# A 1.55 $\rm R_{\oplus}$ habitable-zone planet hosted by TOI-715, an M4 star
near the ecliptic South Pole

Georgina Dransfield,1 Mathilde Timmermans,2 Amaury H.M.J. Triaud,1 Martín
Dévora-Pajares,3 Christian Aganze,4 Khalid Barkaoui,2,5,6 Adam J. Burgasser,4
Karen A. Collins,7 Marion Cointepas,8,9 Elsa Ducrot,10 Maximilian N.
Günther11, Steve B. Howell,12 Catriona A. Murray,13 Prajwal Niraula,5 Benjamin
V. Rackham,5 Daniel Sebastian,1 Keivan G. Stassun14, Sebastián
Zú$\rm\tilde{n}$iga-Fernández,2 José Manuel Almenara,8 Xavier Bonfils,8
François Bouchy,9 Christopher J. Burke,15 David Charbonneau,7 Jessie L.
Christiansen,16 Laetitia Delrez,2 Tianjun Gan,18 Lionel J. García,2 Michaël
Gillon,2 Yilen Gómez Maqueo Chew,19 Katharine M. Hesse,15 Matthew J. Hooton,20
Giovanni Isopi,21Emmanuël Jehin,22 Jon M. Jenkins,12 David W. Latham,7 Franco
Mallia,21 Felipe Murgas,8,6,23 Peter P. Pedersen,20 Francisco J.
Pozuelos,2,22,24 Didier Queloz,20 David R. Rodriguez,25 Nicole
Schanche,17,26,27 Sara Seager,4,23,26 Gregor Srdoc,29 Chris Stockdale,30
Joseph D. Twicken,12,31 Roland Vanderspek,15Robert Wells,17 Joshua N. Winn,32
Julien de Wit,5 Aldo Zapparata,21

A list of affiliations is given at the end of the paper E-mail:
<EMAIL_ADDRESS>Region Fellow, Marie Sklodowska-Curie ActionESA
Research Fellow51 Pegasi b Fellow

(Accepted 2023 May 10. Received 2023 May 09; in original form 2022 November
03)

###### Abstract

A new generation of observatories is enabling detailed study of exoplanetary
atmospheres and the diversity of alien climates, allowing us to seek evidence
for extraterrestrial biological and geological processes. Now is therefore the
time to identify the most unique planets to be characterised with these
instruments. In this context, we report on the discovery and validation of
TOI-715 b, a $R_{\rm b}=1.55\pm 0.06\rm R_{\oplus}$ planet orbiting its nearby
($42$ pc) M4 host (TOI-715/TIC 271971130) with a period $P_{\rm
b}=19.288004_{-0.000024}^{+0.000027}$ days. TOI-715 b was first identified by
TESS and validated using ground-based photometry, high-resolution imaging and
statistical validation. The planet’s orbital period combined with the stellar
effective temperature $T_{\rm eff}=3075\pm 75~{}\rm K$ give this planet an
instellation $S_{\rm b}=0.67_{-0.20}^{+0.15}~{}\rm S_{\oplus}$, placing it
within the most conservative definitions of the habitable zone for rocky
planets. TOI-715 b’s radius falls exactly between two measured locations of
the M-dwarf radius valley; characterising its mass and composition will help
understand the true nature of the radius valley for low-mass stars. We
demonstrate TOI-715 b is amenable for characterisation using precise radial
velocities and transmission spectroscopy. Additionally, we reveal a second
candidate planet in the system, TIC 271971130.02, with a potential orbital
period of $P_{02}=25.60712_{-0.00036}^{+0.00031}$ days and a radius of
$R_{02}=1.066\pm 0.092\,\rm R_{\oplus}$, just inside the outer boundary of the
habitable zone, and near a 4:3 orbital period commensurability. Should this
second planet be confirmed, it would represent the smallest habitable zone
planet discovered by TESS to date.

###### keywords:

exoplanets – planets and satellites: detection – planets and satellites:
terrestrial planets – planets and satellites: fundamental parameters

††pubyear: 2022††pagerange: A 1.55 $\rm R_{\oplus}$ habitable-zone planet
hosted by TOI-715, an M4 star near the ecliptic South Pole–A 1.55 $\rm
R_{\oplus}$ habitable-zone planet hosted by TOI-715, an M4 star near the
ecliptic South Pole

##  1 Introduction

At long last the era of JWST has arrived, and with it the age of detailed
exoplanetary atmospheric characterisation (The JWST Transiting Exoplanet
Community Early Release Science Team et al., 2022). This achievement was
unlocked just as the community hit another significant milestone: the
discovery of the 5000th planet beyond the solar system111Reported on 2022
March 21: https://exoplanetarchive.ipac.caltech.edu/docs/exonews_archive.html.
This ever-growing sample combined with the might of JWST is now set to deepen
our understanding of planets, including those in our solar system, on a sub-
population level.

One sub-population of particularly enduring interest consists of small,
potentially habitable planets orbiting cool M-type stars. With current
instrumentation, M dwarfs represent our best hope of finding temperate
terrestrial planets, the best example being the seven Earth-sized planets of
the TRAPPIST-1 system orbiting an M8V type star (Gillon et al., 2017). The
reduced radii of M dwarfs enable small planets to produce large transit
depths; this combined with the shorter orbital periods of these more compact
systems makes photometric monitoring from ground and space-based facilities
considerably more feasible (e.g. Nutzman & Charbonneau, 2008; Reiners et al.,
2018; Sebastian et al., 2021; Triaud, 2021). In the context of atmospheric
characterisation by transmission spectroscopy, bright, nearby M dwarfs are
ideal planetary hosts as small temperate planets will transit frequently,
enabling high signal-to-noise detections of atmospheric features with fewer
hours of telescope time (Charbonneau & Deming, 2007; Dressing & Charbonneau,
2015; Morley et al., 2017). Additionally, these low mass stars appear more
likely to host small planets from a theoretical and observational point of
view (Montgomery & Laughlin, 2009; Dressing & Charbonneau, 2013; Bonfils et
al., 2013; Mulders et al., 2015; Alibert & Benz, 2017; He et al., 2017;
Sabotta et al., 2021).

The so-called ‘habitable zone’ is a circumstellar region where an Earth-like
planet could sustain liquid water on its surface (Kasting et al., 1993). With
sometimes contradictory definitions of its boundaries, which depend on stellar
spectral type, planetary albedo, mass and even cloud cover (Kopparapu et al.,
2014; Underwood et al., 2003; Ramirez & Kaltenegger, 2016), it can be
challenging to categorise planets using this metric. However, the most widely
applicable and conservative definition comes from Kopparapu et al. (2013),
stating that rocky planets receiving between $0.42-0.842\,S_{\oplus}$ are in
their star’s HZ, irrespective of all other factors.

The activity of the M-dwarf host stars themselves is an additional factor to
consider in the habitability of planets orbiting M dwarfs, as the effect of
their frequent flaring is not fully known (O’Malley-James & Kaltenegger,
2017). Stellar flares could destroy a nascent atmosphere entirely (Davenport
et al., 2016; Dong et al., 2017), or they might provide the energy needed to
catalyse biological processes (Buccino et al., 2007; Patel et al., 2015;
Lingam & Loeb, 2017; Rimmer et al., 2018). Additionally, we must look beyond
our own terrestrial version of habitability, by exploring the possibility that
biology might arise on water-worlds (Madhusudhan et al., 2021) or temperate
sub-Neptunes (Seager et al., 2021), as both of these may have different HZ
boundaries.

We owe much of our current understanding of exoplanetary demographics to the
Kepler mission (Borucki et al., 2010), and one of the most influential results
to come out of statistical studies of the Kepler sample was the bimodal radius
distribution of sub-Neptune sized planets, with a gap between
$1.5-2\,R_{\oplus}$ (Fulton et al., 2017; Van Eylen et al., 2018). The current
best explanations for this bimodality are core-powered mass loss (Lopez &
Fortney, 2013; Ginzburg et al., 2018; Gupta & Schlichting, 2019), photo-
evaporation (Owen & Wu, 2013, 2017) and volatile-poor formation (Lee et al.,
2014; Venturini & Helled, 2017). While the exact location of this so-called
‘radius valley’ depends on stellar mass among other things (Wu, 2019; Gupta &
Schlichting, 2020), it is still unclear whether it is present around M dwarfs
or not (Cloutier & Menou, 2020). Therefore, planets with sizes within the
radius valley (e.g. Cloutier et al., 2020, 2021; Luque et al., 2022) help us
to understand the shape and depth of this gap around M dwarfs. A recent study
by Luque & Pallé (2022), however, indicates that M-dwarf planets may have a
density gap rather than a radius gap separating two populations of small
planets (rocky and water worlds), and that these observations are also well
explained by formation pathways that include disc-driven migration (Venturini
et al., 2020).

At present TESS (Ricker et al., 2015) is providing the community with ample
new planets to improve our understanding of exoplanetary demographics,
including by populating the habitable zone. Notably, while several ‘habitable
zone’ planets discovered by TESS have been confirmed (e.g. Gilbert et al.,
2020; Vach et al., 2022), none yet have fallen within the conservative
habitable zone as described by Kopparapu et al. (2013)—until now.

In this context we report on the discovery of TOI-715 b, a small planet
orbiting a nearby M4 star (TOI-715 / TIC 271971130). The planet’s relatively
long orbital period and cool host provide it with a mild instellation, placing
TOI-715 b comfortably within its star’s conservative habitable zone. We
additionally find a second candidate in the system that, if confirmed, could
be TESS’s smallest habitable zone planet discovered to date.

Our paper is organised as follows: we begin by characterising the host star
using its spectral energy distribution and reconnaissance spectroscopy in
Section 2. We then describe the identification of planetary candidates in the
data, first by TESS followed by our own search, in Section 3. In Section 4 we
describe our ground-based follow-up campaign and our procedures to validate
the system, and in Section 5 we detail our global analysis of all available
data. Finally, we contextualise our results in Section 6 and conclude in
Section 7.

##  2 Stellar Characterisation

TOI-715 (TIC 271971130) is a nearby ($\rm 42\,pc$; Bailer-Jones et al., 2021)
M dwarf of spectral type M4. It is a high-proper-motion target with right
ascension 07:35:24.56 hms and declination -73:34:38.67 dms (J2000, epoch
2015.5), placing it within TESS’s continuous viewing zone (CVZ).

As all our planetary information will be derived using the host star’s
parameters, we begin in the section that follows by characterising TOI-715.
All photometry and stellar parameters adopted for this work can be found in
Table 1.

