# Confirmation and Characterization of the Eccentric, Warm Jupiter TIC
393818343 b with a Network of Citizen Scientists

Lauren A. Sgro SETI Institute, Carl Sagan Center, 339 Bernardo Ave, Suite 200,
Mountain View, CA 94043, USA  Paul A. Dalba Heising-Simons 51 Pegasi b
Postdoctoral Fellow  SETI Institute, Carl Sagan Center, 339 Bernardo Ave,
Suite 200, Mountain View, CA 94043, USA  Department of Astronomy and
Astrophysics, University of California, Santa Cruz, CA 95064, USA  Thomas M.
Esposito SETI Institute, Carl Sagan Center, 339 Bernardo Ave, Suite 200,
Mountain View, CA 94043, USA  Unistellar, 5 allée Marcel Leclerc, bâtiment B,
Marseille, 13008, France  Department of Astronomy, University of California,
Berkeley, CA 94720, USA  Franck Marchis SETI Institute, Carl Sagan Center, 339
Bernardo Ave, Suite 200, Mountain View, CA 94043, USA  Unistellar, 5 allée
Marcel Leclerc, bâtiment B, Marseille, 13008, France  Diana Dragomir
Department of Physics & Astronomy, University of New Mexico, 1919 Lomas Blvd
NE, Albuquerque, NM 87131, USA  Steven Villanueva Jr. NASA Postdoctoral
Program Fellow  NASA Goddard Space Flight Center, 8800 Greenbelt Road,
Greenbelt, MD 20771, USA  Benjamin Fulton NASA Exoplanet Science
Institute/Caltech-IPAC, MC 314-6, 1200 E. California Blvd., Pasadena, CA
91125, USA  Mario Billiani Unistellar Network, Vienna, Austria  Citizen
Scientist  Margaret Loose  Unistellar Network, Santa Fe, New Mexico, USA
Citizen Scientist  Nicola Meneghelli Unistellar Network, Zevio, Verona, Italy
Citizen Scientist  Darren Rivett Unistellar Network, Lake Macquarie, NSW,
Australia  Citizen Scientist  Fadi Saibi Unistellar Network, Sunnyvale, CA,
USA  Citizen Scientist  Sophie Saibi Unistellar Network, Sunnyvale, CA, USA
Citizen Scientist  Bryan Martin  Exoplanet Watch, Great Falls, MT, USA
Citizen Scientist  Georgios Lekkas Exoplanet Watch  Citizen Scientist  Daniel
Zaharevitz  Exoplanet Watch, Frederick, MD, USA  Citizen Scientist  Robert T.
Zellem Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak
Grove Drive, Pasadena, CA 91109, USA  Exoplanet Watch  Ivan A. Terentev Visual
Survey Group, Petrozavodsk, Russia  Citizen Scientist  Robert Gagliano Visual
Survey Group, Glendale, Arizona, USA  Citizen Scientist  Thomas Lee Jacobs
Visual Survey Group, Bellevue, WA, USA  Citizen Scientist  Martti H.
Kristiansen Brorfelde Observatory, Observator Gyldenkernes Vej 7, DK-4340
Tølløse, Denmark  Daryll M. LaCourse Visual Survey Group, Marysville, WA, USA
Citizen Scientist  Mark Omohundro  Department of Physics, University of
Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX13RH, UK  Citizen
Scientist  Hans M. Schwengeler Visual Survey Group, Bottmingen, Switzerland
Citizen Scientist

###### Abstract

NASA’s Transiting Exoplanet Survey Satellite (TESS) has identified over 7,000
candidate exoplanets via the transit method, with gas giants among the most
readily detected due to their large radii. Even so, long intervals between
TESS observations for much of the sky lead to candidates for which only a
single transit is detected in one TESS sector, leaving those candidate
exoplanets with unconstrained orbital periods. Here, we confirm the planetary
nature of TIC 393818343 b, originally identified via a single TESS transit,
using radial velocity data and ground-based photometric observations from
citizen scientists with the Unistellar Network and Exoplanet Watch. We
determine a period of $P$ = 16.24921 $\begin{subarray}{c}+0.00010\\\
-0.00011\end{subarray}$ days, a mass $M_{P}$ = 4.34 $\pm$ 0.15 $M_{J}$, and
semi-major axis $a$ = 0.1291 $\begin{subarray}{c}+0.0021\\\
-0.0022\end{subarray}$ au, placing TIC 393818343 b in the “warm Jupiter”
population of exoplanets. With an eccentricity $e$ = 0.6058 $\pm$ 0.0023, TIC
393818343 b is the most eccentric warm Jupiter to be discovered by TESS
orbiting less than 0.15 au from its host star and therefore an excellent
candidate for follow-up, as it may inform our future understanding of how hot
and warm Jupiter populations are linked.

Extrasolar gaseous giant planets (509) – Transit photometry (1709) – Exoplanet
astronomy (486) – Amateur astronomy (35) – Radial velocity (1132)

††journal: AJ††facilities: Automated Planet Finder (Levy), Keck:I (HIRES),
TESS, Unistellar††software:  astropy (Astropy Collaboration et al., 2013,
2018), EXOFASTv2 (Eastman et al., 2013; Eastman, 2017; Eastman et al., 2019),
lightkurve (Lightkurve Collaboration et al., 2018), SpecMatch (Petigura, 2015;
Petigura et al., 2017), LcTools (Schmitt et al., 2019)

##  1 Introduction

In September 2023, the NASA Exoplanet Archive (NEA) announced the 5,500th
confirmed exoplanet 111https://exoplanetarchive.ipac.caltech.edu/. This
growing number is far from the first exoplanet discovery of 51 Peg b (Mayor &
Queloz, 1995), which we now know as a hot Jupiter. Hot Jupiters are typically
categorized as having $\geq 0.25$ Jupiter masses and orbital periods of $\leq$
10 days, and are among the most easily detected via the transit method due to
their large radii, small semi-major axes, and frequent transits. However, a
growing population of discovered warm Jupiters ($P$ $\approx$ 10-200 days, $a$
= 0.1-0.5 au) has informed our understanding of hot Jupiter formation, as it
is expected that hot Jupiters reach their orbits via migration from outside
the snowline (Dawson & Johnson, 2018). Migration pathways are favored over in
situ formation in part due to the large amount of solids required, ${\sim}10$
$\,M_{\rm E}$, for core accretion (Rafikov, 2006) and the lack of
observational evidence for formation by gravitational instability, although
there is active work investigating this latter scenario (Hall et al., 2020;
Paneque-Carreño et al., 2021). Therefore, this intermediate population of warm
Jupiters may be progenitors of hot Jupiters and links to our solar system’s
distant, cold Jupiter.

