and 1.5-day binned data
respectively.  Table 3: Prior distribution of the parameters used in our
models.

Symbol | Parameter | Distribution
---|---|---
Orbital parameters
$P$ [days] | Orbital period | $\mathcal{G}\,(7.397,0.002)$
$T_{\rm 0}$ [RJD] | Time of inf. conj. | $\mathcal{G}\,(58357.36,0.02)$
$i$ [degrees] | Orbital inclination | $\mathcal{U}\,(50,90)$
$cos(w)\sqrt{e}$ | Ecc. parametrization | $\mathcal{U}\,(-\frac{\sqrt{2}}{2},\frac{\sqrt{2}}{2})$
$sin(w)\sqrt{e}$ | Ecc. parametrization | $\mathcal{U}\,(-\frac{\sqrt{2}}{2},\frac{\sqrt{2}}{2})$
Planet parameters
$R_{\rm p}$/$R_{\rm\star}$ | Planet-to-star radius ratio | $\mathcal{U}\,(0.0,0.1)$
$K$ [$\rm m/s$] | RV semi-amplitude | $\mathcal{U}\,(1,10)$
Stellar parameters
$M_{\rm\star}$ $[\rm M_{\odot}]$ | Stellar Mass | $\mathcal{ZTG}\,(0.427,0.029)$
$R_{\rm\star}$ $[\rm R_{\odot}]$ | Stellar Radius | $\mathcal{ZTG}\,(0.428,0.025)$
$u1$ | LD coefficient | $\mathcal{ZTG}\,(0.2204,0.0011)$
$u2$ | LD coefficient | $\mathcal{ZTG}\,(0.4112,0.0017)$
Matérn-3/2 GP hyperparameters
$\eta_{\sigma_{S2}}$ [$\rm e^{-}/s$] | S2 amplitude | $\mathcal{U}\,(0,10^{2})$
$\eta_{\rho_{S2}}$ [days] | S2 timescale | $\mathcal{U}\,(0,10^{2})$
$\eta_{\sigma_{S29}}$ [$\rm e^{-}/s$] | S29 amplitude | $\mathcal{U}\,(0,10^{2})$
$\eta_{\rho_{S29}}$ [days] | S29 amplitude | $\mathcal{U}\,(0,10^{2})$
Quasiperiodic GP hyperparameters
$\eta_{\rm 1,RV}$ [$\rm m/s$] | Amplitude (RV) | $\mathcal{LU}\,(10^{-2},10)$
$\eta_{\rm 2,RV}$ [days] | Decay timescale | $\mathcal{U}\,(0,5\cdot 10^{2})$
$\eta_{\rm 3,RV}$ [days] | Correlation period | $\mathcal{U}\,(40,80)$
$\eta_{\rm 4,RV}$ | Periodic scale | $\mathcal{LU}\,(10^{-3},10)$
$\eta_{\rm 1,FWHM}$ [$\rm m/s$] | Amplitude (FWHM) | $\mathcal{LU}\,(10^{-2},10)$
Instrument-dependent parameters (TESS)
$F_{\rm 0,S2}$ [$\rm e^{-}/s$] | TESS S2 flux offset | $\mathcal{U}\,(-10^{2},10^{2})$
$F_{\rm 0,S29}$ [$\rm e^{-}/s$] | TESS S29 flux offset | $\mathcal{U}\,(-10^{2},10^{2})$
$\sigma_{\rm TESS,S2}$ [$\rm e^{-}/s$] | TESS S2 jitter | $\mathcal{U}\,(0,10^{2})$
$\sigma_{\rm TESS,S29}$ [$\rm e^{-}/s$] | TESS S29 jitter | $\mathcal{U}\,(0,10^{2})$
Instrument-dependent parameters (ESPRESSO and HARPS)
$\gamma_{\rm ESPR,RV}$ [$\rm m/s$] | ESPR. sys. RV | $\mathcal{U}\,(13000,16000)$
$\gamma_{\rm HAR,RV}$ [$\rm m/s$] | HARPS sys. RV | $\mathcal{U}\,(13\cdot 10^{3},16\cdot 10^{3})$
$\gamma_{\rm ESPR,FWHM}$ [$\rm m/s$] | ESPR. sys. FWHM | $\mathcal{U}\,(0,10^{4})$
$\gamma_{\rm HAR,FWHM}$ [$\rm m/s$] | HARPS sys. FWHM | $\mathcal{U}\,(0,10^{4})$
$\sigma_{\rm ESPR,RV}$ [$\rm m/s$] | ESPR. RV jitter | $\mathcal{U}\,(0,10)$
$\sigma_{\rm HAR,RV}$ [$\rm m/s$] | HARPS RV jitter | $\mathcal{U}\,(0,10)$
$\sigma_{\rm ESPR,FWHM}$ [$\rm m/s$] | ESPR. FWHM jitter | $\mathcal{U}\,(0,10)$
$\sigma_{\rm HAR,FWHM}$ [$\rm m/s$] | HARPS FWHM jitter | $\mathcal{U}\,(0,10)$
Orbital and planet parameters of an additional Keplerian
$P_{\rm 2}$ [days] | Orbital period | $\mathcal{U}\,(0,5\cdot 10^{2})$
$T_{\rm 0,2}$ [RJD] | Time of inf. conj. | $\mathcal{U}\,(58467,59855)$
$cos(w_{2})\sqrt{e_{2}}$ | Ecc. parametrization | $\mathcal{U}\,(-\frac{\sqrt{2}}{2},\frac{\sqrt{2}}{2})$
$sin(w_{2})\sqrt{e_{2}}$ | Ecc. parametrization | $\mathcal{U}\,(-\frac{\sqrt{2}}{2},\frac{\sqrt{2}}{2})$
$K_{2}$ [$\rm m/s$] | RV semi-amplitude | $\mathcal{U}\,(1,10)$
111111The $\mathcal{U}(a,b)$ and $\mathcal{LU}(a,b)$ symbols indicate uniform
and log-uniform distributions, being $a$ and $b$ the lower and upper limits.
The $\mathcal{G}(\mu,\sigma)$ and $\mathcal{ZTG}(\mu,\sigma)$ symbols indicate
Gaussian and zero-truncated Gaussian distributions, being $\mu$ and $\sigma$
the mean and width of the distributions. RJD refers to the Reduced Julian Day
(JD $-$ 2 400 000).

