TOI-1842b: A Transiting Warm Saturn Undergoing Reinflation around an Evolving Subgiant

The tireless efforts of exoplanetary astronomers have, for more than three decades, relentlessly banished the darkness shrouding the mysteries of planetary system formation and evolution in the Solar neighborhood. The stars that our ancestors once thought of as distant campfires are now known to host retinues of worlds in a riotous array of sizes, compositions, and orbital properties. Of late, we have fueled this revolution with deceptively small space-borne robotic sentries; the Kepler (Borucki et al. 2010) and now Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015) spacecraft have delivered 2981 new confirmed exoplanets from over 6000 candidates. ³⁰

The Level 1 mission requirement of TESS is to obtain mass and radius measurements for 50 planets smaller than 4 R_⊕(Ricker et al. 2015), and at the end of its two-year prime mission in 2020 July, nearly 40 such Level 1 planets have been confirmed (e.g., Huang et al. 2018; Luque et al. 2019; Quinn et al. 2019; Vanderburg et al. 2019; Trifonov et al. 2021). As is common in astronomy, where the most attention is lavished on discoveries at the margins of detectability, transits of larger planets are often overshadowed by their smaller kin. However, such warm giants from the TESS mission are also scientifically valuable (e.g., Nielsen et al. 2019; Dalba et al. 2020; Jordán et al. 2020; Schlecker et al. 2020). These giant planets, with orbital periods between about 5 and 10 days, reside far enough from their host star that tidal circularization has not yet erased any orbital eccentricity or inclination imposed by postformation migration mechanisms (Dawson & Johnson 2018). Such planets are thus important laboratories for understanding the means by which close-in giant planets are delivered to their present locations.

In a similar vein, planets orbiting stars of different types than the Sun remain underrepresented. This arose first from the technical requirements of the radial-velocity method, which favored Solar-type stars (late F through K dwarfs) due to the Doppler information contained within their abundant and narrow spectral lines (e.g., Mayor & Queloz 1995; Cochran et al. 1997; Butler et al. 1999; Pepe et al. 2004; Vogt et al. 2010). As the transit technique flowered in the age of Kepler, the bias toward Solar-type stars was reinforced to give the mission the best chance of achieving its goal of detecting Earth-size planets. Again, the hotter and more evolved stars were spurned for their younger and cooler counterparts. Evolved stars in particular are heavily disfavored in transit searches since their larger stellar radii diminish the transit signals of planets. The advent of purpose-built infrared high-precision spectrographs (Mahadevan et al. 2012; Quirrenbach et al. 2014; Kotani et al. 2018; Gilbert et al. 2018) has fueled an outsized interest in the coolest stars, stoked heavily by the profligacy with which the universe produces systems of multiple small planets around these M dwarfs (e.g., Anglada-Escudé et al. 2012; Wright et al. 2016; Dreizler et al. 2020; Cloutier et al. 2019), and because habitable-zone planets are more accessible.

However, the hotter, higher-mass stars (A type through mid-F dwarfs) have yet to receive much attention from the exoplanet community. While on the main sequence, these stars offer a limited number of useful spectral lines for radial-velocity measurement. Some progress toward understanding the exoplanet population hosted by these stars has been achieved by studying their evolved counterparts, since, as subgiants and low-luminosity giants, the stars cool down and slow their rotation, resulting in a forest of narrow spectral lines that permit precise velocity measurements (e.g., Johnson et al. 2007; Bowler et al. 2010; Jones et al. 2011; Sato et al. 2013; Wittenmyer et al. 2020). A conundrum has been the apparent lack of giant planets in orbits shorter than ∼100 days (Johnson et al. 2010; Reffert et al. 2015; Van Eylen et al. 2016; Jones et al. 2021). Contrary to popular belief, the hot- and warm-Jupiter populations have not been engulfed by gastronomically rapacious host stars—the low-luminosity giants targeted by the major surveys have typical radii of less than 10 R_⊙, so close-in giant planets would remain intact until the last ∼1% of their lifetimes (Villaver & Livio 2009; Kunitomo et al. 2011). Transit surveys have been successful in confirming the presence of close-in giant planets around main-sequence A and F stars (e.g., Snellen et al. 2009; Talens et al. 2017; Rodriguez et al. 2019), and preliminary estimates of the occurrence rates suggest that there is no significant dependence on host-star mass (Zhou et al. 2019).

In this paper, we report the discovery and confirmation of TOI-1842b, a rare short-period giant planet orbiting an evolved star. Section 2 details the various photometric and spectroscopic observations, and in Section 3, we describe the properties of the host star. Section 4 gives the results of the joint fitting, and we conclude in Section 5.