Two primary migration pathways have been proposed that may produce hot
Jupiters: disk migration, in which an exchange of angular momentum with the
surrounding protoplanetary disk causes the planet’s migration (Goldreich &
Tremaine, 1980; Baruteau et al., 2014), and high eccentricity (tidal)
migration, where a gas giant on an eccentric orbit undergoes tidal dissipation
due to close periastron passage with the host star, reducing the planets’s
semi-major axis and therefore circularizing the orbit (Rasio & Ford 1996; Wu &
Murray 2003, see Dawson & Johnson 2018 for a recent review). To experience
tidal migration, a warm Jupiter must have a particular combination of
eccentricity and semi-major axis that brings it sufficiently close to its
host. Multiple mechanisms for the origin of this high eccentricity have been
proposed e.g., Kozai-Lidov oscillations (Kozai, 1962; Lidov, 1962; Naoz,
2016), secular interactions, (Wu & Lithwick, 2011; Petrovich & Tremaine,
2016), and planet-planet scattering (Rasio & Ford, 1996; Ford & Rasio, 2006;
Chatterjee et al., 2008; Nagasawa et al., 2008). However, only three warm
Jupiters are known that exhibit an eccentricity large enough to eventually
join the hot Jupiter population via tidal migration, implying an entire
population of hot Jupiters with only suspected origins (Naef et al. 2001; Dong
et al. 2021, Gupta et al. 2024, in prep.). The population of eccentric warm
Jupiters with unknown fates is important to study and add to, as their
evolution history may contain an important piece to the planetary formation
puzzle.

With the conclusion of the Kepler and K2 missions, NASA’s Transiting Exoplanet
Survey Satellite (TESS, Ricker et al., 2015) has become the most promising
observatory with which to detect warm gas giants. There are already 27
confirmed planets in the NASA Exoplanet Archive (NASA Exoplanet Archive,
2024)222Accessed on 2024-05-22 at 20:00 UTC. representative of this population
($a$ = 0.1 – 0.5 au, $M$ $\geq 0.25$ $M_{J}$, and $P$ ranging from 10 to 98
days), all discovered by TESS despite its recent launch in 2018. However, TESS
only observes each sector of the sky for an average of 27.4 days, leaving a
population of TESS candidates for which only a single transit has been
detected. Such candidates have poorly constrained periods and must rely on
follow-up photometric and radial velocity (RV) efforts to confirm system
properties. One such way to detect subsequent transits is via photometry
gathered from the citizen science community. Enlisting such global networks
mitigates the limitations faced by professional telescope facilities, such as
visibility and weather restrictions of a single location and finite available
time. The exoplanet community is already successfully working with these
networks through programs such as the Unistellar Network (Peluso et al.,
2023), Exoplanet Watch (Zellem et al., 2020), and ExoClock (Kokori et al.,
2022, 2023). Here we add to the quickly growing list of citizen science
successes in exoplanet follow-up.

We present the confirmation of a high-eccentricity ($e$ = 0.6058 $\pm$ 0.0023)
warm Jupiter TIC 393818343 b with $P$ = 16.24921
$\begin{subarray}{c}+0.00010\\\ -0.00011\end{subarray}$ days using both
ground-based transit and RV data. We first identified a single transit
signature via visual inspection in Sector 55 TESS data as part of the Visual
Survey Group (Kristiansen et al., 2022). Follow-up observations and analyses
with Lick Observatory, the Unistellar Network of Citizen Astronomers, and
Exoplanet Watch confirm the existence of this exoplanet. We present our
findings in the following order: in Section 2 we present our observations of
TIC 393818343 b, in Section 3 we detail our methods of modeling the system in
order to plan photometric observations and confirm the planet, in Section 4 we
discuss our results, and in Section 5 we discuss the implications of this
discovery and potential future observations. Finally, in Section 6 we give an
overview of our findings.

##  2 Observations

###  2.1 TESS Photometry

TIC 393818343 was observed by TESS at a 2-minute cadence during its extended
mission in Sector 55 from 2022 August 5 to September 1. We use the image data
as processed by the Science Processing Operations Center (SPOC, Jenkins et
al., 2016) at NASA Ames Research Center (Team, 2021). Although no evidence of
two or more transits was found by the Quick Look Pipeline, additional analyses
were carried out by the Visual Survey Group (VSG). Using the LcTools software
package (Schmitt et al., 2019; Schmitt & Vanderburg, 2021), this group of
professional and citizen scientists detected a single transit in TESS data
taken on 2022 August 19, prompting a Doppler monitoring follow-up campaign. We
refer to the candidate exoplanet that caused this transit signature as TIC
393818343.01. The TESS observation, along with all other photometric
observations used in this work, are detailed in Table 1. The candidate TIC
393818343.01 was further categorized as a TESS object of interest, TOI
6883.01, on 2024 February 1 (Conzo & Moriconi, 2024).

Table 1: Summary of Photometric Observations  UTC Date | Transit Epoch | Observer | Telescope | Filter
---|---|---|---|---
2022 Aug 19 | 0 |  $\cdots$ | TESS | TESS
2023 May 23 | 17 | N. Meneghelli | eVscope-A | Clear
2023 May 23 | 17 | M. Billiani | eVscope-B | Clear
2023 Jun 24 | 19 | B. Martin | 12-in Dall-Kirkham | I
2023 Jun 24 | 19 | D. Rivett | eVscope-C | Clear
2023 Jun 24 | 19 | F. & S. Saibi | eVscope-D | Clear
2023 Jun 24 | 19 | M. Loose | eVscope-E | Clear

###  2.2 HIRES & Levy Spectroscopy

After the discovery of the Sector 55 TESS transit by the VSG, we obtained a
high signal-to-noise (S/N ¿ 200) template spectrum of TIC 393818343 taken with
the High Resolution Echelle Spectrometer (HIRES, Vogt et al., 1994) at W. M.
Keck Observatory without the HIRES iodine cell. We analyzed this spectrum
using the synthetic spectral matching tool SpecMatch (Petigura et al., 2017)
to gather basic stellar parameters as priors in our global system modeling
including effective temperature $T_{eff}$, surface gravity $logg$, metallicity
[Fe/H], and projected rotational velocity vsini. After confirming via the
HIRES data that TIC 393818343 was an appropriate target for Doppler
spectroscopy, we proceeded with a follow-up campaign.