###  4.2 ESPRESSO and HARPS radial velocity analysis

We analyzed the ESPRESSO and HARPS RV data sets described in Sects. 2.2 and
2.3 through a model composed of three components: a Keplerian, which models
planetary-induced RV signals, an instrumental component, which models the
systemic velocity as measured by each instrument, and a GP, which models the
RV correlated noise induced by the stellar activity.

We implemented the Keplerian component through the Python package radvel
(Fulton et al. 2018) by using the parametrization
$\left\\{P,T_{0},K,\sqrt{e}cos(w),\sqrt{e}sin(w)\right\\}$, being $P$ the
orbital period of the planet, $T_{0}$ the time of inferior conjunction, $K$
the semi-amplitude, $e$ the orbital eccentricity, and $w$ the planetary
argument of periastron. The instrumental component of our model consists of an
offset that corresponds to the systemic radial velocity of the star as
measured by each instrument ($\gamma_{ins})$. The periodograms of the
spectroscopic data show that the stellar rotation induces significant
activity-related RVs (Sect. 2.2). We modeled this correlated noise through a
GP with a quasiperiodic kernel (Ambikasaran et al. 2015; Faria et al. 2016).
The choice of this kernel is motivated by the fact that although stellar
rotation is a periodic phenomenon, the activity-induced RV signals are
quasiperiodic, given that active regions evolve; that is, they move on the
stellar surface and appear and disappear throughout the magnetic cycle
timescale. The quasiperiodic kernel depends on four hyperparameters
($\eta_{1}$, $\eta_{2}$, $\eta_{3}$, and $\eta_{4}$) and can be written in
terms of the separation between data points $\tau=t_{i}-t_{j}$ as

$K_{QP}(\tau)=\eta_{1}^{2}\rm{exp}\left[-\frac{\tau^{2}}{2\eta_{2}^{2}}-\frac{2sin^{2}\left(\frac{\pi\tau}{\eta_{3}}\right)}{\eta_{4}^{2}}\right].$
(2)

The hyperparameter $\eta_{1}$ scales with the amplitude of the stellar
activity signal. $\eta_{3}$ corresponds to the main periodicity of the signal
and it is considered a measure of the stellar rotation period (e.g., Angus et
al. 2018). $\eta_{2}$ is the lengthscale of exponential decay. For two data
points in the X-axis far from each other, the larger $\eta_{2}$ is, the more
closely correlated those data points are (e.g., Angus et al. 2018); therefore,
$\eta_{2}$ is considered as a measure of the timescale of growth and decline
of the active regions (Haywood et al. 2014; Faria et al. 2016). $\eta_{4}$
controls the amplitude of the $\rm sin^{2}$ term. The smaller $\eta_{4}$ is,
data points separated by one orbital period will be much more closely
correlated than those data points separated by a different period of time;
thus, $\eta_{4}$ indicates the complexity of the harmonic content of the
activity signal. Finally, in order to model the white noise not taken into
account in our model, we included a jitter term per instrument
($\sigma_{ins}$) that we added quadratically to the uncertainties of our RV
measurements.

In order to obtain a more robust estimation of the model parameters, we
included to our data set an activity indicator to be modeled jointly with the
HARPS and ESPRESSO RVs. This approach consists of using two GP kernels with
shared hyperparameters except the signal amplitude (hereafter we differentiate
between $\eta_{\rm 1,RV}$ and $\eta_{\rm 1,indicator}$). This approach has
been used in previous works and relies on the assumption that the variations
on the activity indicators are caused by the stellar activity alone, and that
their periodicity and coherence are the same as those of the activity
component of the RV (e.g., Suárez Mascareño et al. 2020; Lillo-Box et al.
2020a; Barros et al. 2022). From the time series and periodograms of Fig. 3,
we can perceive a certain correlation between the RVs and the activity
indicators. We computed the strength of these correlations by means of the
Pearson product-moment correlation coefficient (Rodgers & Nicewander 1988),
obtaining the following results: 0.35 for the correlation with the FWHM, 0.20
with S-index, -0.07 with $\rm H_{\alpha}$, -0.22 with BIS, 0.02 with NaD, and
0.21 with the contrast of the CCF. Showing a moderate degree of correlation,
we decided to use the FWHM of the CCF as the activity indicator in our
modeling.