Table 1: Stellar parameters adopted for this work. Designations |  TOI-715, TIC 271971130, 2MASS J07352425-7334388, APASS 33649915, Gaia DR2 5262666416118954368, UCAC4 083-012601, WISE J073524.46-733438.7
---|---
Parameter |  Value |  Source
$\alpha$ |  07:35:24.56 |  Gaia Collaboration (2022)
$\delta$ |  -73:34:38.67 |  Gaia Collaboration (2022)
Distance |  42.46$\pm$0.03 pc |  Bailer-Jones et al. (2021)
$\mu_{\alpha}$ |  $\rm 82.67\pm 0.02\,mas\,yr^{-1}$ |  Gaia Collaboration (2022)
$\mu_{\delta}$ |  $\rm 9.92\pm 0.02\,mas\,yr^{-1}$ |  Gaia Collaboration (2022)
$RV$ |  $\rm+55.8\pm 2.7\,km\,s^{-1}$ |  Gaia Collaboration (2022)
$U$ |  $\rm+29.7\pm 0.6\,km\,s^{-1}$ |  This work
$V$ |  $\rm-54.8\pm 2.1\,km\,s^{-1}$ |  This work
$W$ |  $\rm+0.7\pm 0.9\,km\,s^{-1}$ |  This work
SpT |  M4 |  This work
$R_{\star}$ |  $0.240\pm 0.012R_{\odot}$ |  This work
$M_{\star}$ |  $0.225\pm 0.012M_{\odot}$ |  This work
${\rm T_{eff}}$ |  3075$\pm$75 K |  This work
$\log g_{\star}$ |  5.0$\pm$0.2 |  This work
$\rm[Fe/H]$ |  +0.09$\pm$0.20 dex |  This work (spectroscopy)
|  $-$0.25$\pm$0.25 dex |  This work (SED)
Age |  6.2${}^{+3.2}_{-2.2}$ Gyr |  This work
T mag |  13.5308$\pm$0.0073 |  Stassun et al. (2019)
B mag |  18.14$\pm$0.16 |  Zacharias et al. (2013)
V mag |  16.24$\pm$0.01 |  Zacharias et al. (2013)
G mag |  14.8940$\pm$0.0007 |  Gaia Collaboration (2022)
J mag |  11.808$\pm$0.024 |  Cutri et al. (2003)
H mag |  11.264$\pm$0.026 |  Cutri et al. (2003)
K mag |  10.917$\pm$0.019 |  Cutri et al. (2003)
W1 mag |  10.753$\pm$0.023 |  Cutri et al. (2021)
W2 mag |  10.571$\pm$0.020 |  Cutri et al. (2021)
W3 mag |  10.387$\pm$0.049 |  Cutri et al. (2021)
W4 mag |  > 8.92 |  Cutri et al. (2021)

###  2.1 Reconnaissance Spectroscopy

We collected an optical spectrum of TOI-715 on 2022 January 07 (UT) using the
Low Dispersion Survey Spectrograph on the 6.5-m Magellan II (Clay) Telescope
at Las Campanas Observatory in Chile. With its upgraded red-sensitive CCD
(Stevenson et al., 2016), this instrument is now known as “LDSS-3C”. We used
LDSS-3C in long-slit mode with the standard setup (fast readout speed, low
gain, and $1\times 1$ binning) and the VPH-Red grism, OG-590 blocking filter,
and the $0\aas@@fstack{\prime\prime}75\times 0\aas@@fstack{\prime}4$ center
slit. This setup provides spectra covering 6000–10 000 Å with a resolution of
$R\sim 1810$, which is insufficient to produce a notable constraint on $v\sin
i_{\star}$.

We observed TOI-715 during clear conditions with seeing of
$0\aas@@fstack{\prime\prime}5$. We collected six exposures of 300 s each,
totaling 30 min on-source, at an average airmass of 1.445. Afterwards, we
collected three 1-s exposures of the nearby F8 V standard star HR 2283
(Maiolino et al., 1996) at an average airmass of 1.360. At each pointing, we
collected a 1-s HeNeAr arc lamp exposure and three, 10-s flat fields with the
“quartz high” lamp. We reduced the data with a custom, Python-based pipeline,
which includes bias removal, flat field correction, and spectral extraction.
For wavelength calibration, the HeNeAr arc exposure was used, which was
extracted similarly to the science spectrum. For flux calibration, we used the
ratio of the spectrum of the flux standard HR 2283 with an F8 V template from
Pickles (1998) to compute a relative flux correction; no correction was made
to address telluric absorption. The final science spectrum has a maximum SNR
per resolution element of 193 at 9202 Å and a mean SNR per resolution element
of 124 in the 6000–10 000 Å range.

Figure 1: LDSS-3C red optical spectrum of TOI-715 (black line), compared to
its best-fit M4 template (Kesseli et al., 2017, magenta line). Spectra are
normalized in the 7400–7500 Å region, and major absorption features are
labeled, including regions of strong telluric absorption ($\oplus$). The inset
box shows a close-up of the region encompassing H$\alpha$ (6563 Å) and Li i
(6708 Å) features, neither of which is detected in the data.

The reduced spectrum (Figure 1) was analyzed using
kastredux222https://github.com/aburgasser/kastredux.. We compared the spectrum
to Sloan Digital Sky Survey templates from Kesseli et al. (2017), finding a
best match to an M4 dwarf. This classification was verified through spectral
classification indices from Reid et al. (1995); Lépine et al. (2003); and
Riddick et al. (2007), all of which measure consistent spectral
classifications of M4. We evaluated the $\zeta$ metallicity index (Lépine et
al., 2007, 2013), determining a value of 1.066$\pm$0.002 which corresponds to
a metallicity [Fe/H] = $+$0.09$\pm$0.20 dex based on the empirical calibration
of Mann et al. (2013). There is no significant evidence of H$\alpha$ emission,
with an equivalent width limit $|EW|$ $<$ 1.2 Å corresponding to
$\log_{10}{L_{H\alpha}/L_{bol}}<-6.5$ (Douglas et al., 2014). We also find no
evidence of Li I absorption at 6708 Å ($EW<0.7$ Å), ruling out a young age and
substellar mass (Magazzu et al., 1993).

###  2.2 Spectral Energy Distribution

As an independent determination of the basic stellar parameters, we performed
an analysis of the broadband spectral energy distribution (SED) of the star
together with the Gaia DR3 parallax (with no systematic offset applied; see,
e.g., Stassun & Torres, 2021), to determine an empirical measurement of the
stellar radius, following the procedures described in Stassun & Torres (2016)
and Stassun et al. (2017); Stassun et al. (2018). We utilised the highest
quality broadband photometry available, which were the $JHK_{S}$ magnitudes
from 2MASS, the W1–W3 magnitudes from WISE, and the $GG_{\rm BP}G_{\rm RP}$
magnitudes from Gaia. Together, the available photometry spans the full
stellar SED over the wavelength range 0.4–10 $\mu$m (see Figure 2).

Figure 2: Spectral energy distribution of TOI-715. Red symbols represent the
observed photometric measurements, where the horizontal bars represent the
effective width of the passband. Blue symbols are the model fluxes from the
best-fit NextGen atmosphere model (black).

We performed a fit using NextGen stellar atmosphere models (Hauschildt et al.,
1999), with the free parameters being the effective temperature ($T_{\rm
eff}$), surface gravity ($\log g$), and metallicity ([Fe/H]). The remaining
free parameter is the extinction $A_{V}$, which we fixed at zero due to the
star’s proximity. The resulting fit (Figure 2) has a reduced $\chi^{2}$ of
1.3, with best-fit $T_{\rm eff}=3075\pm 75$ K, $\log g=5.0\pm 0.2$, [Fe/H] =
$-0.25\pm 0.25$. Integrating the (unreddened) model SED gives the bolometric
flux at Earth, $F_{\rm bol}=8.21\pm 0.19\times 10^{-11}$ erg s-1 cm-2. Taking
the $F_{\rm bol}$ and $T_{\rm eff}$ together with the Gaia parallax, gives the
stellar radius, $R_{\star}=0.240\pm 0.012$ R⊙. In addition, we can estimate
the stellar mass from the empirical relations of Mann et al. (2019), giving
$M_{\star}=0.225\pm 0.012$ M⊙, which is consistent with the value of $0.21\pm
0.05$ M⊙ determined via $\log g$ and $R_{\star}$.

###  2.3 Estimated age

The lack of detectable H$\alpha$ emission in its optical spectrum, and absence
of any significant flaring activity in its TESS lightcurve, both suggest a
relatively old age for TOI-715 given the persistent emission of most mid-type
M dwarfs (Newton et al., 2017; Kiman et al., 2019). We estimated the age of
TOI-715 by comparing its $UVW$ velocities and metallicity to local stars with
previously-determined ages from isochrone fitting and metallicities from high
resolution spectroscopy (cf. Burgasser & Mamajek 2017; Delrez et al. 2022).
Our comparison sample was drawn from the GALAH Data Release 3 catalogue (Buder
et al., 2021), for which ages are estimated using the Bayesian Stellar
Parameter Estimation (BSTEP) code (Sharma et al., 2018). A matched GALAH
sample was selected by requiring a distance $\leq$200 pc, and agreement with
TOI-715 in individual $UVW$ velocities to within 10 km/s and metallicity to
within 0.20 dex. The age distributions of the full GALAH sample and matched
stars are shown in Figure 3. The latter shows a broad peak with a maximum
probability at 7 Gyr; the median age of the distribution and 25% and 75%
quantiles yields an age estimate of 6.6${}^{+3.2}_{-2.2}$ Gyr. TOI-715 is
likely to be as old or older than the Sun, consistent with its low degree of
magnetic activity.

Analyses of ground-based and Kepler photometry by Newton et al. (2016) and
McQuillan et al. (2014), respectively, show that typical rotational periods of
old M4 dwarfs ($0.4-0.5\,\rm M_{\odot}$) are roughly 15-50 days, most likely
at the longer end of this range (and possibly beyond; longer periods in Newton
et al. (2016) are "grade B" detections due to low amplitude variations). This
range falls within the detectable period range for the full TESS dataset;
however, the absence of detectable H$\alpha$ emission suggests a lack of
significant spotting, so even if the period were within this range TESS
measurements may not be sufficient to detect variability. In either case, the
lack of H$\alpha$ emission and rotationally-induced variability on a time
scale $\leq 50$ days are both consistent with an old age.

Figure 3: Distribution of ages for all sources in GALAH Data Release 3 (light
grey histogram) and those GALAH sources that match the $UVW$ velocities and
metallicity of TOI-715. (dark grey histogram). The latter distribution is
consistent with a median age of 6.6${}^{+3.2}_{-2.2}$ Gyr (green shaded
region, 25% and 75% quantiles), which we adopt as the age of TOI-715.

##  3 Identification of planetary candidates

Figure 4: PDCSAP flux extracted from the short (2 minute) cadence data of the
24 sectors (1–13, 27, 29–37, 39) in which TOI-715 was observed. Light grey
point show the $120\,\rm s$ exposures and the purple line shows the flux in
$3\,\rm minute$ bins. The transit events of TOI-715.01 are shown with the dark
pink arrows and the locations of possible transits of TIC 271971130.02 are
indicated with green arrows; we note that individual transits are not readily
apparent by eye.