From 2022 October to 2023 July, we initiated a Doppler monitoring campaign of
TIC 393818343 using the Levy Spectrograph on the Automated Planet Finder (APF)
telescope at Lick Observatory in Mt Hamilton, California (Radovan et al.,
2014; Vogt et al., 2014). The Levy Spectrograph is a high resolution slit-fed
optical echelle spectrometer with R $\approx$ 114,000 (Radovan et al., 2010)
and an iodine cell for precise wavelength calibration (Butler et al., 1996).
Over this observing period we gathered 56 RV measurements with uncertainties
ranging from 3.6 – 7.8 $ms^{-1}$. For each spectrum, we deduced RV using a
forward modeling procedure (Butler et al., 1996; Fulton et al., 2015) in which
we used the Keck HIRES spectrum as a template. Obtaining a template spectrum
of the necessary quality was more efficient with Keck-HIRES due to the
aperture, which is 10 m as opposed to APF’s 2.4-m aperture. All RV
measurements from APF-Levy are presented in Table 2 and time series RVs are
shown in Figure 1.

We combined these RV measurements with the TESS photometry using EXOFASTv2
(Eastman et al., 2013, 2019) to predict future transit windows, which enabled
a ground-based photometric campaign for TIC 393818343.01 transits. The follow-
up photometry is described in Section 2.3 and 2.4, while system modeling is
described in Section 3.

Table 2: RV Measurements of TIC 393818343 From APF-Levy.  BJDTDB | RV (m s-1)
---|---
$2459858.773894$ | $679.6\pm 4.0$
$2459861.776856$ | $6.4\pm 4.3$
$2459864.782496$ | $-110.4\pm 4.4$
$2459868.654536$ | $-128.9\pm 3.9$
$2459872.684238$ | $48.9\pm 5.8$
$2459897.626448$ | $-128.3\pm 3.9$
$2459904.706206$ | $4.5\pm 3.8$
$2460044.993459$ | $-127.8\pm 3.7$
$2460050.016955$ | $-53.0\pm 3.6$
$2460055.956763$ | $84.3\pm 4.3$
$2460060.938359$ | $-143.0\pm 4.1$
$2460061.929630$ | $-126.5\pm 4.6$
$2460062.935416$ | $-123.3\pm 4.5$
$2460063.943645$ | $-124.6\pm 4.0$
$2460064.929811$ | $-90.1\pm 6.0$
$2460072.904212$ | $10.6\pm 6.5$
$2460073.922257$ | $-59.6\pm 4.5$
$2460074.940567$ | $-99.8\pm 7.8$
$2460075.891244$ | $-114.0\pm 4.1$
$2460076.895873$ | $-135.5\pm 4.5$
$2460078.891853$ | $-126.6\pm 3.7$
$2460081.896280$ | $-77.8\pm 4.4$
$2460083.911727$ | $37.0\pm 3.6$
$2460084.875597$ | $211.9\pm 4.6$
$2460085.866808$ | $534.0\pm 4.0$
$2460087.860502$ | $193.1\pm 3.6$
$2460088.945967$ | $11.3\pm 3.9$
$2460089.862875$ | $-59.5\pm 4.2$
$2460091.854226$ | $-121.5\pm 4.9$
$2460093.968028$ | $-138.0\pm 6.5$
$2460095.840985$ | $-142.0\pm 6.8$
$2460096.851126$ | $-102.3\pm 4.5$
$2460097.835640$ | $-89.7\pm 4.8$
$2460098.891130$ | $-53.1\pm 3.9$
$2460104.828737$ | $55.1\pm 4.1$
$2460105.964703$ | $-41.2\pm 5.0$
$2460106.848495$ | $-69.4\pm 7.7$
$2460107.823347$ | $-98.4\pm 7.2$
$2460108.871167$ | $-123.8\pm 3.8$
$2460109.810574$ | $-126.8\pm 4.7$
$2460110.805436$ | $-128.9\pm 4.8$
$2460111.796328$ | $-119.6\pm 5.9$
$2460112.950157$ | $-107.5\pm 4.0$
$2460113.824174$ | $-89.9\pm 5.0$
$2460115.789096$ | $3.5\pm 5.6$
$2460118.864396$ | $704.4\pm 4.2$
$2460119.859491$ | $321.7\pm 4.4$
$2460121.786157$ | $-11.4\pm 4.2$
$2460123.811574$ | $-115.6\pm 4.1$
$2460125.961876$ | $-130.1\pm 3.7$
$2460128.882675$ | $-112.7\pm 4.3$
$2460131.882008$ | $-22.3\pm 3.7$
$2460133.902699$ | $272.0\pm 5.0$
$2460135.963745$ | $382.6\pm 4.2$
$2460138.837038$ | $-49.3\pm 4.2$
$2460140.992830$ | $-118.3\pm 3.8$
Figure 1: APF-Levy RVs of TIC 393818343. Panel a is the time series. Panel c
is phase-folded on the best-fit ephemeris. Panel b shows that an additional
long-term RV trend may be present, although from this data set alone it is
statistically consistent with no trend.

###  2.3 Unistellar Network Observations

Using the Unistellar Network of citizen scientists, we obtained photometry of
TIC 393818343 in accordance with our predicted transit windows calculated as
described in Section 3. This network consists of observers around the world
who operate mobile, 114-mm Newtonian imaging telescopes, called eVscopes,
produced by Unistellar (Marchis et al., 2020). The two models of eVscope both
utilize a SONY CMOS sensor, either IMX224 or IMX347, with a 28′ $\times$ 37′
or 34′ $\times$ 47′ field of view, respectively. See Peluso et al. (2023) for
an in-depth review of the Unistellar Network Exoplanet program.