We explored the constraining capacity of the HARPS data, motivated by their
shorter time coverage and smaller precision than ESPRESSO data. To do so, we
ran an MCMC fit considering a circular Keplerian model ($e$ = 0, $w$ = 0) to
the ESPRESSO and ESPRESSO+HARPS data sets. Besides, for each data set, we
performed a complementary fit with zero planets in order to obtain the
significance against the null hypothesis. To do so, we used the Perrakis
algorithm through the bayev implementation (Díaz et al. 2016) to compute the
logarithm of the Bayesian evidence of the model ln($\mathcal{Z}$) based on the
15$\%$ of the final flattened chain. In the same way as in the previous
section, we used wide uniform priors for all the parameters, except for those
for which we have prior information (see Table 3 for the detailed prior
distributions). As a result, the inferred parameters are compatible in both
tested data sets (ESPRESSO and ESPRESSO+HARPS). However, we obtain a larger
Bayesian evidence against the null hypothesis when using the ESPRESSO+HARPS
data set: $\rm\Delta ln(\mathcal{Z})_{ESPR}$ = +16.5, $\rm\Delta
ln(\mathcal{Z})_{ESPR+HARPS}$ = +17.7. This way, the inclusion of HARPS data
slightly increases the significance of the detection. Consequently, due to the
higher evidence, together with the advantage of having a longer time span to
search for additional signals, we used the ESPRESSO+HARPS data set for the
subsequent analysis.

We tested six different models in order to assess whether we can detect
additional planetary signals, and to select the simplest model that best
represents our data. The models involve one and two planets in all the
possible combinations of circular and eccentric orbits. To identify the
models, we use the nomenclature Xp[$\rm P_{i}c$], where X is the total number
of planets considered in the system, and $\rm P_{i}$ indicates which planets
have assumed circular orbits. In this work, planet ”1” corresponds to
TOI-244.01, and planet ”2” to an additional planet without prior orbital
information (see Table 3). This way, we have tested the following models:
1p1c, 1p, 2p1c2c, 2p, 2p1c, and 2p2c. In Fig. 7, we compare the obtained
difference of the log-evidence of each model and the log-evidence of the 0
planet model $\rm ln(\mathcal{Z}_{\rm Xp[\rm P_{i}c]})-ln(\mathcal{Z}_{0p})$.
We assume that a difference of +6 (i.e., $\rm\Delta ln\mathcal{Z}$ ¿ 6)
indicates strong evidence in favor of the largest evidence model (Trotta
2008). For our data set, there is no model meeting the condition when compared
to a simpler model, so we chose the simplest model as the one that best
represents our data set, which, in this case, coincides with the one with the
largest evidence: the 1 planet model in a circular orbit (1p1c).

Figure 7: Differences of the log-evidences of different models (labeled in the
X axis) and the 0 planet model, tested on the ESPRESSO+HARPS data set. The
gray shaded region indicates the $\rm 0\geq\Delta ln\mathcal{Z}\geq$ -6 region
from the largest evidence model (1p1c).

###  4.3 Joint analysis

We inferred the final parameters of the system by modeling jointly the TESS
photometry (same way as in Sect. 4.1) and ESPRESSO+HARPS radial velocities
(same way as in Sect. 4.2; considering the 1p1c model). In Fig. 8 (top
panels), we show the complete RV data set together with the median posterior
Keplerian+GP model. In the lower panels, we include the ESPRESSO+HARPS RVs and
TESS photometry folded to the inferred orbital period together with the median
posterior model, being both subtracted from their corresponding GP components.
In Table 4, we present the main parameters of TOI-244 b. The complete list of
fitted parameters can be found in Table 12.

Table 4: TOI-244 b main parameters obtained from the fitted parameters in the joint analysis (Sect. 4, Table 12) and the computed stellar parameters (Sect. 3, Table 2). Symbol | Parameter | Value
---|---|---
$P$ [days] | Orbital period | $7.397225^{+0.000026}_{-0.000023}$
$T_{0}$ [RJD] | Time of inf. conj. | 58357.3627 $\pm$ 0.0020
$R_{\rm p}$ [$\rm R_{\oplus}$] | Planet radius | 1.52 $\pm$ 0.12
$M_{\rm p}$ [$\rm M_{\oplus}$] | Planet mass | 2.68 $\pm$ 0.30
$\rho_{\rm p}$ [$\rm g\cdot cm^{-3}$] | Planet density | 4.2 $\pm$ 1.1
$a/R_{\star}$ |  $a$ relative to $R_{\star}$ | 28.1 $\pm$ 1.8
$a$ [AU] | Orbit semi-major axis | 0.0559 $\pm$ 0.0013
$S$ [$\rm S_{\oplus}$] | Insolation flux | 7.3 $\pm$ 0.4
$T_{\rm eq}$ [K] | Equilibrium temperature | 458 $\pm$ 20
$g$ [$\rm m\cdot s^{-2}$] | Surface gravity | 11.3 $\pm$ 2.2
$b$ | Impact parameter | 0.61 $\pm$ 0.31
$\delta$ [ppt] | Transit depth | 1.06 $\pm$ 0.12
$T_{\rm 14}$ [hours] | Transit duration | 1.7 $\pm$ 0.5

Figure 8: Top panel: ESPRESSO and HARPS radial velocities of GJ 1018. The
purple solid line indicates the median posterior global model, and the black
and gray solid lines indicate the median posterior GP and Keplerian models,
respectively. The shaded purple region indicates the 68.7$\%$ confidence
interval of the global model. Bottom panel: ESPRESSO and HARPS RVs (left) and
TESS photometry (right) subtracted from their corresponding GP components and
folded to the orbital period of TOI-244 b. The solid lines indicate the median
posterior models.