TOI-715 is in TESS’s southern continuous viewing zone (CVZ), meaning that in
principle, it would be observed in all southern sectors. In practice, it was
not optimally placed on the CCD during sectors 28 and 38, so at the time of
writing, there are 24 sectors of short-cadence (2 minute) data for this
target333There are additionally two sectors of fast cadence (20 second) data
available for TIC 271971130 (27 and 28). Observations at 2 minute cadence were
requested by two General Observer programs: G011180 (PI: Dressing) and G03278
(PI: Mayo)..

In the following sections we describe first the initial candidate
identification in the TESS data, followed by our own search for further
candidates.

###  3.1 TESS Candidate Identification

All TESS 2-minute cadence data are processed in the first instance by the SPOC
(Science Processing Operations Center) pipeline, presented in Jenkins et al.
(2016). Data products are then available to the community in the form of
Simple Aperture Photometry (SAP Twicken et al., 2010; Morris et al., 2020) or
Presearch Data Conditioning Simple Aperture Photometry (PDCSAP Stumpe et al.,
2012; Smith et al., 2012; Stumpe et al., 2014), the latter having been
corrected for instrument systematics and for crowding effects. Data products
are downloadable from the NASA Mikulski Archive for Space Telescopes (MAST)
and available via the Lightkurve package (Lightkurve Collaboration et al.,
2018). All lightcurves are additionally searched by SPOC for periodic transit-
like signals and candidates with $\rm SNR\,>\,7.1$ are reported as threshold
crossing events (TCEs).

TOI-715.01 was first reported as a candidate on 2019 May 24 (Guerrero et al.,
2021) following a multi-sector transit search (Jenkins, 2002; Jenkins et al.,
2010, 2020) conducted on 2019 May 5 for sectors 1–9. The candidate was already
identified in the multi-sector search of Sectors 1–-6 conducted on 2019 April
18, but did not at this time pass the necessary tests to be reported as a TOI.
The transit signal was fitted with an initial limb-darkened transit model (Li
et al., 2019) and subjected to a suite of diagnostic tests (Twicken et al.,
2018) to help determine whether the signature is from an exoplanet. The
transit signature passed all of the tests reported in the Data Validation
report for Sectors 1-9444All data validation reports (Twicken et al., 2018)
referred to in this section are downloadable at
https://tev.mit.edu/data/search/?q=271971130, and the difference image
centroiding test located the source of the transit signal to within
$3\aas@@fstack{\prime\prime}7\pm 3\aas@@fstack{\prime\prime}5$ in the
subsequent data validation reports for the multi-sector searches of Sectors
1–13 and 27–39.

At the time of the first reported TCE555Multi-sector TCE statistics available
at
https://archive.stsci.edu/missions/tess/catalogs/tce/tess2018206190142-s0001-s0006_dvr-
tcestats.csv, the TESS Input Catalog (TIC) in use was version 7, which had not
yet incorporated the Gaia-DR2 data; as such, TOI-715 did not have a stellar
radius and the planet candidate was reported as a $\rm 7.1\,R_{\oplus}$ object
with a period of $\rm\sim 19.2\,days$. Following the update to the TICv8
(Stassun et al., 2019) the planet candidate’s radius estimate was revised,
demonstrating that TOI-715.01 was likely a super-Earth and thus a high
priority target.

SPOC TCEs are subject to the TESS-ExoClass
666https://github.com/christopherburke/TESS-ExoClass automated classifier that
reduces the number of TCEs that undergo the manual TOI vetting procedure.
TESS-ExoClass applies a series of tests that are similar to the Kepler
Robovetter (Coughlin et al., 2016; Thompson et al., 2018). TOI 715.01 passes
all the tests of TESS-ExoClass and has been placed in the Tier 1 (highest
quality candidate) category, and has been classified as a Tier 1 candidate in
the subsequent SPOC multi-sector 1–13, 1–36, and 1–39 searches. Further
details for the TOI assignment process are available in Guerrero et al.
(2021).

We present the PDCSAP TESS lightcurves for all 24 sectors of 2-minute cadence
data in Figure 4; the timings of the 29 transits are indicated with dark pink
arrows.

###  3.2 Search for additional candidates

We make use of the custom pipeline Sherlock777Sherlock is publicly available
at https://github.com/franpoz/SHERLOCK, presented in Pozuelos et al. (2020)
and Demory et al. (2020), to search the TESS data for additional transiting
candidates, as was done in Dransfield et al. (2022a). Sherlock downloads all
lightcurves from MAST and, using Wotan (Hippke et al., 2019), applies a bi-
weight function with varying window sizes to detrend the data. Each detrended
lightcurve and the original PDCSAP lightcurve are then searched for transit-
like signals using Transit Least Squares (Hippke & Heller, 2019). We use only
the 2-minute cadence in our candidate search, applying a Savitzky–Golay (SG)
digital filter (Savitzky & Golay, 1964) previously following the strategy
described in Delrez et al. (2022)888We use SciPy’s implementation of the SG
filter, using a 3rd order polynomial and a window size of 11 points. and we
test 11 window sizes between 0.19 and 1.9 days when detrending with the bi-
weight filter. We thus carry out our transit search on the PDCSAP lightcurve
and the 11 detrended lightcurves and only consider signals with SNR>7 for
further investigation.

We recover TOI-715.01 at a period of $\rm 19.29\,d$ in the first instance in
all 12 lightcurves, and we find that the highest SNR and SDE are achieved in
the PDCSAP flux without any detrending with Wotan.

We additionally find two other periodic signals in the data. Of these, the
signal with highest SNR is at a period of $\rm 25.61\,d$, putting it within
$0.4\%$ of the first order 4:3 commensurability. The signal is detected in
nine of the searched lightcurves with a maximum SNR of 13.81 and SDE of 11.6;
it has a depth of $\rm\sim\,1\,ppt$, making this a $\rm\sim\,1.16\,R_{\oplus}$
candidate. In order to ensure this signal is not an artefact produced by the
SG filter, we additionally search the untreated PDCSAP lightcurve and recover
the signal with an SNR of 6.81. We adopt this candidate as TIC
271971130.02999This candidate has been submitted as a Community TESS Object of
Interest (CTOI): https://exofop.ipac.caltech.edu/tess/target.php?id=271971130
and highlight the positions of transit events on Figure 4 with dark green
arrows. In Figure 5 we present the TESS lightcurve folded on this signal,
along with the Lomb-Scargle periodogram.

Given the large amount of data in Figure 4, it is not apparent by eye how much
pre- and post-transit baseline each transit has. We therefore note that the
total number of in-transit points for TIC 271971130.02 is 1371, while the
total number of points before and after the transits are 1349 and 1369
respectively (counted up to 1 transit duration). Thus on average the pre-
transit baseline is 98.4% of a transit duration, while the post-transit
baseline is 99.9%.

The SPOC ran the Data Validation module at the ephemeris for the second signal
and obtained an SNR of 5.5 sigma, and additionally identified other sub-
threshold transit-like signals at higher SNR.

Figure 5: Results of our search for additional candidates in the data, using
Transit Least Squares as implemented by Sherlock. Upper panel: TLS periodogram
showing the detected 25.61 day period of TIC 271971130.02 and its harmonics.
Lower panel: TESS PDCSAP lightcurve phase-folded on this period, with a
transit model overplotted. We note that the errorbars on the binned points are
smaller than the markers, and that the lightcurve shown here is the one
treated with the SG Filter

.

The second signal we recover has a period of $\rm 7.17\,d$ and a depth of
$<1\,\rm ppt$; it has a maximum SNR of 9.63 and SDE of 8.17. The very low SNR
of the signal makes it challenging to discern by eye whether or not the shape
is consistent with that of a planetary transit. We also note that this signal
is only found in four of the detrended lightcurves (with window sizes between
0.38–0.65 days, but not the PDCSAP lightcurve, indicating that the signal
could be dependent on detrending. The duration of a transit at this period on
a circular orbit is should be of order 0.065 days, and all tested windows are
at least $3\times$ this duration. We therefore do not believe the detrending
will have suppressed the transit in other lightcurves if it is real. When TESS
returns to the southern skies in its second extended mission, the additional
photometry will provide further insight into the nature of this signal.

It is important to note that the SNR and SDE yielded by SHERLOCK are inherited
from the Transit Least Squares algorithm, which uses a simple estimation that
can not be compared with values provided by other pipelines, such as SPOC.
Instead, these values are used internally in SHERLOCK to compare the signals
found and select the most prominent among them.

##  4 Vetting and Validation

In this section we describe the results of the multi-facility follow-up
campaign conducted between May 2020 and April 2022. We begin with the high-
resolution imaging observations, and then outline the photometric observations
collected from five southern observatories. Finally, we describe how all our
follow-up observations were used to validate the planetary nature of
TOI-715.01 and TIC 271971130.02.

All follow-up observations are summarised in Table 2.

Table 2: Summary of ground-based follow-up observations carried out for the
validation of TOI-715.01 and TIC 271971130.02

.  Follow-up Observations High Resolution Imaging Observatory Filter Date
Sensitivity Limit Result Gemini South $562~{}{\rm nm}$ 2020 December 26
$\Delta m=4.69$ at $0.5\arcsec$ No sources detected Gemini South $832~{}{\rm
nm}$ 2020 December 26 $\Delta m=5.07$ at $0.5\arcsec$ No sources detected
Photometric Follow-up Observatory Filter Date (Candidate) Coverage Result LCO-
SAAO Sloan-$i^{\prime}$ 2020 May 13 Ingress Transit ruled out on or off target
during the time covered LCO-SAAO Sloan-$i^{\prime}$ 2020 October 15 (.01)
Ingress Transit detected on target ExTrA $\rm 1.21\mu m$ 2021 February 26
(.01) Full Detection ExTrA $\rm 1.21\mu m$ 2021 April 25 (.01) Full Detection
TRAPPIST-South $I+z^{\prime}$ 2021 April 25 (.01) Full Detection LCO-CTIO
Sloan-$i^{\prime}$ 2021 April 25 (.01) Full Detection SSO-Callisto
Sloan-$r^{\prime}$ 2021 September 26 (.01) Full Detection SSO-Io
Sloan-$r^{\prime}$ 2021 September 26 (.01) Full Detection SSO-Europa
Sloan-$r^{\prime}$ 2021 September 26 (.01) Full Detection SSO-Ganymede
Sloan-$r^{\prime}$ 2021 September 26 (.01) Gapped Interruption due to weather
- ingress detected TRAPPIST-South $I+z^{\prime}$ 2021 September 26 (.01) Full
Detection OACC-CAO Sloan-i’2 2021 November 24 (.01) Full Detection SSO-
Callisto $I+z^{\prime}$ 2021 November 24 (.01) Full Detection SSO-Io
$I+z^{\prime}$ 2021 November 24 (.01) Full Detection SSO-Ganymede
$I+z^{\prime}$ 2021 November 24 (.01) Full Detection TRAPPIST-South
$I+z^{\prime}$ 2021 November 24 (.01) Full Detection ExTrA $\rm 1.21\mu m$
2022 February 08 (.01) Full Detection TRAPPIST-South $I+z^{\prime}$ 2022 April
07 (.01) Full Detection ExTrA $\rm 1.21\mu m$ 2022 April 07 (.01) Full
Detection TRAPPIST-South $I+z^{\prime}$ 2022 October 25 (.02) Egress
Inconclusive (high airmass) Spectroscopic Observations Instrument Wavelength
Range Date Number of Spectra Use Magellan/LDSS3 $380-1000~{}\rm nm$ 2022
January 06 1 Stellar characterisation