On 2023 May 23, two citizen scientists observed TIC 393818343 during a
predicted transit window from Austria and Italy. This prompted another follow-
up campaign (see Section 3) on 2023 June 24 that included three participating
Unistellar Network telescopes located in California, U.S., and Australia.

For each eVscope observation, which varied in duration, off-target or
saturated frames were removed from the data set. All other images were dark
subtracted and plate solved. Calibrated frames were then aligned and averaged
together up to a total integration time of two minutes each to increase the
star S/N and reduce computation time in subsequent steps. Following the
procedure of Dalba & Muirhead (2016) and Dalba et al. (2017), differential
photometry was performed on each dataset using an optimized combination of up
to ten reference stars. The pool of potential reference stars is created by
identifying all stars in each image (other than the target), measuring their
fluxes, and removing those which are saturated or do not appear in $\geq 50$%
of the frames. We then select up to ten of the brightest stars and generate
individual reference light curves for each of them. Next, we generate
“composite reference star” light curves from every unique combination of those
individual reference light curves, where the combination is a median across
stars (not time). As a result, we produce up to 1,023 composite reference star
light curves with which to normalize the target star’s flux. The best
normalized target light curve from that group is selected as the one with the
smallest residuals in the out-of-transit measurements when compared to a flat
model. An optimal aperture size is deduced for each observation by fitting a
2-D Gaussian function to the target across all images in the dataset and
calculating a weighted average standard deviation across all frames, where the
weights are inversely proportional to the variance. For the only Unistellar
dataset crossing ingress, we tested a variety of aperture sizes from 14′′ \-
24′′ to ensure that the ingress signal was not introduced by a varying PSF as
seeing conditions changed. With this additional analysis, we confirmed that
the ingress signal was present in all iterations of the photometry and that
therefore our detection was robust. We do not detrend for airmass for any of
the four datasets that contain in-transit data since they do not have both
pre- and post-transit data points.

We note that there is a spatially coincident, unbound star, Gaia DR3
4233021429372256512, seen 6.5′′ from TIC 393818343 that can appear blended
with the target in ground-based images. We include this nearby source in the
target aperture when performing differential photometry and discuss its
potential impact in Section 4.

###  2.4 Exoplanet Watch Observations

Several citizen scientists associated with Exoplanet Watch observed during
both campaigns with telescopes other than eVscopes. Two observations on 2023
May 23 were taken outside of the transit window and therefore contained only
baseline data. We do not include these observations in Table 1 or in
subsequent modeling. However, a 4.5-hour observation taken 2024 June 24 from
Utah, U.S., did cover the predicted transit window and is detailed in Table 1.
The observer used a Johnson I filter on a 305-mm Dall-Kirkham telescope
equipped with a ZWO ASI 294mm CMOS camera (FOV 63′ $\times$ 31′) and an
exposure time of 90 s per image. The data were $2\times 2$ binned and
calibrated with dark, flat, and bias frames using EXOTIC (Zellem et al.,
2020). EXOTIC was also used by the observer to perform plate solving and
differential photometry with a 19.5′′-diameter target aperture.

##  3 Methods

In this section we describe the global system modeling, the two follow-up
photometric campaigns, and the subsequent modeling once additional data were
received.

###  3.1 Initial System Modeling

Using the single TESS transit and the subsequent APF-Levy RV observations, we
initially modeled the stellar and planetary parameters for the TIC 393818343
system via the EXOFASTv2 suite (Eastman et al., 2013, 2019), which jointly
considers photometry and RV data when modeling a system. Preliminary modeling
included TESS photometry and APF-Levy RV data through May of 2023 along with
archival broadband photometry from Gaia DR2 (Gaia Collaboration et al., 2016,
2018), the Two Micron All Sky Survey (2MASS, Cutri et al., 2003), and the
Wide-field Infrared Survey Explorer (WISE, Cutri et al., 2014). These archival
datasets, as well as Gaia parallax measurements, were used by EXOFASTv2 to fit
spectral energy distributions (SEDs) from MIST stellar evolutionary models
(Dotter, 2016) to gather host star properties.

Based on the Specmatch analysis of the HIRES TIC 393818343 template spectrum,
we placed normal host-star priors on $T_{eff}$ and [Fe/H], as well as on
parallax using the Gaia DR3 parallax data corrected for the zero-point offset
(Vallenari et al., 2023; Lindegren et al., 2021). We also placed a uniform
prior on extinction in the V-band ($A_{V}\leq 0.2455$) derived from galactic
dust maps (Schlafly & Finkbeiner, 2011). These informative priors are listed
at the top of Table 3, and noninformative priors used by EXOFASTv2 can be
found in Table 3 of Eastman et al. (2019).

We consider the fit—including archival, TESS, and RV data—converged in
accordance with the default EXOFASTv2 statistics, where the number of
independent draws of the underlying posterior probability distribution is
$T_{z}$ ¿ 1000 (Ford & Rasio, 2006), and the Gelman-Rubin statistic (Gelman &
Rubin, 1992) ¡ 1.01 for every fitted parameter. This EXOFASTv2 analysis
initially returned a posterior period with 68% confidence of 16.2511 $\pm$
0.0046 days and duration of 4.2 hours, predicting the next mid-transit time to
be $T_{o}$ = 2460087.50889 $\pm$ 0.08 BJD_TDB, or UTC 2023-05-23 00:09:19.

###  3.2 Initial Photometric Campaign & Subsequent Modeling

We refer to this particular transit window with mid-transit time $T_{o}$ =
2460087.50889 $\pm$ 0.08 BJD_TDB as Epoch 17, where Epoch 0 is defined as the
TESS Sector 55 transit. We prompted the Unistellar Network and Exoplanet Watch
to observe this 2$\sigma$ window plus half the transit time, a 12-hour span
during which we obtained four multi-hour observations. Two of these
observations consisted only of baseline data, thus they are not included in
our tables or figures (see Section 2.4).