In this section, we describe the TESS photometry, follow-up ground-based photometry, and spectroscopic observations used to establish the planetary nature of TOI-1842b.

2.1. Photometric Observations

2.2. Spectroscopic Observations

3.1. Spectroscopic Stellar Parameter Determination

We derived stellar parameters for TOI-1842 using spectra from Minerva-Australis as fully detailed in Addison et al. (2021). The spectroscopic parameters T_eff, $\mathrm{log}g$, and overall metallicity [M/H] were derived using iSpec (Blanco-Cuaresma et al. 2014; Blanco-Cuaresma 2019). Our derived T_eff, $\mathrm{log}g$, and [M/H] values were then used as input for the Bayesian isochrone modeler isochrones (Morton 2015; Montet et al. 2015) that uses the Dartmouth Stellar Evolution Database (Dotter et al. 2008). The isochrones modeling also included Gaia G, G_BP, G_RP magnitudes; 2MASS J, H, and K_S magnitudes along with Gaia EDR3 parallax values and their respective errors.

The extracted FIES spectrum was compared to a library of synthetic templates to measure effective temperature T_eff, surface gravity $\mathrm{log}g$, projected rotational velocity $v\sin i$, and metallicity [M/H] (a solar mix of metals, rather than Fe only). Our analysis follows Buchhave et al. (2012) and Buchhave et al. (2014), using five spectral orders spanning a wavelength range from 5065 to 5320 Å.

The spectroscopic and derived physical parameters for TOI-1842 are given in Table 2. In summary, we find that TOI-1842 is a slightly evolved subgiant star which is somewhat more massive and metal-rich than the Sun. While the stellar mass from the TIC is 2σ different from that derived with the Minerva-Australis spectrum, we note that the TIC mass does not use a spectroscopic log g, which can lead to discrepancies in derived masses (e.g., Clark et al. 2021). The isochrones-derived stellar mass and radius are adopted for the joint modeling as presented in the next section.

Table 2. Stellar Parameters for TOI-1842

Parameter	Value	Reference
R.A. (h:m:s)	13:27:51.04	1
decl. (d:m:s)	+09:01:50.32	1
Distance (pc)	223.47 ± 2.24	1
V (mag)	9.81 ± 0.03	2
T (mag)	9.2673 ± 0.0061	3
Teff (K)	6115 ± 119	3
	6239 ± 50	4
	6230 ± 50	5
$\mathrm{log}g$ (cm s−2)	3.876 ± 0.080	3
	4.25 ± 0.10	4
	4.10 ± 0.10	5
R⋆ (R⊙)	2.05 ± 0.09	3
	2.02 ± 0.05	5
	2.013 ± 0.045	6
L⋆ (L⊙)	5.29 ± 0.22	3
	5.50 ± 0.31	5
M⋆ (M⊙)	1.15 ± 0.15	3
	1.46 ± 0.03	5
	1.78 ± 0.42	6
Metallicity, [M/H]	0.39 ± 0.08	4
	0.22 ± 0.05	5
Age (Gyr)	2.49 ± 0.22	5
ρ⋆ (g cm−3)	0.189 ± 0.041	3
	0.237 ± 0.021	6
	0.25 ± 0.02	5
$v\sin i$ (km s−1)	4.23 ± 0.20	5
	7.6 ± 0.5	4

Notes. Adopted values are shown in bold.

References. 1. Gaia Collaboration (2018); 2. Høg et al. (2000); 3. Stassun et al. (2019); 4. FIES spectrum (this work); 5. Minerva-Australis spectrum (this work); 6. SED fit (Section 3.2).

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3.2. Spectral Energy Distribution

In order to provide an initial empirical constraint on the stellar radius and mass, we performed an analysis of the broadband spectral energy distribution (SED) together with the Gaia DR2 parallax following the procedures described in Stassun & Torres (2016) and Stassun et al. (2017, 2018). We used the NUV flux from GALEX, the B_T V_T magnitudes from Tycho-2, the JHK_S magnitudes from 2MASS, the W1–W4 magnitudes from WISE, and the GG_BP G_RP magnitudes from Gaia. Together, the available photometry spans the full stellar SED over the wavelength range 0.2–22 μm (see Figure 1).