###  4.1 High resolution imaging

Close stellar companions (bound or in the line of sight) can confound derived
exoplanet properties in a number of ways. The detected transit signal might be
a false positive due to a background eclipsing binary and even real planet
discoveries will yield incorrect stellar and exoplanet parameters if a close
companion exists and is unaccounted for (e.g., Ciardi et al., 2015; Furlan &
Howell, 2017, 2020). Additionally, the presence of a close companion star
leads to the non-detection of small planets residing within the same
exoplanetary system (Lester et al., 2021). Approximately 25% of M dwarfs are
part of binary or multiple star systems (e.g., Cortes Contreras et al., 2015;
Winters et al., 2019), though fewer than 5% of known spectroscopic binaries
include an M-dwarf primary star (Pourbaix et al.,
2004)101010https://sb9.astro.ulb.ac.be. Nonetheless, high resolution imaging
provides crucial information toward our understanding of exoplanetary
formation, dynamics and evolution (Howell et al., 2021).

TOI-715 was observed on 2020 December 26 UT using the Zorro speckle instrument
on the Gemini South 8-m telescope (Scott et al., 2021). Zorro provides
simultaneous speckle imaging in two bands ($562~{}\rm nm$ and $832~{}\rm nm$)
with output data products including a reconstructed image with robust contrast
limits on companion detection (see Howell & Furlan, 2022). TOI-715 was found
to be a single star to within the angular and brightness contrast levels
achieved. Eight sets of $1000\times 0.06$ s images were obtained and processed
by our standard reduction pipeline (Howell et al., 2011). Figure 6 shows our
final contrast curves and the $832~{}\rm nm$ reconstructed speckle image.
These high-resolution observations revealed no companion star brighter than 5
magnitudes below that of the target star from the 8-m telescope diffraction
limit ($20~{}\rm mas$) out to $1.2\arcsec$. At the distance of TOI-715
($42.4~{}\rm pc$) these angular limits correspond to spatial limits of $0.84$
to $50.9~{}\rm AU$.

Figure 6: Plot showing the $5\sigma$ speckle imaging contrast curves in both
filters as a function of the angular separation out to $1.2\arcsec$, the end
of speckle coherence. The inset shows the reconstructed $832~{}\rm nm$ image
with a $1\arcsec$ scale bar. The star, TOI-715, was found to have no close
companions to within the angular and brightness contrast levels achieved.

###  4.2 Photometric follow-up

####  4.2.1 Las Cumbres Observatory

We used the Las Cumbres Observatory Global Telescopes (LCOGT; Brown et al.,
2013) 1.0 m network nodes at South Africa Astronomical Observatory (SAAO) and
Cerro Tololo Inter-American Observatory to observe three transits of
TOI-715.01. First and second transits were observed with LCO-SAAO on 2020 May
13 and 2020 October 15, and third was observed with LCO-CTIO on 2021 April 25.
All observations were carried out with the Sloan-$i^{\prime}$ and an exposure
time of 180 s.

Photometric brightness was measured using an uncontaminated target aperture of
4.3″(11 pixels). We used the TESS Transit Finder, which is a customized
version of the Tapir software package (Jensen, 2013), to schedule our
photometric time series. The 1.0 m telescopes are equipped with $4096\times
4096$ SINISTRO cameras with an image scale of $0\aas@@fstack{\prime\prime}389$
per pixel, resulting in a $26\arcmin\times 26\arcmin$ field-of-view. The raw
images were calibrated with the standard LCO BANZAI pipeline (McCully et al.,
2018), and photometric data were extracted with AstroImageJ (Collins et al.,
2017).

The observation of 2020 May 13 only provided $\sim 20\%$ in-transit coverage
as well as 1.2 hours of pre-transit baseline. The transit was ruled out on or
off target during the window covered by this observation111111With the updated
ephemeris we confirm that transit would have started just after the end of the
window covered by this observation.. The subsequent observation of 2020
October 15 confirmed the transit event on target with the detection of an
ingress that was 29 minutes late relative to the ephemeris. The observation of
2021 April 25 resulted in the detection of a ful transit.

####  4.2.2 SPECULOOS-Southern Observatory

The SPECULOOS Southern Observatory (SSO) is comprised of four Ritchey-Chrétien
1.0 m-class telescopes installed at ESO Paranal in the Atacama desert (Delrez
et al., 2018). Designed to hunt for small, habitable-zone planets orbiting
ultra-cool stars (Sebastian et al., 2021), all four telescopes are equipped
with a deep-depletion Andor CCD camera with $2048\times 2048$ 13-$\micron$
pixels. Each telescope therefore has a field of view of
$12\arcmin\,\times\,12\arcmin$ and a pixel scale of
$0\aas@@fstack{\prime\prime}35$ (Burdanov et al., 2018).

All SPECULOOS observations are scheduled using the python package
SPOCK121212https://github.com/educrot/SPOCK (Sebastian et al., 2021), and
processed in the first instance by an automatic data reduction pipeline,
described in detail in Murray et al. (2020). Successful observations of TESS
targets are then reprocessed using prose, a publicly available python
framework for processing astronomical
images131313https://github.com/lgrcia/prose as described in Garcia et al.
(2021, 2022). Images are calibrated and aligned before performing aperture
photometry on the 500 brightest sources detected; prose then performs
differential photometry (Broeg et al., 2005) on the target star to extract the
lightcurve. Individual lightcurves are detrended for airmass, sky background
and FWHM (full width at half maximum) using second order polynomials in time,
and they are then modelled using exoplanet (Foreman-Mackey et al., 2021b).

We observed TOI-715.01 for the first time on the night of 2021 September 26
with all four telescopes simultaneously (Io, Europa, Ganymede, Callisto) using
the Sloan-$r^{\prime}$ filter and $\rm 120$-s exposures. Cloudy skies during
the transit caused some large systematics in three out of four telescopes,
although this did not prevent the transit from being detected. The fourth
telescope, Ganymede, closed during the weather alert, causing only the ingress
to be detected.

We re-observed TOI-715.01 on the night of 2021 November 24 with three of our
telescopes. We collected $\rm 13$-s exposures using the custom filter
$I+z^{\prime}$ (Sebastian et al., 2021). Observations were once again hampered
by the untimely appearance of clouds, which affected the in-transit precision
of our observations. Fortunately, the transit was still detected with all
three instruments despite the overcast conditions.

####  4.2.3 TRAPPIST-South

We observed four transits of TOI-715.01 with TRAPPIST-South (TS) (Jehin et
al., 2011; Gillon et al., 2011), located in ESO La Silla Observatory in Chile.
This 0.6-m telescope is equipped with a FLI ProLine PL3041-BB camera and a
back-illuminated CCD with a pixel size of $0\aas@@fstack{\prime\prime}64$,
providing a total field of view of $22\arcmin\,\times\,22\arcmin$ for an array
of $2048\,\times\,2048$ pixels. TS is a Ritchey-Chrétien telescope with F/8
and is equipped with a German equatorial mount.

All four transits were observed with the custom $I+z^{\prime}$ filter to
maximize the photometric precision. The observations took place on 2021 April
25, 2021 September 26, 2021 November 24 and 2022 April 07, with exposure times
of 50s, 70s, 120s and 90s respectively. We reduced the images using prose
pipeline (Garcia et al., 2022, 2021) to extract optimal light curves.
Individual lightcurves were then modeled using the exoplanet (Foreman-Mackey
et al., 2021b) package and detrended using a second order polynomial of
airmass, FWHM (full width at half maximum) and sky background. None of the
observations suffered weather losses.

We additionally observed TOI-715 on 2022 October 25 during a partial transit
of the second candidate (.02) but the target was too low during the event. The
observation was therefore inconclusive.

####  4.2.4 ExTrA

ExTrA (Bonfils et al., 2015) is a near-infrared (0.85–1.55 $\rm\mu m$) multi-
object spectrograph fed by three 0.6 m telescopes located at La Silla
observatory. We observed four full transits of TOI 715.01 on 2021 February 26,
2021 April 25, 2022 February 08, and 2022 April 07. We observed the first
three transits using two telescopes, and the fourth using three; all
observations were carried out with $8\arcsec$ aperture fibers. For both
nights, we used the spectrograph’s low resolution mode ($R\sim 20$) and 60s
exposures. Five fibre positioners are used at the focal plane of each
telescope to collect light from the target and four comparison stars. As
comparison stars, we also observed 2MASS J07381369-7347351, 2MASS
J07410628-7341297, 2MASS J07393394-7330535, and 2MASS J07372328-7321411 with
J-magnitude (Skrutskie et al., 2006) and $T_{\rm eff}$ (Gaia Collaboration et
al., 2021) similar to TOI-715. The resulting ExTrA data were analyzed with
custom data reduction software, detailed in Cointepas et al. (2021).

####  4.2.5 OACC-CAO

We observed a transit of TOI-715.01 with Campo Catino Austral Observatory
(OACC-CAO), located in El Sauce Observatory in the Atacama desert in Chile, on
the night of 2021 November 24. The telescope is a $\rm 0.6\,m$ Planewave CDK
$24\arcsec$ with a Planewave L600 mount, equipped with an FLI filter wheel
with Sloan filters and an FLI PL16803 camera with $4096\times 4096$
$9~{}\rm\mu m$ pixels. It has a field of view of $32\arcmin$ and a pixel scale
of $0\aas@@fstack{\prime\prime}48$.

TOI-715.01 was observed with the Sloan-i’2 filter using 180s exposures. The
images were reduced using AstroImageJ (Collins et al., 2017) and we detect the
full transit in an uncontaminated aperture.

###  4.3 Statistical Validation

We make use of the statistical validation package Triceratops141414Version
1.0.17. (Giacalone et al., 2021; Giacalone & Dressing, 2020) to assess the
probability of the planet hypothesis for TOI-715.01 and TIC 271971130.02.
Triceratops calculates the flux contribution from nearby stars to check if any
could be responsible for the transit signal. It then calculates the relative
probabilities of a range of transiting planet (TP) and eclipsing binary (EB)
scenarios using lightcurve models fitted to the phase-folded photometry.