Two of the Epoch 17 observations from the Unistellar Network spanned the
projected egress time. For each light curve we use a custom transit fitting
routine adapted for eVscope data that uses the pycheops Python package and a
least-squares minimization to model the transit (Peluso et al., 2023). Due to
the partial nature of the individual light curves, the transit duration and
period were fixed to those values determined by EXOFASTv2, while the mid-
transit time and depth were allowed to vary within 2$\sigma$ and 40% of the
expected value, respectively. Our custom routine then uses the Schwarz
criterion rankings from Kass & Raftery (1995) to determine if the fitted
transit is statistically favored over a flat line, among other tests that
ensure the model is reasonable. As a result of the analyses included in this
routine, observation during Epoch 17 resulted in a preliminary detection of
egress.

The two Epoch 17 datasets containing egress signatures were then combined into
the EXOFASTv2 fit with the previous TESS photometry and continued RV
monitoring data through June of 2023 in order to refine the period to $P$ =
16.24927 $\pm$ 0.0046 days.

###  3.3 Confirmation Photometric Campaign

We initiated a confirmation campaign to observe a predicted transit with mid-
transit time at $T_{o}$ = 2460119.97632 $\pm$ 0.08 BJD_TDB, or 2023-06-24
11:18:53 UTC, two periods after our initial endeavor. We refer to this transit
window, which spanned eight hours to allow for two-hour baseline observations
in addition to the expected transit duration of $\sim$4 hours, as Epoch 19.

Four observers took part in our Epoch 19 confirmation campaign, with their
data covering predicted ingress and egress. Light curves from all photometric
campaign epochs are presented individually in Figure 2. The Epoch 19 campaign
was successful, as the transit was detected in a joint light curve combining
the three Unistellar eVscope data sets, shown in Figure 3. Another observation
from Exoplanet Watch also detected ingress independently. These four datasets
were then combined with the complete APF RV dataset once Doppler monitoring
concluded in 2023 July, as well as with the ground-based Epoch 17, TESS Epoch
0, and archival photometry, to be used as input for a final EXOFASTv2 analysis
following the process described in Section 3.1. We discuss the results of this
analysis in Section 4.

Table 3: Median values and 68% confidence interval for TIC393818343

Parameter | Units | Values
---|---|---
Informative Priors: |  |  |
$T_{\rm eff}$ | Effective Temperature (K) | $\mathcal{N}(5800,100)$ |  |
$[{\rm Fe/H}]$ | Metallicity (dex) | $\mathcal{N}(0.32,0.06)$ |  |
$\varpi$ | Parallax (mas) | $\mathcal{N}(10.644,0.397)$ |  |
$A_{V}$ | V-band Extinction (mag)) | $\mathcal{U}(0,0.2455)$ |  |
Stellar Parameters: |  |  |
$M_{*}$ | Mass ($\,M_{\sun}$) | $1.082^{+0.055}_{-0.056}$ |  |
$R_{*}$ | Radius ($\,R_{\sun}$) | $1.086\pm 0.020$ |  |
$L_{*}$ | Luminosity ($\,L_{\sun}$) | $1.168^{+0.065}_{-0.063}$ |  |
$F_{Bol}$ | Bolometric Flux (cgs) | $0.00000000423\pm 0.00000000023$ |  |
$\rho_{*}$ | Density (cgs) | $1.198^{+0.040}_{-0.058}$ |  |
$\log{g}$ | Surface gravity (cgs) | $4.402^{+0.013}_{-0.018}$ |  |
$T_{\rm eff}$ | Effective Temperature (K) | $5756^{+67}_{-66}$ |  |
$[{\rm Fe/H}]$ | Metallicity (dex) | $0.318\pm 0.058$ |  |
$[{\rm Fe/H}]_{0}$ | Initial Metallicity1 | $0.299\pm 0.057$ |  |
$Age$ | Age (Gyr) | $3.8^{+2.6}_{-2.1}$ |  |
$EEP$ | Equal Evolutionary Phase2 | $355^{+33}_{-22}$ |  |
$A_{V}$ | V-band extinction (mag) | $0.127^{+0.073}_{-0.076}$ |  |
$\sigma_{SED}$ | SED photometry error scaling | $1.28^{+0.49}_{-0.30}$ |  |
$\varpi$ | Parallax (mas) | $10.643\pm 0.039$ |  |
$d$ | Distance (pc) | $93.96^{+0.35}_{-0.34}$ |  |
$\dot{\gamma}$ | RV slope3 (m/s/day) | $0.001\pm 0.015$ |  |
Planetary Parameters: | b |  |
$P$ | Period (days) | $16.24921^{+0.00010}_{-0.00011}$ |  |
$R_{P}$ | Radius ($\,R_{\rm J}$) | $1.087^{+0.023}_{-0.021}$ |  |
$M_{P}$ | Mass ($\,M_{\rm J}$) | $4.34\pm 0.15$ |  |
$T_{C}$ | Time of conjunction ($\rm{BJD_{TDB}}$) | $2459811.24020^{+0.00025}_{-0.00024}$ |  |
$a$ | Semi-major axis (AU) | $0.1291^{+0.0021}_{-0.0022}$ |  |
$i$ | Inclination (Degrees) | $89.57^{+0.30}_{-0.38}$ |  |
$e$ | Eccentricity | $0.6058\pm 0.0023$ |  |
$\omega_{*}$ | Argument of Periastron (Degrees) | $1.69\pm 0.49$ |  |
$T_{eq}$ | Equilibrium temperature4 (K) | $805.5^{+9.6}_{-9.4}$ |  |
$\tau_{\rm circ}$ | Tidal circularization timescale (Gyr) | $26.2^{+2.1}_{-2.6}$ |  |
$K$ | RV semi-amplitude (m/s) | $414.8\pm 2.8$ |  |
$R_{P}/R_{*}$ | Radius of planet in stellar radii | $0.10295^{+0.00065}_{-0.00062}$ |  |
$a/R_{*}$ | Semi-major axis in stellar radii | $25.60^{+0.28}_{-0.42}$ |  |
$\delta$ | $\left(R_{P}/R_{*}\right)^{2}$ | $0.01060\pm 0.00013$ |  |
$\delta_{\rm I}$ | Transit depth in I (fraction) | $0.01237^{+0.00038}_{-0.00037}$ |  |
$\delta_{\rm Clear}$ | Transit depth in Clear (fraction) | $0.01205\pm 0.00025$ |  |
$\delta_{\rm TESS}$ | Transit depth in TESS (fraction) | $0.01270\pm 0.00022$ |  |
$\tau$ | Ingress/egress transit duration (days) | $0.01634^{+0.00067}_{-0.00024}$ |  |
$T_{14}$ | Total transit duration (days) | $0.17294^{+0.00089}_{-0.00082}$ |  |
$b$ | Transit impact parameter | $0.119^{+0.10}_{-0.083}$ |  |
$b_{S}$ | Eclipse impact parameter | $0.123^{+0.11}_{-0.086}$ |  |
$\tau_{S}$ | Ingress/egress eclipse duration (days) | $0.01698^{+0.00069}_{-0.00033}$ |  |
$T_{S,14}$ | Total eclipse duration (days) | $0.1791\pm 0.0020$ |  |
$\rho_{P}$ | Density (cgs) | $4.22^{+0.16}_{-0.24}$ |  |
$logg_{P}$ | Surface gravity | $3.962^{+0.010}_{-0.018}$ |  |
$\Theta$ | Safronov Number | $0.952^{+0.019}_{-0.020}$ |  |
$\langle F\rangle$ | Incident Flux (109 erg s-1 cm-2) | $0.0682^{+0.0033}_{-0.0031}$ |  |
$T_{P}$ | Time of Periastron ($\rm{BJD_{TDB}}$) | $2459810.145^{+0.015}_{-0.016}$ |  |
$T_{S}$ | Time of eclipse ($\rm{BJD_{TDB}}$) | $2459808.973\pm 0.019$ |  |
$e\cos{\omega_{*}}$ |  | $0.6055\pm 0.0023$ |  |
$e\sin{\omega_{*}}$ |  | $0.0179\pm 0.0052$ |  |
Wavelength Parameters: | Dall-Kirkham (I) | eVscope (Clear) | TESS
$u_{1}$ | linear limb-darkening coeff | $0.294\pm 0.050$ | $0.425\pm 0.026$ | $0.339\pm 0.033$
$u_{2}$ | quadratic limb-darkening coeff | $0.269\pm 0.050$ | $0.271\pm 0.024$ | $0.273\pm 0.046$
Telescope Parameters: | APF-Levy |  |
$\gamma_{\rm rel}$ | Relative RV Offset3 (m/s) | $33.1\pm 1.5$ |  |
$\sigma_{J}$ | RV Jitter (m/s) | $7.23^{+1.1}_{-0.97}$ |  |