##  5 Discussion

###  5.1 Detectability of additional planets in the RV data set

Figure 9: Detectability map for the ESPRESSO+HARPS data set obtained from the
injection-recovery test. The redder values correspond to false alarm
probabilities (FAPs) larger than 1% while the blue color indicates that the
signal is recovered with FAP $<1$%. The location of TOI-244 b is indicated by
the green symbol. The green dotted vertical lines indicate the optimistic HZ
around GJ 1018 according to Kopparapu et al. (2013). The bottom dashed line
indicates the limit imposed by the median radial velocity uncertainty of the
original data set (i.e., before removing the planet and activity contribution)
while the upper dashed line indicates the limit corresponding to the rms of
the data.

We constrained the possibility of additional planets in the radial velocity
data set by following the injection-recovery procedure described in Suárez
Mascareño et al. (2022). In brief, we first removed the radial velocity
contribution from the confirmed planet TOI-244 b. The residuals from this
include the activity of the star. We then injected a Keplerian model from a
grid of periods (from 1 to 1000 days, 100 bins in log-space) and planet masses
(from 0.1 to 30 $\rm M_{\oplus}$, 100 bins in log-space) and random phases. We
assumed coplanar orbits with TOI-244 b. From the resulting RVs we computed the
activity model using the hyper-parameters determined in the joint analysis
section and removed its contribution. We then computed the false alarm
probability of the power at the injected period. We consider the injected
planet signal is detected if the FAP is below 1%. Figure 9 shows the result of
this exercise. All injections implying a root mean square (rms) larger than
1.5 times the original rms are assumed as detected. We repeated the process
five times and average all the iterations to obtain the final detectability
matrix shown in Fig. 9. The current ESPRESSO+HARPS data set allows us to
detect planets down to 2 $\rm M_{\oplus}$ at orbital periods up to around 22
days.

It is of special interest to constrain the presence of planets in the
habitable zone (HZ) of GJ 1018, given that late K-dwarfs and early M-dwarfs
represent an ideal trade-off between detectability and true habitability of
their planets (see the KOBE experiment; Lillo-Box et al. 2022). For the HZ of
GJ 1018, which is located in the period range 24-102 days (Kopparapu et al.
2013), the current data allows us to discard planets with masses above 20 $\rm
M_{\oplus}$.

###  5.2 Internal structure of TOI-244 b

####  5.2.1 TOI-244 b in the mass-radius diagram

Figure 10: Mass-radius diagrams for the small planet sample with measured
dynamical masses with a precision better than 20$\%$. Left: Color coding
indicates the stellar host metallicities. TOI-244 b is not filled with any
color since we get inconsistent metallicities with the different methods used
(see Sect. 3.2). The dashed and dotted lines correspond to theoretical
interior models that consider different mass percentages of Fe and $\rm
MgSiO_{3}$ (Zeng et al. 2016, 2019). Right: Color coding indicates the stellar
insolation received by the planets. The dashed lines correspond to theoretical
models from Zeng et al. (2019), which consider planets without a significant
amount of volatiles (back and red lines), planets with condensed water (dark
blue), and planets with $\rm H_{2}$ atmospheric envelopes (light blue). The
dotted lines correspond to the Turbet et al. (2020) theoretical models for
Earth-like planets with $\rm H_{2}O$-dominated atmospheres. These plots have
been prepared using mr-plotter, which is available at
https://github.com/castro-gzlz/mr-plotter.

In Fig. 10, we plot the radius versus the mass for all the known planets from
the NASA Exoplanet Archive with measured dynamical masses with a precision
better than $20\%$. TOI-244 b is located in an unpopulated region,
significantly separated (1.8$\sigma$ and 6.7$\sigma$ in radius and mass
respectively) from the Earth-like composition curve (33$\%$ Fe and 66$\%$ $\rm
MgSiO_{3}$ in mass; Zeng et al. 2019), where rocky planets typically reside.
Being located above that curve, TOI-244 b has a lower density than expected.
We highlight other five planets in this particular region of the parameter
space: TOI-561 b (Lacedelli et al. 2021; Weiss et al. 2021; Brinkman et al.
2023), L 98-59 c and d (Demangeon et al. 2021), HD 260655 c (Luque et al.
2022), and TOI-4481 b (Palle et al. 2023). The existence of these planets
might be explained by the presence of lighter elements than those expected for
Earth-like compositions. This translates into three possible scenarios:
planets with a scarcity or complete absence of iron on their cores (being thus
practically composed of silicates), planets with a significant amount of
volatile elements, or planets in which both scenarios coexist and have a
significant effect on the density of the planet. In the following, we discuss
each scenario separately.