In the first instance we find that the false positive probabilities (FPP) are
0.403 and 0.592 for candidates .01 and .02 respectively when examining the
TESS data, while the threshold for validation is 0.015. For candidate .01, we
make use of our follow-up photometry by folding in the four TRAPPIST-South
lightcurves. We choose these as they were observed with the custom I+z’ filter
(which is very similar to the TESS bandpass) and all four are at different
epochs.

With this phase-folded subset of our follow-up photometry, we find that the
false positive probability reduces to 0.0107, placing it below the threshold
for statistical validation. To similarly reduce the FPP for TIC 271971130.02,
we will need to collect ground-based photometry to confirm the event on-target
or rule out events on nearby stars (such an observation was scheduled from
Hazelwood Observatory but cancelled due to bad weather).

####  4.3.1 Statistical validation conclusions

With a FPP of 0.0107 TOI-715.01 is now below the customary threshold for
statistical validation (0.015; Giacalone et al., 2021); we therefore consider
the planet validated and refer to it as TOI-715 b. TIC 271971130.02 does not
yet meet the criterion for statistical validation, and is therefore classified
as a ‘likely transiting planet’ until more evidence such as ground-based
photometry is collected.

##  5 Global photometric analysis

Figure 7: Photometry of TOI-715 b along with best fitting models. Grey points
are raw flux and dark pink circles are 15-minute binned points. The dark pink
lines are 20 fair draws from the posterior transit model. The flux and the
transit models have been corrected by subtracting the baseline models. The
TESS, ExTrA 2 and ExTrA 3 photometry are phase-folded, while the others are
single transits. Figure 8: Photometry of TOI-715 b along with best fitting
models. Grey points are raw flux and dark pink circles are 15-minute binned
points. The dark pink lines are 20 fair draws from the posterior transit
model. The flux and the transit models have been corrected by subtracting the
baseline models. All transits shown here are single transits. For the TRAPPIST
lightcurves, the number in the figure titles denotes the visit number. These
are treated as single transits due to the different exposure times of each
visit. Increased scatter during the transit observed by SSO-Io (bottom right
panel) cause the middle binned point to have a large errorbar.

We carried out a global photometric analysis of the TESS photometry and the
datasets described in Section 4.3 using Allesfitter (Günther & Daylan, 2021,
2019), a flexible and publicly available python-based inference package. We
exclude all partial transits, as well as the transit obtained from OACC (due
to large scatter in the data) from our analysis, leaving 15 full transits from
our ground-based facilities. Allesfitter generates lightcurve models using
Ellc (Maxted, 2016), and Gaussian Process (GP) models using Celerite (Foreman-
Mackey et al., 2017). The best fitting models are then chosen using either a
nested sampling algorithm via Dynesty (Speagle, 2020), or MCMC sampling with
Emcee (Foreman-Mackey et al., 2013). For all modeling in this paper, we make
use of the nested sampling algorithm as it calculates the Bayesian evidence at
each step in the sampling; this allows us to compare the Bayesian evidence for
different models by calculating the Bayes Factor (Kass & Raftery, 1995). All
prior distributions and their bounds can be found in Table 3.

We adopt the signal parameters from Section 3 as uniform priors, and the
stellar parameters from Section 2 as normal priors and fit for all planetary
parameters ($R_{\rm p}/R_{\star}$, $(R_{\star}+R_{\rm p})/a$, $\cos\,i$,
$T_{0}$, and $P$). We fit two models in the first instance: one where
eccentricity is constrained to zero and another where eccentricity is allowed
to vary, parametrised as $\sqrt{e_{\rm b}}\cos{\omega_{\rm b}}$ and
$\sqrt{e_{\rm b}}\sin{\omega_{\rm b}}$ (as in Triaud et al., 2011).

The observations described in Section 4.3 span several photometric filters
from the blue to the near-infrared ends of the spectrum. To allow the colour-
dependent transit depth to be a free parameter in our models, we also fit each
lightcurve for a dilution parameter ($D$) which we give a uniform prior
between -1 and 1. We fix the dilution of the TESS observations at 0 as the
PDCSAP lightcurve is already corrected for crowding (Stumpe et al., 2012), and
use the transit depth as the reference depth; we then use Allesfitter’s ‘$\rm
coupled\\_with$’ functionality to ensure that the dilution and quadratic limb-
darkening coefficients are fitted together for all observations taken in the
same photometric band.

We use the python package PyLDTK (Parviainen & Aigrain, 2015) and the PHOENIX
stellar atmosphere library (Husser et al., 2013) to calculate quadratic limb-
darkening coefficients for each photometric band. These are also adopted as
normal priors in our models after we reparameterise them in the Kipping (2013)
parametrisation by converting from $u_{1},u_{2}$ to $q_{1},q_{2}$.

Finally, due to the complex instrument systematics present in the data
obtained with the ExTrA telescopes, we model the correlated ‘red’ noise in
these observations using a GP. We select the Matérn 3/2 kernel as implemented
by Celerite due to its versatility and its ability to model both long and
short term trends. We fit for two GP hyperparameters for each ExTrA telescope:
the amplitude scale, $\sigma$, and the length scale, $\rho$. We place wide
uniform priors on these terms. For all other telescopes, we make use of a
hybrid spline to model the baseline. Allesfitter also fits an ‘error scaling’
term to account for white noise in the data.

The initial fits (1-planet circular, and 1-planet eccentric) both confirm that
TOI-715 b is a $\rm 1.550\pm 0.061\,\rm R_{\oplus}$ planet with an equilibrium
temperature of $\rm 234\pm 12\,\rm K$. We find that the eccentric model has
slightly higher evidence, with a logged Bayes Factor of $\sim 6.5\pm 1.1$,
although the eccentricity derived from this more complex model,
$e=0.31_{-0.20}^{+0.38}$, is consistent with zero at the $2\sigma$ level, and
we do not consider this a detection of orbital eccentricity. All modelled
ground-based transits of TOI-715 b, as well as the phase-folded TESS
photometry, can be found in Figures 7 and 8. In Figure 10 we present the
averaged depths, in each photometric band, demonstrating the achromaticity of
the observed transits.

We remind the reader that in Section 3, we identified a second transiting
planet candidate in the TESS data, which we now incorporate into the model,
and test this two-planet hypothesis. We test two further models to seek
evidence of a second planet in the system, starting by fixing the linear
ephemerides of planet b and allowing the mid-time of each individual transit
to vary. This model is motivated by the second candidate’s proximity to a
$1^{\rm st}$ order 4:3 mean-motion resonance with the first planet. If the
second candidate is real, we might expect for these planets to experience
mutual gravitational interactions exciting transit timing variations (TTVs).

We place a wide uniform prior on each transit mid-time, corresponding to the
linear predicted mid-time $\pm\,60$ minutes.We find that due to the low SNR of
individual transits in the TESS data, the transit mid-times are not well
constrained for events observed only by TESS. However, transits observed
simultaneously by several ground-based observatories have significantly less
scatter and smaller error bars. In these events, we see evidence suggesting
TTVs of up to 5 minutes, although the Bayesian evidence for this model is
lower than for the two linear models tested. Models with linear ephemerides
are preferred, with Bayes Factors of $\sim 9.5$ and $\sim 16$ for the circular
and eccentric models respectively. We note that we did not test a fixed linear
ephemerides fit with a 2-planet model as the individual transits in TESS for
the second candidate have even lower SNR and we have no ground-based transits
at the time of writing.

Figure 9: Phase folded TESS photometry of TIC 271971130.02. Grey points are
raw flux and green circles are 15-minute binned points. The green lines are 20
fair draws from the posterior transit model. The flux and the transit models
have been corrected by subtracting the baseline models. Figure 10: Measured
transit depths vs. wavelength. The dark grey horizontal line indicates the
depth of the TESS transits, while other depths are highlighted with circles
for comparison.

The final model we test is a 2-planet circular Keplerian model. We fit for all
the same parameters as before, but now additionally include the transit
parameters for the second candidate. The phase-folded TESS photometry for the
second candidate is presented in Figure 9; the fitted and derived parameters
for planet b resulting from the 2-planet model are consistent with those
emerging from all previous models. Also noteworthy is that the host density as
calculated by Allesfitter following Seager & Mallén-Ornelas (2003) with the
transit parameters of the second candidate is $\rm 22.7\pm 3.0\,\rm
g\,cm^{-3}$, which is within $1\sigma$ of the stellar density prior of $\rm
23.1\pm 3.2\,\rm g\,cm^{-3}$, suggesting that both signals are produced over
the same star. The results of this fit indicate that if TIC 271971130.02 is
confirmed, it is likely a $1.066\pm 0.092~{}\rm R_{\oplus}$ planet with an
orbital period of $25.60712_{-0.00036}^{+0.00031}$ and an equilibrium
temperature of $215\pm 12~{}\rm K$.

All fitted and derived parameters from the linear 2-planet circular model are
presented in Table 3.

Table 3: Priors used in our fit, along with fitted and derived parameters.
Uniform priors are indicated as
$\rm\mathcal{U}(lower~{}bound,~{}upper~{}bound)$ and normal priors are
indicated as $\rm\mathcal{N}(mean,~{}standard~{}deviation)$. ⋆Equillibrium
temperature is calculated assuming an albedo of 0.3 and emissivity of 1.