††footnotetext: See Table 3 in Eastman et al. (2019) for a detailed
description of all parameters and default, noninformative priors. Informative
priors listed as $\mathcal{N}(a,b)$ indicates a normal distribution with mean
$a$ and standard deviation $b$, while $\mathcal{U}(a,b)$ indicates a uniform
distribution over the interval [$a$, $b$].

11footnotetext: The metallicity of the star at birth

22footnotetext: Corresponds to static points in a star’s evolutionary history.
See §2 in Dotter (2016).

33footnotetext: Reference epoch = 2459999.883362

44footnotetext: Assumes no albedo and perfect redistribution

Figure 2: Individual transit observations of TIC 393818343 b and the best-fit
model from EXOFASTv2 shown in red. The names of observers and relevant epochs
are listed in Table 1. Figure 3: Light curve of observations taken by
Unistellar Network observers during the second photometric confirmation
campaign (Epoch 19, described in Section 3.3). Note that observer Margaret
L.’s data were affected by fog near the expected mid-transit time.

##  4 Results

From the best-fit EXOFASTv2 solution using all of the available data outlined
in Section 3.3, we confirm that TIC 393818343.01, hereafter TIC 393818343 b,
is a giant planet with an eccentric orbit at $e$ = 0.6058 $\pm$ 0.0023. The
best-fit period $P$ = 16.24921 $\begin{subarray}{c}+0.00010\\\
-0.00011\end{subarray}$, in close agreement with the initial estimates, and
best-fit mass and radius of $M_{P}$ = 4.34 $\pm$ 0.15 $M_{J}$ and $R_{P}$ =
1.087$\begin{subarray}{c}+0.023\\\ -0.021\end{subarray}$ $M_{J}$,
respectively, place this planet in the warm Jupiter category of gas giants.
Posteriors of the EXOFASTv2 modeling are listed in Table 3.

There is a foreground star, Gaia DR3 4233021429372256512, at a distance of
84.5 pc compared to the target’s distance of 93.5 pc (Bailer-Jones et al.,
2018), with which the target is blended in ground-based images. We included
this star in the target aperture when performing differential photometry,
running the risk that the transit depth could be diluted when compared to TESS
observations, which are corrected for this dilution, due to the excess
baseline flux from the companion. However, we found that the modeled transit
depth in Clear, the filter representative of the eVscope, is in agreement with
the transit depth measured from the TESS light curve (see Table 3).

To test this agreement between the corrected TESS and the ground-based data,
we assume that the depth of the measured TESS transit is 1.27% of the host
star flux. Note that we used the lightkurve package (Lightkurve Collaboration
et al., 2018) as described in Peluso et al. (2024) to reproduce the TESS light
curve discovered by the VSG and deduce the transit depth, which uses the TESS
PDCSAP flux that has already been corrected for crowding. The crowding metric
from the TESS TargetPixelFile for the Sector 55 observation, or CROWDSAP, is
0.77034879, indicating that approximately 23% of flux in the TESS aperture is
from objects other than the host star. If we consider this additional flux in
our depth calculation, the resultant ”diluted” depth is 1.03%. This depth is
only 0.24% from the corrected transit depth, which is well within the noise
floor of the eVscope observations. We obtain a similar result when considering
the ”diluted” depth in relation to the difference in magnitude between the
host and foreground star (1.38 mag, or 28% of the host star magnitude in Gaia
$G_{rp}$). Therefore, we ignore the effects of dilution from Gaia DR3
4233021429372256512 in our modeling using ground-based data.