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We performed a fit using Kurucz stellar atmosphere models, adopting the effective temperature (T_eff), surface gravity ($\mathrm{log}g$), and metallicity ([Fe/H]) from the spectroscopically determined values. The extinction (A_V ) was allowed to vary but limited to the maximum line-of-sight extinction from the dust maps of Schlegel et al. (1998). The resulting fit is very good (Figure 1) with a reduced χ² of 1.4. Integrating the (unreddened) model SED gives the bolometric flux at Earth of F_bol = 3.278 ± 0.076 × 10⁻⁹ erg s⁻¹ cm⁻². Taking the F_bol and T_eff together with the Gaia DR2 parallax, adjusted by + 0.08 mas to account for the systematic offset reported by Stassun & Torres (2018), gives the stellar radius as R_⋆ = 2.013 ± 0.045 R_⊙.

In addition, we can compute an empirical estimate for the stellar mass via the empirical stellar radius above together with the spectroscopic $\mathrm{log}g$, which gives M_⋆ = 1.8 ± 0.4 M_⊙, consistent with the empirical eclipsing-binary-based relations of Torres et al. (2010), which give M = 1.37 ± 0.08 M_⊙. The mass and radius together give a mean stellar density of ρ_⋆ = 0.24 ± 0.02 g cm⁻³. These estimates are also consistent with the mass and radius derived from the Minerva-Australis spectra.

Finally, from the spectroscopic $v\sin i$ and the stellar radius we obtain an upper limit on the stellar rotation period, ${P}_{\mathrm{rot}}/\sin i=13.4\pm 0.9$ days. Using the empirical gyrochronology relations of Mamajek & Hillenbrand (2008), this then provides upper limits on the stellar age of ≈1.9 Gyr (1σ) and ≈1.5 Gyr (3σ). The lack of lithium absorption in the FIES spectrum also places a lower age limit of ∼800 Myr.

3.3. High-resolution Imaging

We observed TOI-1842 with optical speckle interferometric imaging as part of our standard process for validating transiting exoplanets to assess the possible contamination of bound or unbound companions on the derived planetary radii (Ciardi et al. 2015). Speckle imaging observations of TOI-1842 were performed on 2021 March 1 UT using the Zorro speckle instrument on Gemini-South. ³² Zorro simultaneously provides speckle imaging in two bands, 562 and 832 nm, with output data products including a reconstructed image, and robust limits on companion detections (Howell et al. 2011). Figure 2 shows our results for TOI-1842 from both bandpasses and the reconstructed 832 nm speckle image. We find that TOI-1842 is indeed a single star with no detected companion brighter than Δ_mag ∼ 4–7.5 magnitudes (M0V) detected within 12. The angular resolution limits of the speckle observations, at the distance of TOI-1842 (d = 223.5 pc), correspond to spatial limits of 4.5 to 268 au.

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We used the Exo-Striker exoplanet toolbox ³³ (Trifonov 2019) to perform a signal analysis and a joint fit of the transit light curve and radial-velocity data. For the photometric transit period search, Exo-Striker uses the transitleastsquares package (TLS; Hippke & Heller 2019), and for the RV-period search analysis, it uses the generalized Lomb–Scargle periodogram (GLS; Zechmeister & Kürster 2009). Exo-Striker allows for wide variety of joint-modeling schemes of multitelescope transit and RV data consistent with planetary signals. The modeling can be performed either by "best-fit" optimization schemes (i.e., Levenberg-Marquardt, Nelder-Mead, Newton, etc.), or sampling schemes such as an affine-invariant ensemble Markov chain Monte Carlo sampler (Goodman & Weare 2010) via the emcee package (Foreman-Mackey et al. 2013), and the nested sampling technique (Skilling 2004) via dynesty sampler (Speagle 2020), which allow detailed posterior probability analysis.

4.1. Signal Search

Figure 3 shows the TLS signal detection efficiency power spectra of the detrended relative flux of the TESS and the LCOGT data of TOI-1842. We detect a significant transit event on the combined light-curve data with a period of 9.5742 ± 0.0024 days, t₀ = 2458933.32187, a mean transit duration of 4.272 hr, and a mean transit depth of 0.9967 in relative flux units. Figure 3 also shows that TLS detects other significant periodicities in the data, which, however, are attributed to the 1/2, 1/3, 1/4, etc, low-frequency harmonics of the actual transit period. Our independent transit-search analyses are in agreement with the available SPOC DV estimates based on the TESS Sector 23 data of TOI-1842.

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Figure 4 shows the period search analysis of the combined Minerva-Australis and NRES RV data set. The top panel of Figure 4 shows the window function of the available Doppler data of TOI-1842. The observational scheduling results in strong window function power at periods of 1, 20.54, 29.98 days, and lower frequencies. The middle panel of Figure 4 shows the GLS power spectra of the Doppler data of TOI-1842. Horizontal dashed lines indicate GLS false-positive thresholds of 10%, 1%, and 0.1%. We detect a significant periodic signal with a period of 9.34 ± 0.15 days, which is near the period of the transit signal. The remaining significant signals are aliases of the window function and the Doppler-induced planetary signal. The bottom panel of Figure 4 shows the GLS power spectrum of the RV residuals of the joint fit (see Section 4.2). We did not detect any significant residual RV periodicity suggesting the presence of any additional planet.