| TOI-715 b | TIC 271971130.02 |
---|---|---|---
Fit Parametrisation and Priors
Transit Depth; $R_{\rm p}/R_{\star}$ | $\mathcal{U}(0.01,0.1)$ | $\mathcal{U}(0.01,0.1)$ |
Inverse Scaled Semi-major Axis; $(R_{\star}+R_{\rm p})/a$ | $\mathcal{U}(0.01,0.05)$ | $\mathcal{U}(0.008,0.04)$ |
Orbital Inclination; $\cos{i}$ | $\mathcal{U}(0.000,0.04)$ | $\mathcal{U}(0.000,0.04)$ |
Transit Epoch; $T_{0}$ (BJD) | $\mathcal{U}(2\,458\,327.3,2\,458\,327.7)$ | $\mathcal{U}(2\,458\,342.0,2\,458\,342.4)$ |
Period; $P$ (days) | $\mathcal{U}(19.0,19.4)$ | $\mathcal{U}(25.4,25.8)$ |
Limb Darkening Coefficients
TESS $\rm u_{1}$ | $0.3322\pm 0.0013$ |  TESS $\rm q_{1}$ | $\mathcal{N}(0.413,0.050)$
TESS $\rm u_{2}$ | $0.3103\pm 0.0047$ |  TESS $\rm q_{2}$ | $\mathcal{N}(0.259,0.050)$
I+z $\rm u_{1}$ | $0.2942\pm 0.0014$ |  I+z $\rm q_{1}$ | $\mathcal{N}(0.453,0.050)$
I+z $\rm u_{2}$ | $0.3791\pm 0.0052$ |  I+z $\rm q_{2}$ | $\mathcal{N}(0.218,0.050)$
Sloan-i’ $\rm u_{1}$ | $0.3863\pm 0.0018$ |  Sloan-i’ $\rm q_{1}$ | $\mathcal{N}(0.532,0.050)$
Sloan-i’ $\rm u_{2}$ | $0.3431\pm 0.0060$ |  Sloan-i’ $\rm q_{2}$ | $\mathcal{N}(0.265,0.050)$
Sloan-r’ $\rm u_{1}$ | $0.6408\pm 0.0032$ |  Sloan-r’ $\rm q_{1}$ | $\mathcal{N}(0.768,0.050)$
Sloan-r’ $\rm u_{2}$ | $0.2273\pm 0.0079$ |  Sloan-r’ $\rm q_{2}$ | $\mathcal{N}(0.218,0.050)$
ExTra (1.2$\mu$m) $\rm u_{1}$ | $0.2058\pm 0.0007$ |  ExTra (1.2$\mu$m) $\rm q_{1}$ | $\mathcal{N}(0.142,0.050)$
ExTra (1.2$\mu$m) $\rm u_{2}$ | $0.1708\pm 0.0028$ |  ExTra (1.2$\mu$m) $\rm q_{2}$ | $\mathcal{N}(0.273,0.050)$
External Priors | GP Priors
Stellar Mass; $M_{\star}$ ($\mathrm{M_{\odot}}$) | $\mathcal{N}(0.225,0.012)$ | Amplitude Scale $\mathrm{GP\ln\sigma(flux)}$ | $\mathcal{U}(-7,-7)$
Stellar Radius; $R_{\star}$ ($\mathrm{R_{\odot}}$) | $\mathcal{N}(0.240,0.012)$ | Length scale $\mathrm{GP\ln\rho(flux)}$ | $\mathcal{U}(-7,2)$
Stellar Effective Temperature; $T_{\rm eff}$ (K) | $\mathcal{N}(3075,75)$ |  |
Fitted Parameters | Source
$R_{\rm p}/R_{\star}$ | $0.0618\pm 0.0017$ | $0.0425\pm 0.0034$ | 2-planet linear fit
$(R_{\star}+R_{\rm p})/a$ | $0.01367\pm 0.00017$ | $0.01129_{-0.00047}^{+0.00055}$ | 2-planet linear fit
$\cos{i}$ | $0.00252_{-0.00032}^{+0.00030}$ | $0.0048_{-0.0024}^{+0.0021}$ | 2-planet linear fit
$T_{0}$ $(\mathrm{BJD})$ | $2\,459\,002.63051_{-0.00074}^{+0.00070}$ | $2\,459\,007.9879_{-0.0041}^{+0.0045}$ | 2-planet linear fit
$P$ $(\mathrm{d})$ | $19.288004_{-0.000024}^{+0.000027}$ | $25.60712_{-0.00036}^{+0.00031}$ | 2-planet linear fit
Fitted GP Hyperparameters | Source
ExTrA 2 |  $\mathrm{gp\ln\sigma}$ $-3.77_{-0.19}^{+0.24}$ |  $\mathrm{gp\ln\rho}$ $-2.73_{-0.17}^{+0.26}$ | 2-planet linear fit
ExTrA 3 |  $\mathrm{gp\ln\sigma}$ $-4.60_{-0.19}^{+0.22}$ |  $\mathrm{gp\ln\rho}$ $-2.77_{-0.15}^{+0.24}$ | 2-planet linear fit
Derived Parameters | Source
Companion radius; $R_{\mathrm{p}}$ ($\mathrm{R_{\oplus}}$) | $1.550\pm 0.064$ | $1.066\pm 0.092$ | 2-planet linear fit
Instellation; $S_{\mathrm{p}}$ ($\mathrm{S_{\oplus}}$) | $0.67_{-0.20}^{+0.15}$ | $0.48_{-0.17}^{+0.12}$ | 2-planet linear fit
Semi-major axis; $a$ (AU) | $0.0830\pm 0.0027$ | $0.0986\pm 0.0054$ | 2-planet linear fit
Inclination; $i$ (deg) | $89.856_{-0.017}^{+0.018}$ | $89.72_{-0.12}^{+0.14}$ | 2-planet linear fit
Impact parameter; $b$ | $0.195_{-0.024}^{+0.023}$ | $0.44_{-0.22}^{+0.18}$ | 2-planet linear fit
Total transit duration; $T_{\mathrm{tot}}$ (h) | $1.980\pm 0.025$ | $1.99_{-0.21}^{+0.14}$ | 2-planet linear fit
Full-transit duration; $T_{\mathrm{full}}$ (h) | $1.741\pm 0.022$ | $1.79_{-0.25}^{+0.15}$ | 2-planet linear fit
Equilibrium temperature⋆; $T_{\mathrm{eq}}$ (K) | $234\pm 12$ | $215\pm 12$ | 2-planet linear fit
Transit depth TESS; $\delta_{\mathrm{tr;TESS}}$ (ppt) | $4.48_{-0.23}^{+0.26}$ | $2.03_{-0.27}^{+0.31}$ | 2-planet linear fit
Transit depth i+z; $\delta_{\mathrm{tr;i+z}}$ (ppt) | $4.48_{-0.48}^{+0.54}$ | - | 2-planet linear fit
Transit depth Sloan-i’; $\delta_{\mathrm{tr;Sloan-i^{\prime}}}$ (ppt) | $4.60_{-1.20}^{+1.50}$ | - | 2-planet linear fit
Transit depth Sloan-r’; $\delta_{\mathrm{tr;Sloan-r^{\prime}}}$ (ppt) | $4.81_{-0.90}^{+0.78}$ | - | 2-planet linear fit
Transit depth ExTra ($1.2~{}\rm\mu m$); $\delta_{\mathrm{tr;ExTra(1.2\mu m)}}$ (ppt) | $4.20_{-0.96}^{+1.10}$ | - | 2-planet linear fit

##  6 Discussion

TOI-715 is host to at least one planet (TOI-715 b), with $R_{\rm b}=\rm
1.550\pm 0.064\,R_{\oplus}$, receiving an instellation $S_{\rm b}=\rm
0.67_{-0.20}^{+0.15}\,S_{\oplus}$, which places it within the ‘conservative
habitable zone’ defined by Kopparapu et al. (2013) as the circumstellar region
where a rocky planet recieves and instellation flux of
$0.42-0.842\,S_{\oplus}$.

Our model suggests that there is also a possibly second, smaller planet in the
system, with size $R_{02}=1.066\pm 0.092~{}\rm R_{\oplus}$, just within the
outer edge of the host’s habitable zone, at an instellation $S_{\rm 02}=\rm
0.48_{-0.17}^{+0.12}\,S_{\oplus}$.

Expectation of planet yield by TESS prior to launch showed TESS would detect
of order $70\pm 9$ Earth-sized planets ($<1.25~{}\rm R_{\oplus}$) and $14\pm
4$ conservative ‘habitable zone’ planets $<2~{}\rm R_{\oplus}$ (as in
Kopparapu et al., 2013), but that TESS would identify of order $0$ Earth-sized
($<1.25~{}\rm R_{\oplus}$) ‘conservative habitable zone planets’ (Sullivan et
al., 2015). More recent yield estimates (post-launch) by Kunimoto et al.
(2022) appear more pessimistic, however, finding only $5\pm 2$ ‘conservative
habitable zone planets’ $<2~{}\rm R_{\oplus}$ would be detected within the
primary mission and first extended mission of TESS, a factor nearly three
times lower than Sullivan et al. (2015). Should the second planet candidate,
TIC 271971130.02 be fully confirmed, its existence would thus represent an
unexpectedly important discovery made possible by the TESS mission, with the
contributions of many ground-based facilities.

In this section, we contextualise the system by examining other factors
required for habitability, and assessing its suitability for further in-depth
characterisation. We begin by revisiting the radius valley to understand what
can be learned about the mass of TOI-715 b from current mass-radius relations.
We then explore the prospects for a precise mass measurement of this planet
with current spectroscopic instrumentation, followed by a discussion of the
potential for atmospheric characterisation with HST and JWST.

###  6.1 TOI-715 b and the radius valley

The radius valley, as identified in the Kepler sample by Fulton et al. (2017),
shows a lack of close-in ($P<100~{}\rm d$) planets with sizes between
$1.5-2~{}\rm R_{\oplus}$; TOI-715 b’s $1.55~{}\rm R_{\oplus}$ appears just
inside this underpopulated region of parameter space. The importance of the
radius valley lies in its potential to teach us about planetary formation and
post-formation evolution (e.g. Owen & Wu, 2017; Ginzburg et al., 2018;
Venturini et al., 2020), and hence planets inside this gap are crucial in
furthering our understanding of the factors that sculpt it.

The location and slope of the radius valley has been shown to depend on the
properties of the host star (e.g. Fulton & Petigura, 2018; Gupta &
Schlichting, 2019; Berger et al., 2020), and Cloutier & Menou (2020) and Van
Eylen et al. (2021) recently re-examined this for low-mass stars. Cloutier &
Menou (2020) looked at the small, close-in planet distribution around stars
cooler than $4700\,\rm K$ and found that the slope is opposite in sign
compared with FGK stars, and also that the peaks of the planet radius
distributions are shifted toward smaller planets. According to this, the
center of the radius valley also shifts towards smaller radii. The sample
examined in this study contained planets discovered by Kepler and K2, and was
corrected for completeness. This work states that planets falling between
their measured radius valley and the one measured by Martinez et al. (2019)
for Sun-like stars are so-called ‘keystone planets’, crucial for further
understanding the radius valley around low-mass stars. By this metric TOI-715
b’s radius of $\rm 1.55\,R_{\oplus}$ is just below the boundary of $\rm
1.66\,R_{\oplus}$ for a period of 19.288 days, falling within the super-Earth
category.

Van Eylen et al. (2021), however, used a significantly smaller sample,
restricted to confirmed and well-characterised planets, with masses and radii
measured to at least 20% precision, orbiting stars cooler than 4000 K. They
found a slope in radius-period space opposite in sign compared with Cloutier &
Menou (2020), and therefore consistent in sign with FGK stars as measured by,
for instance, Martinez et al. (2019). They do, however, find that the valley
shifts towards smaller radii for later spectral types. Contrary to the work of
Cloutier & Menou (2020), the functional form of the radius valley derived in
this study places TOI-715 b above the radius valley and with the population of
sub-Neptunes.