Additionally, the foreground star is identified as a variable in the Gaia DR3
catalog via the phot_variable_flag; however, it is unreasonable that the star
could cause the observed transit. Gaia DR3 4233021429372256512 is a 10.31
magnitude star in the Gaia $G_{RP}$ band, while TIC 393818343 $G_{RP}$ = 8.93
(the TESS bandpass is centered on the traditional Cousins-I band at 786.5 nm,
while $G_{RP}$ is centered at 797 nm). Although the Gaia data indicate a range
in $G_{RP}$ of 0.0163 for the foreground star, the duration of the variability
is listed as time_duration_rp = 928.9 days. As this value is over 5,000 times
the transit duration, it is unlikely that the variability of the foreground
star is responsible for the photometric transit signature that was observed
and predicted via the EXOFASTv2 analysis.

##  5 Discussion

###  5.1 Eccentricity and Tidal Circularization

With an eccentricity $e$ = 0.6058 $\pm$ 0.0023, TIC 393818343 b occupies a
sparsely populated orbital parameter space. This feature is evident in Figure
4, which compares $e$ and $a$ for a subset of confirmed planets with mass
measurements from the NASA Exoplanet Archive (NASA Exoplanet Archive,
2024).333Accessed on 2024-05-22 at 07:30 UTC. The most eccentric known
exoplanets include HD 80606 b ($e$ = 0.93, Naef et al., 2001), TOI-3362 b ($e$
= 0.81, Dong et al., 2021) and another warm Jupiter with $e$ = 0.93 (Gupta et
al. 2024, in prep.), all of which are expected to tidally circularize before
their host star evolves off the main sequence. To compare, we calculate via
EXOFASTv2 the tidal circularization timescale, $\tau_{\rm circ}$, to be
26.2$\begin{subarray}{c}+2.1\\\ -2.6\end{subarray}$ Gyr (see Equation 3 from
Adams & Laughlin (2006), assuming $Q$ = 106). This timescale is longer than
the main sequence lifetime of an early G-type star, which is expected to be
$\sim$10 Gyr; however, this lifetime and $\tau_{\rm circ}$ are on the same
order of magnitude. The full picture of evolution from warm to hot Jupiter is
not fully understood, and TIC 393818343 b may help complete a picture that
suggests tidal migration is more of a spectrum than has previously been
understood. Yet despite its sizeable eccentricity, TIC 393818343 b is not
eccentric or close enough to its host to migrate inward and become a hot
Jupiter under the stated assumptions in a single-planet system.

Tidal circularization might still be possible if tidal dissipation was
expedited, but TIC 393818343 b is not well-suited for this scenario. Wu (2018)
proposes this enhancement is possible through energy exchange between the
orbit and the planet’s degree-2 fundamental-mode (f-mode) during periastron
passage if the planet passes within four times the host’s tidal radius,
$r_{\tau}$ = $R_{P}$($M_{*}$/$M_{P}$)1/3. Using the methods from Gupta et al.
(2023), we calculate a periastron distance $a_{peri}$ $\approx$ 7.5$R_{*}$.
This value is 2.9 times greater than the maximum distance, 4$r_{\tau}$ =
2.6$R_{*}$, at which Wu (2018) states that f-mode dissipation would be
effective to boost tidal dissipation efficiency; therefore, TIC 393818343 b’s
orbit could not be circularized more quickly through this channel.

Considering that only three planets are expected to tidally circularize into
hot Jupiters, there is a disconnect between the observed warm and hot Jupiter
populations. However, Petrovich & Tremaine (2016) propose a mechanism that can
account for much of the hot Jupiter population via secular planet-planet
interactions; i.e., in which Kozai-Lidov oscillations occur due to the
presence of an outer planetary companion. In this case, warm Jupiters migrate
in intervals of high eccentricity but spend more time in low eccentricity
phases and are therefore observed largely at lower eccentricities. As an
exercise, we can consider a case in which TIC 393818343 b does circularize via
this or some other unknown pathway, despite the values of $\tau_{\rm circ}$
and $a_{peri}$. Again following the analysis of Gupta et al. (2023) and
assuming that angular momentum is conserved, the final semi-major axis if TIC
393818343 b were to circularize would be $a_{final}$ = 0.0817 au. Along with
the resultant orbital period in this scenario, $P_{final}$ = 8.17 days, this
final orbital configuration would land TIC 393818343 b in the hot Jupiter
population. Although it is not currently expected to become a hot Jupiter, TIC
393818343 b is an interesting case for further research due to its potential
to enter this prolific population if an outer companion is discovered.

Note that the presence of a perturbing outer planet is beyond the scope of
this work. Despite no clear evidence from our RV data, the existence of a
distant companion cannot be discounted and we leave this investigation to
future studies.

Figure 4: Confirmed planets with 10 ¡ $P$ ¡ 200 days and 0.1 $\leq$ $a$ $\leq$
0.5 au from the NASA Exoplanet Archive that have available mass measurements.
Giant planets with $M_{P}$ $\geq$ 0.25 $M_{J}$ are plotted such that colors
denote the mass of the planet, while planets below this mass cutoff are
plotted in gray. The yellow star indicates TIC 393818343 b at $4.34\pm
0.15$$\,M_{\rm J}$.

###  5.2 Opportunities for Atmospheric Characterization

Atmospheric characterization via emission and transmission spectroscopy have
become more feasible with the launch of the James Webb Space Telescope (JWST)
and other space observatories. To determine the feasibility of follow-up, we
can look at the Emission Spectroscopy Metric (ESM) developed by Kempton et al.
(2018) based upon the work of Zellem et al. (2017), which is proportional to
the S/N that JWST would be expected to achieve with a mid-infrared secondary
eclipse detection using MIRI/LRS. Using the properties of TIC 393818343 b
derived from this work, including the dayside temperature as 1.10 $T_{eff}$,
the ESM = 57, indicating that this planet is a reasonable candidate for
emission spectroscopy. Kempton et al. (2018) recommends a threshold of ESM =
7.5 for candidates, although only terrestrial targets are considered in their
analysis for emission spectroscopy.

However, according to the Transmission Spectroscopy Metric (TSM) (Zellem et
al., 2017; Kempton et al., 2018), TIC 393818343 b is not a suitable target for
this type of observation using JWST/NIRISS. For the target, TSM = 20, which is
lower than the suggested threshold of 96 for sub Jovians, as TIC 393818343 b
is not large or hot enough to be an ideal transmission spectroscopy target.