####  5.2.2 TOI-244 b as an iron-free planet

Figure 11: Planet density normalized by the expected density of an Earth-like
composition (Dorn et al. 2017) versus the estimated iron-to-silicate mass
fraction of the original protoplanetary disk (Santos et al. 2015, 2017) for
the sample studied in Adibekyan et al. (2021) and for TOI-244 b considering
the metal-poor and solar metallicity characterizations (see Sect. 3.2). The
Earth and Mercury are indicated with their respective symbols in black. All
error bars show 1$\sigma$ uncertainties.

The abundance of refractory elements such as Mg, Si, and Fe of solar-type
stars is considered a proxy of the composition of the initial protoplanetary
disk from which stars and planets were formed (e.g., Dorn et al. 2015;
Unterborn et al. 2016). In a recent study, Adibekyan et al. (2021) used
stellar abundances of 22 stars to estimate the iron-to-silicate mass fractions
of their initial protoplanetary disks ($f^{\rm star}_{\rm iron}$) based on the
stoichiometric model presented by Santos et al. (2015, 2017). The authors
found a relationship between $f^{\rm star}_{\rm iron}$ and the densities and
iron mass fractions of their hosted planets, disclosing a compositional link
between the rocky planets and their host stars. In other words, stars with
lower $f^{\rm star}_{\rm iron}$ have lighter planets with lower iron-to-
silicate mass fractions121212The sample analyzed in Adibekyan et al. (2021) is
composed of highly irradiated planets, which are assumed to have negligible
atmospheres, similar to the scenario we are discussing.. In Fig. 11, we plot
the planet density normalized by the expected density of an Earth-like
composition ($\rho$/$\rho_{Earth-like}$; Dorn et al. 2017) as a function of
$f^{\rm star}_{\rm iron}$ for the same planet sample as in Adibekyan et al.
(2021) and for TOI-244 b. For the metal-poor stellar characterization, we
estimate $f^{\rm star}_{\rm iron}$ = 29.2 $\pm$ 4.2 $\%$, which would place
TOI-244 b in agreement with the correlation. For the solar metallicity
characterization, we estimate $f^{\rm star}_{\rm iron}$ = 32.0 $\pm$ 6.5 $\%$,
which would place TOI-244 b slightly more deviated from the trend.

Overall, the low density of the planet together with the possibility of having
a relatively low $f^{\rm star}_{\rm iron}$ (in the metal-poor scenario)
suggests that TOI-244 b might have an iron mass fraction smaller than that
expected for an Earth-like planet. However, according to Fig. 10 (left),
TOI-244 b has a density below what is expected for a 100$\%$ silicate
composition at the 1$\sigma$ level. Hence, the most likely scenario to explain
the low-density of TOI-244 b is that it has a nonnegligible amount of volatile
elements in its composition. The color coding of Fig. 10 (left) represents the
metallicity of the stellar hosts, showing that the growing population of low-
density super-Earths tend to be formed around metal-poor stars (see Sect.
5.3.1 for a more extended discussion). Interestingly, the presence of
volatiles in planets orbiting metal-poor stars is very expected, given that
the building blocks of the original protoplanetary disks that form metal-poor
stars are expected to have lower iron and higher water content than the
expected for the disks that form stars with solar metallicities (Santos et al.
2017).

####  5.2.3 TOI-244 b as a volatile-rich planet

The most abundant volatiles on protoplanetary disks are $\rm H_{2}$/He and
$\rm H_{2}O$ (e.g., Lodders 2003; Terada et al. 2007). In interior modeling,
it is common to consider a hydrogen-rich envelope, which is representative of
a primordial atmosphere. However, water is commonly considered in condensed
form (either liquid or solid), based on traditional mass-radius relationships
(e.g., Seager et al. 2007; Swift et al. 2012; Zeng et al. 2016). In recent
work, Turbet et al. (2020) computed new mass-radius relationships taking into
account that for $\rm H_{2}O$-rich rocky planets more irradiated than the
runaway greenhouse limit (i.e $S$ ¿ 1.1 $\rm S_{\oplus}$), water is unstable
in condensed form, so it would form a thick $\rm H_{2}O$-dominated atmosphere.
Hence, given the much lower density of water vapor than liquid or solid water,
the water content of highly irradiated $\rm H_{2}O$-rich planets is expected
to be dramatically lower than the commonly computed ones (Mousis et al.
2020a). In the right-hand mass-radius diagram of Fig. 10, we include a
theoretical composition model consistent with Earth-like planets with $\rm
H_{2}$-dominated atmospheres contributing 0.1$\%$ to the total planetary mass
(Zeng et al. 2019), showing that a small amount of hydrogen would
significantly increase the radius of a rocky planet. We also include three
theoretical models of Earth-like planets with an additional $\rm
H_{2}O$-dominated atmosphere (Turbet et al. 2020). In this case, a 4$\%$ water
mass fraction would reproduce the observed density of TOI-244 b. In addition,
we include the 50$\%$ $\rm H_{2}O$-condensed and 50$\%$ $\rm MgSiO_{3}$
theoretical model from Zeng et al. (2019), which is widely used to describe
planets in the higher mode of the bimodal radius distribution (2 $\rm
R_{\oplus}$ ¡ $R_{\rm p}$ ¡ 4 $\rm R_{\oplus}$). This model allows us to
illustrate the huge amount of condensed water ($\sim$20$\%$ in mass) that
would be needed to explain the low density of TOI-244 b, in contrast with the
4$\%$ mass fraction when considering steam. In this diagram, the color coding
indicates the isolation fluxes received by the planets. Interestingly, TOI-244
b receives a lower insolation than the typically received by planets over the
Earth-like composition curve, which could be interpreted as a hint that it
could have maintained an atmospheric envelope (we discuss a possible density-
insolation relation on a population level in Sect. 5.3.2). To infer whether
the atmosphere of TOI-244 b could be a primary H2-dominated atmosphere, or a
secondary outgassed potentially water-dominated atmosphere (Kite & Barnett
2020), we estimated the amount of material that could be removed by
atmospheric loss processes such as photoevaporation and Jeans escape.