In Figure 11 we present the current sample of exoplanets orbiting stars cooler
than $T_{\mathrm{eff}}\,=\,4000\,\rm K$ with planetary radii measured to
better than 10% precision151515Retrieved from the NASA Exoplanet Archive on
2022 October 05, https://exoplanetarchive.ipac.caltech.edu/cgi-
bin/TblView/nph-tblView?app=ExoTbls&config=PSCompPars in radius-period space.
We highlight the positions of the radius valleys as measured by Martinez et
al. (2019); Cloutier & Menou (2020) and Van Eylen et al. (2021) and the
positions of TOI-715 b and TIC 271971130.02. In this sample, a clear radius
bimodality for planets orbiting M dwarfs is not visible. What is evident is
that precise characterisation of planets such as TOI-715 b that fall between
the various definitions of the low-mass star radius valley is essential to
understand whether or not this bimodality will eventually be borne out by the
data.

Figure 11: Plot of the current sample of planets orbiting hosts cooler than
4000 K in radius-period space. The grey circles are confirmed planets with
radii measured to better than 10% precision, and the dark pink and green
circles are TOI-715 b and TIC 271971130.02 respectively. We note that some
errorbars are smaller than the markers. The blue line indicates the measured
radius valley according to Van Eylen et al. (2021) for hosts cooler than 4000
K. The black dashed line is the low-mass star radius valley as measured by
Cloutier & Menou (2020) for hosts cooler than 4700 K, while the dotted black
line indicated the radius valley for Sun-like stars as measured by Martinez et
al. (2019). The yellow shaded area indicates where planets referred to as
‘keystone planets’ can be found. The histogram in the right-hand panel does
not include TOI-715 b or TIC 271971130.02.

###  6.2 TOI-715 b and the density gap

Luque & Pallé (2022) recently demonstrated using a sample of planets with
precisely measured radii (to at least <8%) and masses (to at least <25%) that
small planets orbiting low-mass stars in fact likely have a density gap rather
than a radius valley. The results of that study indicate that small planets
come in three flavours: rocky, water-worlds and gassy. For planets smaller
than $2\,\rm R_{\oplus}$ they find a clear bi-modality in density which allow
us to estimate the mass of TOI-715 b in both the rocky
($\rm\sim\,7M_{\oplus}$) and water-world ($\rm\sim\,2M_{\oplus}$) scenarios;
both different from the Chen & Kipping (2017) estimate of
$\rm\sim\,3.5M_{\oplus}$ and the rocky and volatile-rich predictions from
Otegi et al. (2020) ($\rm\sim\,4.1M_{\oplus}$ and $\rm\sim\,3.5M_{\oplus}$
respectively). While this result is very promising for our understanding of
the composition of small planets around low-mass stars, their sample contained
only 34 planets; therefore continued systematic characterisation of these
systems with consistent sample selection criteria remains crucial to
understand this distribution and ultimately the composition of planets around
M dwarfs.

###  6.3 Prospects for a mass measurement of TOI-715 b

Given the faintness of TOI-715, the only southern hemisphere instrument
currently capable of the precision required to measure the mass of planet b is
ESPRESSO at the VLT (Very Large Telescope Pepe et al., 2010). If the planet is
rocky and has a mass of $\rm\sim 7\,M_{\oplus}$, then we calculate a predicted
RV semi-amplitude of $\rm 4.5\,m\,s^{-1}$. From ESPRESSO’s exposure time
calculator (ETC) we find that we could achieve a precision of $\rm
7.4\,m\,s^{-1}$ per measurement, meaning that for a 25% precision on the mass
of the planet, 44 spectra need to be collected, with $\rm 1800\,s$ exposures
and SNR $\rm\sim$ 14\. For this, we assume that TOI-715 is a relatively slow
rotator ($\rm vsini\lesssim 2\,km\,s^{-1}$) which is not unusual for old M
dwarfs that do not show significant stellar activity (Moutou et al., 2017;
Reiners et al., 2022).

However the ETC is usually pessimistic. Based on observed radial-velocity
precision of mid-M type planet-host stars obtained with ESPRESSO we expect a
slightly better performance for this instrument. Observations of Proxima Cen
(M5V) resulted in a mean precision of $\rm 0.6\,m\,s^{-1}$ (Suárez Mascareño
et al., 2020), which is about 40% better than the ETC predicts. A similar case
is LHS 1140 (M3V), for which a mean precision of $\rm 0.8\,m\,s^{-1}$ has been
achieved (Lillo-Box et al., 2020), which is a precision twice better than
expected. Empirical results161616Empirical RV Uncertainties - MAROON-X for M
dwarfs using the MAROON-X spectrograph at Gemini North (Seifahrt et al., 2020)
found a more than 40% increase in precision between M0 and M6 dwarf stars. The
ESPRESSO ETC assumes an M2 spectral type to predict the RV precision. Thus,
this apparent increase in RV precision for mid-M dwarfs, compared to early M
dwarfs can explain the observed difference in performance.

Since TOI-715 is an M4 dwarf, we can conservatively expect a 20% better RV
precision compared to the ETC (thus a precision of $\rm\sim 5.9\,m\,s^{-1}$).
This means a 25% precision on the mass of the planet can be reached with only
$\rm\sim 30$ spectra ($\rm 1800\,s$ exposures; SNR $\sim$ 14). This amounts to
15 hours of telescope time, which is not unrealistic for a planet of this
importance.

If however, TOI-715 b is in fact a ‘water world’, then a mass of $\rm\sim
2\,M_{\oplus}$ would produce an RV semi-amplitude of just $\rm
1.3\,m\,s^{-1}$. In this case 150 hours of telescope time are needed to
constrain the mass with 25% precision. Should the density bi-modality for
small planets around M dwarfs suggested by Luque & Pallé (2022) be confirmed,
a non-detection might enable us to infer the planet’s mass.

In Section 5 we found that there were small hints of transit timing variations
(TTVs) in the ground-based transits of TOI-715 b. If the second planet is
confirmed and the orbits are commensurate, the masses of both planets could be
suitable for characterisation using dynamical modelling. Given the low SNR of
individual transits and the long orbital periods, such a campaign would be
well suited to ASTEP (Antarctic Search for Transiting Exoplanets, Daban et
al., 2010; Schmider et al., 2022), given its convenient Antarctic location
(Dransfield et al., 2022b).

###  6.4 Prospects for detailed atmospheric characterisation of TOI-715 b

Figure 12: Transmission spectroscopy metric vs. instellation for transiting
planets orbiting hosts cooler than 4000 K. The blue shaded area shows the
conservative habitable zone as defined by Kopparapu et al. (2013). The two
dark pink circles highlight the positions of TOI-715 b in the rocky and water
world scenarios, while the green circle indicates the position of TIC
271971130.02. We highlight in yellow the positions of the seven planets of
TRAPPIST-1. The sizes of the circles are scaled with planetary radius.

The Transmission Spectroscopy Metric (TSM) as established by Kempton et al.
(2018) is often used to quantify the suitability of planets for atmospheric
characterisation via transmission spectroscopy. We calculate the TSM for
TOI-715 b in both mass limits171717The formula for TSM calculations includes a
planetary radius-dependent scale factor, with the cutoff for ‘terrestrial
planets’ at 1.5 R⊕. As the radius of TOI-715 b is $1.55\,\rm R_{\oplus}$ we
use the terrestrial scale factor (0.190) for the ‘rocky world’ TSM, and the
‘sub-Neptune’ scale factor (1.26) for the ‘water world’ TSM to enable a fair
comparison. and for TIC 271971130.02, as well as the published sample of
planets with $R_{p}\,<\,2~{}\rm R_{\oplus}$ and $S_{p}\,<\,1.6~{}\rm
S_{\oplus}$181818Retrieved from the NASA Exoplanet Archive on 2022 September
14, https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-
tblView?app=ExoTbls&config=PSCompPars. The comparison sample contained 34
planets initially; we then removed the seven planets discovered via radial
velocity as these are not known to transit. In Figure 12 we present this
sample of small habitable zone and temperate exoplanets, highlighting the
location of the conservative habitable zone (Kopparapu et al., 2013). We also
highlight the positions of the five TRAPPIST-1 planets that fall in this
parameter space (d-h) (Gillon et al., 2017), the recently discovered LP 890-9
c (SPECULOOS-2 c/TOI-4306 c; Delrez et al., 2022), K2-3 d (Crossfield et al.,
2015), and the well characterised mini-Neptune, LHS 1140 b (Dittmann et al.,
2017).

In the case of a ‘water world’ composition, TOI-715 b could have a
transmission signal comparable to LHS-1140 b if it does indeed have detectable
atmospheric features. In the higher mass limit we find that TOI-715 b has a
TSM of approximately 1.9, below the suggested cutoff of 12 for follow-up of
terrestrial planets.

We use Tierra (Niraula et al., 2022) to calculate simulated transmission
spectra for TOI-715 b in the ‘rocky’ ($\sim 7\,\rm M_{\oplus}$) and ‘water-
world’ ($\sim 2\,\rm M_{\oplus}$) mass scenarios in the case of a primordial
hydrogen/helium-dominated atmosphere. We then use Pandexo (Batalha et al.,
2017) to simulate JWST observations in both cases to assess the detectability
of atmospheric features. We find that in the low-mass case atmospheric
features could be detectable with a single JWST transit if the atmosphere is
cloudless. In the higher mass case, we find that extracting meaningful
features would require at least five transits, also in the cloud-free case. A
secondary atmosphere with higher mean molecular mass could suppress the scale
height by more than an order of magnitude, in turn making it challenging to
detect atmospheric features of these planets (de Wit et al., 2016).

In both mass cases, the carbon dioxide feature centered on $\rm 4.5\,\mu m$,
the methane feature at $\rm 3.3\,\mu m$ and multiple water bands are some of
the most accessible atmospheric features with the NIRSpec PRISM, the most
suitable instrument for JWST follow-up given the similar brightness of TOI-715
compared to TRAPPIST-1. Detection of these features will constrain the
metallicity of the putative planetary atmospheres, providing first hints of
the carbon-chemistry of the atmosphere itself (Madhusudhan, 2012), and
insights into their formation history (Öberg et al., 2011).

###  6.5 Flares and habitability

Both TOI-715 b and the candidate TIC 271971130.02 lie in an interesting
location of the parameter space with regard to insolation, radius, and
potential rocky composition, naturally posing questions of their habitability.
Many recent prebiotic chemistry and astrobiology studies investigated how life
may have originated on Earth and other planets (see e.g. Patel et al. 2015;
Airapetian et al. 2016; Xu et al. 2018 and reviews by Sutherland 2017; Kitadai
& Maruyama 2018). The first processes leading from inorganic compounds towards
RNA precursors likely require a liquid solution (e.g. liquid surface water)
and an energy source. For the latter, the host star’s flaring may provide the
necessary UV radiation (in the 200-280 nm range) to trigger these early steps
(e.g. Todd et al., 2018; Rimmer et al., 2018; Rimmer et al., 2021).