To compare, Mann et al. (2023) plot various ESM values according to planet
mass and temperature for confirmed giant planets ($R>0.5R_{J}$) to assess
follow-up for the cool giant TOI-2010 b. From their analysis, we can see that
TIC 393818343 b’s ESM falls among the median for all giant planets as well as
those of a similar temperature. Emission spectroscopy of TIC 393818343 b could
both expose the temperature structure of its atmosphere via a P-T profile as
well as atmospheric abundances, which would add valuable information to the
connection between warm and hot Jupiters (Madhusudhan, 2019; Fortney et al.,
2021). Chemical composition and abundances may indicate whether a gas giant
formed in situ or migrated inwards, a key question in the general formation of
warm Jupiters and, although emission spectra are available for many hot
Jupiters, there is a paucity of this data for their cooler cousins.

Considering the ESM and an extremely likely occultation of TIC 393818343 b
($b_{s}$ = 0.123 $\begin{subarray}{c}+0.11\\\ -0.086\end{subarray}$, see
Equation 7 of Winn et al., 2010), we believe it is a suitable candidate for
emission spectroscopy follow-up. Secondary eclipse timing, $T_{S}$, is
available in Table 3. Note that, due to its small semi-major axis $a$, TIC
393818343 b is not a reasonable target for direct imaging observations.

##  6 Summary

Here we present a confirmation of the TESS-discovered exoplanet TIC 393818343
b. First identified as a candidate from a single TESS transit in Sector 55, we
gathered follow-up RV and photometric observations using both the APF at Lick
Observatory and a network of citizen scientists. Our analyses show that TIC
393818343 b is a warm Jupiter, with $P$ = 16.25 days and $a$ = 0.1291 au, on a
highly eccentric orbit, $e$ = 0.6058, around an early G-type star. Its orbital
parameters indicate that, if the orbit were to circularize, TIC 393818343 b
would join the hot Jupiter population; however, the timescale for which this
would take place via the tidal migration pathway is longer than the main
sequence lifetime of the host star.

We note that this planet is a candidate for atmospheric characterization via
emission spectroscopy, although it is not a feasible target for transmission
spectroscopy or direct imaging. Future studies of TIC 393818343 b’s atmosphere
with space telescopes such as JWST or the Hubble Space Telescope, could add to
the ongoing investigation as the scientific community continues to
characterize the warm Jupiter population.

The crucial data from ground-based observations to confirm the planetary
nature of TIC 393818343 b demonstrate the impact of the burgeoning field of
citizen science. These data constrained the orbital parameters modeled with
EXOFASTv2 and confirmed the period derived from the RV analysis. Due to the
global nature of the Unistellar Network and Exoplanet Watch, weather effects
were mitigated and multiple observations were possible during each campaign,
furthering the significance of our detections. This discovery adds to the
multiple publications over various fields resulting from such citizen science
observations; e.g. Zellem et al. (2020); Pearson et al. (2022); Perrocheau et
al. (2022); Graykowski et al. (2023); Sgro et al. (2023); Peluso et al.
(2024). These networks of amateur and professional astronomers will continue
to be vital in keeping ephemerides fresh for TIC 393818343 b and other
confirmed exoplanets, as well as in the confirmation of future candidates.

##  7 Acknowledgements

L.A.S. and T.M.E. were partially supported during this work by the NASA
Citizen Science Seed Funding Program via grant number 80NSSC22K1130 and the
NASA Exoplanets Research Program via grant 80NSSC24K0165. Those NASA grants
also support the UNITE (Unistellar Network Investigating TESS Exoplanets)
program, under the auspices of which the Unistellar data were collected.
P.A.D. acknowledges support by a 51 Pegasi b Postdoctoral Fellowship from the
Heising-Simons Foundation. D.D. acknowledges support from the NASA Exoplanet
Research Program grant 18-2XRP18_2-0136. This study also benefited from the
kind contributions of the Gordon and Betty Moore Foundation.

The scientific data highlighted in this paper were partially sourced from the
Unistellar Network, a collaborative effort between Unistellar and the SETI
Institute. We extend our gratitude to Frédéric Gastaldo for financially
supporting the foundational phase of the Unistellar Citizen Science project.
We express our heartfelt appreciation to the citizen astronomers who shared
their invaluable data for these observations. This publication also makes use
of data products from Exoplanet Watch, a citizen science project managed by
NASA’s Jet Propulsion Laboratory on behalf of NASA’s Universe of Learning.
This work is supported by NASA under award number NNX16AC65A to the Space
Telescope Science Institute, in partnership with Caltech/IPAC, Center for
Astrophysics—Harvard & Smithsonian, and NASA Jet Propulsion Laboratory. Part
of this research was carried out at the Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National Aeronautics and
Space Administration (80NM0018D0004).

We thank Ken and Gloria Levy, who supported the construction of the Levy
Spectrometer on the Automated Planet Finder. We thank the University of
California and Google for supporting Lick Observatory and the UCO staff for
their dedicated work scheduling and operating the telescopes of Lick
Observatory. Some of the data presented herein were obtained at the W. M. Keck
Observatory, which was made possible by the generous financial support of the
W. M. Keck Foundation and is operated as a scientific partnership among the
California Institute of Technology, the University of California, and NASA.
The authors wish to recognize and acknowledge the very significant cultural
role and reverence that the summit of Mauna Kea has always had within the
indigenous Hawaiian community. We are most fortunate to have the opportunity
to conduct observations from this mountain.

This paper includes data collected by the TESS mission. Funding for the TESS
mission is provided by the NASA’s Science Mission Directorate. This research
also made use of the NASA Exoplanet Archive, which is made available by the
NASA Exoplanet Science Institute at IPAC, operated by the California Institute
of Technology under contract with the National Aeronautics and Space
Administration. Additionally, this research made use of the Exoplanet Follow-
up Observation Program (ExoFOP; DOI: 10.26134/ExoFOP5) website, which is
operated by the California Institute of Technology, under contract with the
National Aeronautics and Space Administration under the Exoplanet Exploration
Program.

Data: All the TESS data used in this paper can be found in MAST:
doi:10.17909/t9-nmc8-f686. All NASA Exoplanet Archive data used in this study
can be found on the NASA Exoplanet Archive: 10.26133/NEA12.

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