Planets in close proximity to their host stars are exposed to high levels of
X-ray and extreme UV radiation (XUV), which can lead to partial or total loss
of a hydrogen envelope by photoevaporation. If the escape is energy-limited,
the atmospheric mass loss rate by photoevaporation can be written as (Erkaev
et al. 2007; Owen & Wu 2013)

$\dot{M}=\epsilon\frac{\pi
F_{\mathrm{XUV}}R_{\mathrm{p}}^{3}}{GM_{\mathrm{p}}},$ (3)

where $F_{\mathrm{XUV}}$ is the XUV flux received by the planet, $G$ the
gravitational constant and $\epsilon$ is an efficiency parameter. Stars emit
the most XUV at a young age, during the so-called saturation regime (Pizzolato
et al. 2003), which is particularly long for M-dwarfs. This makes
photoevaporation extremely efficient in the vicinity of M-dwarfs at a young
age. Using the analytical fits of Sanz-Forcada et al. (2011), we estimated
that GJ 1018 emitted $3.9\times 10^{29}$ erg$\cdot$s-1 of XUV during its
saturation regime, that lasted for $\tau_{\mathrm{sat}}=350$ Myr. This
translates into a mass loss rate during the saturation regime of $\rm
0.074\leavevmode\nobreak\ M_{\oplus}$/Gyr, during which the planet lost $\rm
0.026\leavevmode\nobreak\ M_{\oplus}$ of hydrogen. Following the approach of
Aguichine et al. (2021), we integrated Eq. 3 in time assuming that, at first
order, the mass and radius of the planet remain roughly constant, and only
$F_{\mathrm{XUV}}$ varies in time, following the fit of Sanz-Forcada et al.
(2011). We then find that, in total, TOI-244 b could have lost $\rm
0.1\leavevmode\nobreak\ M_{\oplus}$ of hydrogen by photoevaporation. This
value is most likely a lower estimate, since an initially greater H2 content
would produce a larger planetary radius, greatly increasing the mass loss rate
(see Eq. 3). This estimate is consistent with more refined models of hydrogen
mass loss. For example, the work by Rogers et al. (2023b) also predicts that
planets with core masses $\rm<3\leavevmode\nobreak\ M_{\oplus}$ at
$T_{\mathrm{eq}}=500$ K are entirely stripped of their envelopes, if the
latter are made of pure H2.

Despite the efficient loss of hydrogen, recent studies indicate that secondary
atmospheres, made of a mixture of volatile gases, can be restored by
outgassing from the magma after the photoevaporation phase (Kite & Barnett
2020; Tian & Heng 2023). The equilibrium temperature of TOI-244 b is greater
than that of the Earth, but so is its surface gravity. The characteristics of
TOI-244 b are such that its Jeans escape parameter
$\Lambda_{\mathrm{p}}=GM_{\mathrm{p}}m_{\mathrm{H}}/(k_{\mathrm{B}}T_{\mathrm{eq}}R_{\mathrm{p}})$
(Fossati et al. 2017) has a value consistent with the Jeans escape parameter
of the Earth within the measurements uncertainty range:
$\Lambda_{\mathrm{p}}/\Lambda_{\oplus}=1.07^{+0.28}_{-0.23}$.

Our computations suggest that even if hydrogen was initially present in the
secondary atmosphere of TOI-244 b, it has possibly been removed by Jeans
escape, as it happened on Earth (see Catling & Kasting 2017). Both Jeans
escape and photoevaporation appear to be efficient mechanisms to entirely
remove hydrogen from the atmosphere of TOI-244 b. This favors the modeling of
its interior with an atmosphere of high mean molecular weight volatiles, such
as water vapor. However, we cannot rule out the possibility that its
atmosphere contains other molecules, for instance carbon-rich species such as
CH4, CO or CO2. Spectroscopic measurements are necessary to break the
compositional degeneracy, but the bulk water content can be used as a proxy
for all heavy volatile species (CH4, O2, CO, CO2, etc).

####  5.2.4 Internal structure modeling

We performed a retrieval on the mass and radius data shown in Table 4 with a
1D interior-atmosphere model to estimate TOI-244 b’s compositional parameters.
Our interior model is stratified in three layers: a Fe-rich core, a silicate-
dominated mantle (Brugger et al. 2016, 2017), and a water layer (hydrosphere).
Given the irradiation conditions of TOI-244 b, volatiles such as water cannot
condense out, and is therefore in gaseous and supercritical phases (Mousis et
al. 2020b). Our interior structure model takes into account the low density of
these phases of water by using an equation of state adequate in this region of
the water phase diagram. In addition, we establish the coupling interface
between the interior and the atmosphere at 300 bar. The temperature at this
pressure constitutes the boundary condition for our interior model, and it is
calculated self-consistently by a k-correlated atmospheric model. Furthermore,
the atmospheric thickness is calculated by the atmospheric model by
integrating the hydrostatic equilibrium equation, and then added to the
interior model’s radius to obtain the total planetary radius (Acuña et al.
2021; Acuna et al. 2023).