On the other hand, stellar activity can also pose a significant danger to
exoplanets. Strong stellar winds and XUV outbursts can contribute to
atmospheric loss (e.g. Atri & Mogan, 2020). Even if the atmosphere prevails,
coronal mass ejections (charged particle streams) may interact with and
dissociate atmospheric ozone (e.g. Tilley et al., 2019). Without the major
atmospheric absorber of harmful UV radiation, the next flares might sterilize
existing surface biology.

We investigate TOI-715’s 24 sectors of TESS data for signs of stellar flaring.
We find an average rate of 7 flares per 100 days of observations, with a
maximum flare amplitude of 5% in relative flux over nearly 2 years of data.
This analysis was performed using the stella neural network (Feinstein et al.,
2020) and as part of the TESS flare catalog (Günther et al., in prep.),
building on (Günther et al., 2020). We also compare this flare frequency
distribution with the theoretical potential for ozone sterilisation (Tilley et
al., 2019) and laboratory thresholds for prebiotic chemistry (Rimmer et al.,
2018). We find that the flaring is likely too rare and not energetic enough to
influence either of these effects.

As TOI-715 is an older star (6.6${}^{+3.2}_{-2.2}$ Gyr), its flaring is likely
not as prominent as in younger years, when its planets were forming and
prebiotic chemistry steps might have been triggered. Demographic studies of
young (< 100 Myr) and older M dwarfs show that flaring activity decreases with
stellar age, likely due to spin-down and a decreasing stellar dynamo (e.g.
Günther et al., 2020; Feinstein et al., 2020). Thus, one could carefully
speculate: if stronger flaring or other processes (volcanoes, impacts, or
lightning) had led to astrobiology on TOI-715 b or TIC 271971130.02 several
Gyr ago, there might be a chance it could still exist (pending, of course, all
other criteria such as atmospheric composition, liquid water, etc. are
fulfilled).

##  7 Conclusions

In this work we have presented the discovery, validation and characterisation
of TOI-715 b, a $R_{\rm b}=\rm 1.550\pm 0.064\,R_{\oplus}$ habitable zone
planet orbiting an M4 star with a period of
$19.288004_{-0.000024}^{+0.000027}$ days. We also demonstrated that there is
possibly a second, smaller planet with radius $R_{02}=1.066\pm 0.092~{}\rm
R_{\oplus}$ at a period of $25.60712_{-0.00036}^{+0.00031}$ days, placing it
just inside the outer edge of the circumstellar habitable zone. This system
represents the first TESS discovery to fall within this most conservative and
widely applicable ‘habitable zone’.

We have also demonstrated that TOI-715 b is amenable to further
characterisation with precise radial velocities and transmission spectroscopy;
detailed follow-up of this planet is crucial to further our understanding of
the formation and evolution of small, close-in planets. Given its location in
radius-instellation space, TOI-715 b could also help us understand the
characteristics of the M-dwarf radius valley.

Confirmation of the existence of the TIC 271971130.02 in the coming months
will also be crucial in planning further characterisation of planet b, as
disentangling the two planets in a putative radial velocity campaign could
prove challenging if the orbit of a second planet is not well known.

## List of affiliations

1School of Physics & Astronomy, University of Birmingham, Edgbaston,
Birmingham B15 2TT, United Kingdom
2Astrobiology Research Unit, University of Liège, Allée du 6 août, 19, 4000
Liège (Sart-Tilman), Belgium
3Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071, Granada,
Spain
4Center for Astrophysics and Space Sciences, UC San Diego, UCSD Mail Code
0424, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
5Department of Earth, Atmospheric and Planetary Sciences, MIT, 77
Massachusetts Avenue, Cambridge, MA 02139, USA
6Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200, La
Laguna, Tenerife, Spain
7Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
8Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France
9Observatoire de Genève, Département d’Astronomie, Université de Genève,
Chemin Pegasi 51b, 1290 Versoix, Switzerland
10AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, F-91191 Gif-
sur-Yvette, France
11European Space Agency (ESA), European Space Research and Technology Centre
(ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
12NASA Ames Research Center, Moffett Field, CA 94035, USA
13Department of Astrophysical and Planetary Sciences, University of Colorado
Boulder, Boulder, CO 80309, USA
14Department of Physics & Astronomy, Vanderbilt University, 6301 Stevenson
Center Ln., Nashville, TN 37235, USA
15Department of Physics and Kavli Institute for Astrophysics and Space
Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
16NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E.
California Blvd., Pasadena, CA 91125, USA
17University of Bern, Center for Space and Habitability, Gesellschaftsstrasse
6, 3012 Bern, Switzerland
18Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua
University, Beijing 100084, China
19Instituto de Astronomía, Universidad Nacional Autónoma de México, Ciudad
Universitaria, Ciudad de México, 04510, México
20Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
21Campo Catino Astronomical Observatory, Regione Lazio, Guarcino (FR), 03010
Italy
22Space Sciences, Technologies and Astrophysics Research (STAR) Institute,
Université de Liège, Allée du 6 Août 19C, B-4000 Liège, Belgium
23Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206 La
Laguna, Tenerife, Spain
24Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía
s/n, 18008 Granada, Spain
25Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD,
21218, USA
26Department of Astronomy, University of Maryland, College Park, MD 20742, USA
27NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771,
USA
28Department of Aeronautics and Astronautics, Massachusetts Institute of
Technology, Cambridge, MA 02139, USA
29Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
30Hazelwood Observatory
31SETI Institute, Mountain View, CA 94043, USA
32Department of Astrophysical Sciences, Princeton University, Princeton, NJ
08544, USA

## Acknowledgements

We thank the reviewer for their comments and feedback as these helped clarify
and streamline the maniscript significantly. Funding for the TESS mission is
provided by NASA’s Science Mission Directorate. We acknowledge the use of
public TESS data from pipelines at the TESS Science Office and at the TESS
Science Processing Operations Center. This research has made use of the
Exoplanet Follow-up Observation Program website, which is operated by the
California Institute of Technology, under contract with the National
Aeronautics and Space Administration under the Exoplanet Exploration Program.
This paper includes data collected by the TESS mission that are publicly
available from the Mikulski Archive for Space Telescopes (MAST). Based on data
collected by the SPECULOOS-South Observatory at the ESO Paranal Observatory in
Chile.The ULiege’s contribution to SPECULOOS has received funding from the
European Research Council under the European Union’s Seventh Framework
Programme (FP/2007-2013) (grant Agreement n∘ 336480/SPECULOOS), from the
Balzan Prize and Francqui Foundations, from the Belgian Scientific Research
Foundation (F.R.S.-FNRS; grant n∘ T.0109.20), from the University of Liege,
and from the ARC grant for Concerted Research Actions financed by the
Wallonia-Brussels Federation. This work is supported by a grant from the
Simons Foundation (PI Queloz, grant number 327127). This research is in part
funded by the European Union’s Horizon 2020 research and innovation programme
(grants agreements n∘ 803193/BEBOP), and from the Science and Technology
Facilities Council (STFC; grant n∘ ST/S00193X/1, and ST/W000385/1). The
material is based upon work supported by NASA under award number 80GSFC21M0002
Based on data collected by the TRAPPIST-South telescope at the ESO La Silla
Observatory. TRAPPIST is funded by the Belgian Fund for Scientific Research
(Fond National de la Recherche Scientifique, FNRS) under the grant FRFC
2.5.594.09.F, with the participation of the Swiss National Science Fundation
(SNF). Based on data collected under the ExTrA project at the ESO La Silla
Paranal Observatory. ExTrA is a project of Institut de Planétologie et
d’Astrophysique de Grenoble (IPAG/CNRS/UGA), funded by the European Research
Council under the ERC Grant Agreement n. 337591-ExTrA. This work has been
supported by a grant from Labex OSUG@2020 (Investissements d’avenir – ANR10
LABX56). This work has been carried out within the framework of the NCCR
PlanetS supported by the Swiss National Science Foundation. Some of the
observations in the paper made use of the High-Resolution Imaging instrument
Zorro obtained under Gemini LLP Proposal Number: GN/S-2021A-LP-105. Zorro was
funded by the NASA Exoplanet Exploration Program and built at the NASA Ames
Research Center by Steve B. Howell, Nic Scott, Elliott P. Horch, and Emmett
Quigley. Zorro was mounted on the Gemini South telescope of the international
Gemini Observatory, a program of NSF’s OIR Lab, which is managed by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation. on behalf of the
Gemini partnership: the National Science Foundation (United States), National
Research Council (Canada), Agencia Nacional de Investigación y Desarrollo
(Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina),
Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and
Korea Astronomy and Space Science Institute (Republic of Korea). The Digitized
Sky Surveys were produced at the Space Telescope Science Institute under U.S.
Government grant NAG W-2166. The images of these surveys are based on
photographic data obtained using the Oschin Schmidt Telescope on Palomar
Mountain and the UK Schmidt Telescope. The plates were processed into the
present compressed digital form with the permission of these institutions.
This work makes use of observations from the LCOGT network. Part of the LCOGT
telescope time was granted by NOIRLab through the Mid-Scale Innovations
Program (MSIP). MSIP is funded by NSF. This publication benefits from the
support of the French Community of Belgium in the context of the FRIA Doctoral
Grant awarded to MT. MNG acknowledges support from the European Space Agency
(ESA) as an ESA Research Fellow. BVR thanks the Heising-Simons Foundation for
support. MG is F.R.S.-FBRS Research Director. YGMC acknowledges support from
UNAM-PAPIIT-IG101321 FJP acknowledges financial support from the grant
CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033. This research made
use of Lightkurve, a Python package for Kepler and TESS data analysis
(Lightkurve Collaboration, 2018). This work made use of
Astropy:191919http://www.astropy.org a community-developed core Python package
and an ecosystem of tools and resources for astronomy (Astropy Collaboration
et al., 2013a, 2018b, 2022). This research has made use of the NASA Exoplanet
Archive, which is operated by the California Institute of Technology, under
contract with the National Aeronautics and Space Administration under the
Exoplanet Exploration Program. This research made use of exoplanet (Foreman-
Mackey et al., 2021a) and its dependencies (Agol et al., 2020; Kumar et al.,
2019; Astropy Collaboration et al., 2013b, 2018a; Luger et al., 2019;
Salvatier et al., 2016; Theano Development Team, 2016). Resources supporting
this work were provided by the NASA High-End Computing (HEC) Program through
the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for
the production of the SPOC data products.

## Data Availability

TESS data products are available via the MAST portal at
https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html. Follow-up
photometry and high resolution imaging data for TOI-715 are available on
ExoFOP at https://exofop.ipac.caltech.edu/tess/target.php?id=271971130. These
data are freely accessible to ExoFOP members immediately and are publicly
available following a one-year proprietary period.

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