The two compositional parameters that are free in our MCMC retrieval are the
core mass fraction (CMF) and water mass fraction (WMF). The mean and
uncertainties of their 1$\sigma$ intervals are shown in Table 5. Our
observable parameters are the mass and the radius. The Fe/Si mole ratio is not
considered as an observable, not only because of the difficulty of obtaining
an accurate metallicity for GJ 1018, but also because the assumption that the
composition of rocky planets reflects the stellar one is being questioned
(Adibekyan et al. 2021). Our CMF is compatible within uncertainties with that
of the Earth, as well as with the derived CMF distribution of a sample of
Super-Earths, between 0.1 and 0.5 (Plotnykov & Valencia 2020). The 1$\sigma$
confidence interval of the WMF of TOI-244 b spans from 0.04 to 0.20, being in
the transition between super-Earths (WMF ¡ 0.05) and sub-Neptunes (WMF ¿
0.20). Overall, our internal structure modeling suggests that TOI-244 b has a
$479^{+128}_{-96}$ km thick hydrosphere over a 1.17 $\pm$ 0.09 $\rm
R_{\oplus}$ solid structure composed of a Fe-rich core and a silicate-
dominated mantle compatible with that of the Earth.

Figure 12: Ternary diagram of the sampled 2D marginal posterior distribution for the CMF and WMF of TOI-244 b in our interior structure retrieval. The color code displays the probability density function (PDF). Table 5:  Composition of TOI 244 b assuming a Fe-rich core, a silicate-dominated mantle, and a water layer (hydrosphere). Errors are the 1$\sigma$ confidence intervals of the interior and atmosphere output parameters. Parameters |
---|---
Core Mass fraction (CMF) |  0.43 $\pm$ 0.16
Water mass fraction (WMF) |  0.12 $\pm$ 0.08
Temperature at 300 bar [K] |  2427$\pm$37
Thickness at 300 bar [km] |  479${}_{-96}^{+128}$
Albedo |  0.28 $\pm$ 0.27
Core + Mantle radius [$\rm R_{\oplus}$] |  1.17 $\pm$ 0.09

###  5.3 Emerging trends in the growing population of low-density super-Earths

Table 6:  Normalized density to the density expected for a planet composed
100$\%$ of silicates ($\rho$/$\rho_{rock}$; Zeng et al. 2019), stellar host
metallicity ([Fe/H]), insolation flux ($S$), deviation from the expected
density of an Earth-like composition ($\sigma_{Earth}$), spectral type (SpT),
and reference papers for all the known planets with $R_{\rm p}<2\,\rm
R_{\oplus}$, $M_{p}<3.5\,\rm M_{\oplus}$, and a mass precision better than
30$\%$.

Planet |  $\rho$/$\rho_{rock}$ | [Fe/H] (dex) |  $S$ ($\rm S_{\oplus}$) | $\sigma_{Earth}\,(\sigma)$ | SpT | Ref.
---|---|---|---|---|---|---
L 98-59 c | 0.90 $\pm$ 0.20 | -0.46 $\pm$ 0.26 | $12.8^{+2.6}_{-2.1}$ | 1.4 | M3 | [1]-[3]
L 98-59 d | 0.61 $\pm$ 0.15 | -0.46 $\pm$ 0.26 | $5.0^{+1.0}_{-0.8}$ | 3.3 | M3 | [1]-[3]
HD 260655 c | 0.87 $\pm$ 0.16 | -0.43 $\pm$ 0.04 | 16.1 $\pm$ 0.3 | 1.9 | M0 | [4]
TOI-4481 b | 0.89 $\pm$ 0.12 | -0.28 $\pm$ 0.07 | 130 $\pm$ 6 | 3.3 | M1.5 | [5]
HD 23472 c | 0.52 $\pm$ 0.17 | -0.20 $\pm$ 0.05 | $8.0^{+0.9}_{-0.8}$ | 3.5 | K4 | [6], [7]
TOI-561 b | 0.63 $\pm$ 0.15 | -0.40 $\pm$ 0.05 | 4745 $\pm$ 269 | 3.4 | G9 | [8]-[11]
Kepler-138 b | 0.43 $\pm$ 0.08 | -0.18 $\pm$ 0.10 | 9.9 $\pm$ 0.7 | 8.0 | M1 | [12]-[20]
Kepler-138 c | 0.71 $\pm$ 0.18 | -0.18 $\pm$ 0.10 | 6.8 $\pm$ 0.5 | 2.3 | M1 | [12], [14]-[24]
Kepler-138 d | 0.66 $\pm$ 0.18 | -0.18 $\pm$ 0.10 | 3.4 $\pm$ 0.2 | 2.6 | M1 | [12], [15]-[23]
GJ 1252 b | 0.93 $\pm$ 0.23 | +0.10 $\pm$ 0.10 | $233^{+48}_{-41}$ | 1.0 | M3 | [25], [26]
Kepler-114 c | 0.71 $\pm$ 0.27 | -0.20 $\pm$ 0.10 | 29 $\pm$ 7 | 1.6 | M0 | [27], [28]
TOI-244 b (1) | 0.79 $\pm$ 0.19 | -0.39 $\pm$ 0.07 | 7.3 $\pm$ 0.4 | 1.8 | M2.5 | This work
TOI-244 b (2) | 0.79 $\pm$ 0.19 | -0.03 $\pm$ 0.11 | 7.3 $\pm$ 0.4 | 1.8 | M2.5 | This work

131313Metallicity from (1) SteParSyn (2) ODUSSEAS. References. [1] Kostov et
al. (2019); [2] Cloutier et al. (2019); [3] Demangeon et al. (2021); [4] Luque
et al. (2022); [5] Palle et al. (2023); [6] Trifonov et al. (2019); [7] Barros
et al. (2022); [8] Lacedelli et al. (2021); [9] Weiss et al. (2021); [10]
Lacedelli et al. (2022); [11] Brinkman et al. (2023); [12] Mann et al. (2013);
[13] Rowe et al. (2014); [14] Hadden & Lithwick (2014); [15] Jontof-Hutter et
al. (2015); [16] Morton et al. (2016); [17] Mann et al. (2017); [18] Almenara
et al. (2018); [19] Berger et al. (2018); [20] Piaulet et al. (2023); [21]
Muirhead et al. (2012); [22] Kipping et al. (2014); [23] Holczer et al.
(2016); [24] Gajdoš et al. (2019); [25] Shporer et al. (2020); [26] Crossfield
et al. (2022); [27] Xie (2014); [28] Dressing & Charbonneau (2015).

Figure 13: Normalized density to the density expected for a planet composed
100$\%$ of silicates (Zeng et al. 2019) versus the stellar metallicity of the
stellar hosts (left) and the stellar insolation flux (right) for all the
confirmed planets with $R_{\rm p}<2\,\rm R_{\oplus}$, $M_{p}<3.5\,\rm
M_{\oplus}$, and a mass precision better than 30$\%$. The color coding
indicates the effective temperature of the host stars.

We now discuss the possible existence of trends in the growing population of
low-density super-Earths. In particular, we investigated the possibility of a
density-metallicity and a density-insolation relation within the super-Earth
population. Our sample consists of all known planets with $R_{\rm p}<2\,\rm
R_{\oplus}$, $M_{p}<3.5\,\rm M_{\oplus}$, and a mass precision better than
30$\%$, making a total of 40 well-characterized super-Earths. The threshold in
mass aims at discarding puffy sub-Neptunes above the 0.1$\%$ $\rm H_{2}$
theoretical model from Zeng et al. (2019). In Table 6, we show the
metallicities of the stellar hosts and the received insolation fluxes of the
low-density planets in our sample; that is, those with a measured density
lower than expected for a planet composed entirely of silicates (i.e.,
$\rho$/$\rho_{rock}$ $<$ 1).

####  5.3.1 Density-metallicity

All the stars hosting published low-density super-Earths are metal-poor
([Fe/H] between -0.20 and -0.45 dex) except GJ 1252, which has a slightly
supersolar metallicity ([Fe/H] = +0.10 $\pm$ 0.10 dex). In Fig. 13, we plot
$\rho$/$\rho_{rock}$ versus [Fe/H] for all the planets in our sample and for
TOI-244 b, considering our two estimated metallicities (see Sect. 3.2). We see
that both Earth-like and low-density planets are found around metal-poor
stars. However, there is a scarcity of low-density planets around stars with
supersolar metallicities (corresponding to the lower-right parameter space in
Fig. 13). In order to quantify this possible trend, we carried out a
Kolmogorov–Smirnov (KS) statistical test between the supersolar and subsolar
populations, which allowed us to determine how likely is it that we would see
both populations if they were drawn from the same probability distribution. We
obtain a D statistic of $0.43\pm 0.05$, which corresponds to a p-value of
$0.0013^{+0.0055}_{-0.0011}$. This result allows us to confidently reject the
null-hypothesis (i.e., both samples are drawn from the same distribution).
Although statistically significant, we only propose this trend as a possible
or tentative trend. This is because the sample of well-characterized rocky
planets is still small, and most of those planets orbit M-dwarf stars, whose
computed metallicities are less reliable than those of FGK stars.

According to several previous works, this possible trend is expected to exist.
Metal-poor stars are known to have an enhancement of Mg and Si (e.g., Bensby
et al. 2003), and, as discussed in Sect. 5.2.2, Adibekyan et al. (2021) found
that stars with higher Mg/Fe and Si/Fe ratios host lighter planets with higher
Mg/Fe and Si/Fe ratios. However, the correlation from Adibekyan et al. (2021)
would not explain the possible metallicity-density trend for planets with
$\rho$/$\rho_{rock}$ ¡ 1, since the densities of those planets must be
affected not only by the core and mantle composition, but also by the presence
of volatile elements. Interestingly, the water mass fractions of
protoplanetary disks forming metal-poor stars are known to be higher than the
ones expected for stars with solar metallicities (Santos et al. 2017), so