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4.2. Combined Transit and RV-orbital Analysis

We use the TLS and GLS results from Section 4.1 as a starting point for our combined transit and RV-orbital modeling. From these estimates and the following intermediate transit and RV parameter optimization analysis with the Exo-Striker, we define informative and noninformative parameter priors needed for our global parameter search. We performed multidimensional parameter fitting, constructing the posterior probability distribution by employing a nested sampling scheme using the dynesty sampler. We set 100 "live points" per fitted parameter, sampled via random walk, dynamic nested sampling with a 100% weight on the posterior convergence (see Speagle 2020 for details). Our adopted dynamic nested sampling scheme allows us to construct a posterior probability distribution, from which we adopted the 68.3 % confidence levels as 1σ parameter uncertainties. In addition, the nested sampling analysis is very convenient for an adequate model comparison via the difference of their Bayesian log-evidence ${\rm{\Delta }}\mathrm{ln}Z$. ³⁴ Our adopted parameter priors, the resulting median values and their 1σ uncertainties drawn from the posterior probability distribution, and the sample with the maximum $-\mathrm{ln}{ \mathcal L }$ (i.e., best-fit) for our final orbital analysis are summarized in Table 3. Figure 5 shows the best-fit transit model component applied to Sector 21 and the LCOGT data, whereas Figure 6 shows the RV component of the joint-fit model applied to the combined RVs. Our combined transit and RV-orbital analyses are performed as follows.

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Table 3. Nested Sampling Posteriors and Maximum $-\mathrm{ln}{ \mathcal L }$ of the Orbital and Nuisance Parameters of the TOI-1842 System

Parameter	Median and 1σ	Max. $-\mathrm{ln}{ \mathcal L }$	Adopted Priors
K [m s−1]	${15.9}_{-2.80}^{+2.9}$	16.8	${ \mathcal U }$(5.0,30.00)
P [day]	${9.5739}_{-0.0001}^{+0.0002}$	9.5739	${ \mathcal U }$(9.565,9.580)
t0 [deg]	${2459402.4575}_{-0.0024}^{+0.0026}$	2459402.4572	${ \mathcal U }$(2459402.2,2459402.6)
i [deg]	${88.4}_{-1.5}^{+0.9}$	88.3	${ \mathcal U }$(80.0,90.0)
a/R⋆	${16.2}_{-2.9}^{+1.6}$	16.1	${ \mathcal U }$(5.0,15.00)
R/R⋆	${0.052}_{-0.002}^{+0.003}$	0.052	${ \mathcal U }$(0.01,0.10)
a [au]	${0.1001}_{-0.0007}^{+0.0007}$	0.0925	(derived)
mp [Mjup]	${0.214}_{-0.038}^{+0.040}$	0.225	(derived)
Rp [Rjup]	${1.04}_{-0.05}^{+0.06}$	1.05	(derived)
RV off.MINERVA TEL 1 [m s−1]	${3603.0}_{-11.6}^{+11.2}$	3595.3	${ \mathcal U }$(3200,3800)
RV off.MINERVA TEL 4 [m s−1]	${3652.5}_{-11.4}^{+10.8}$	3646.0	${ \mathcal U }$(3200,3800)
RV off.MINERVA TEL 5 [m s−1]	${3665.9}_{-10.5}^{+9.9}$	3659.3	${ \mathcal U }$(3200,3800)
RV off.${}_{\mathrm{NRES}}$ [m s−1]	${3337.5}_{-28.3}^{+34.9}$	3332.9	${ \mathcal U }$(3200,3800)
RV jitterMINERVA TEL 1 [m s−1]	${15.2}_{-4.6}^{+3.9}$	15.7	${ \mathcal J }$(1.00,25.00)
RV jitterMINERVA TEL 4 [m s−1]	${18.7}_{-5.8}^{+3.4}$	17.5	${ \mathcal J }$(1.00,25.00)
RV jitterMINERVA TEL 5 [m s−1]	${9.5}_{-3.5}^{+3.3}$	8.8	${ \mathcal J }$(1.00,25.00)
RV jitter${}_{\mathrm{NRES}}$ [m s−1]	${43.2}_{-16.6}^{+22.1}$	45.4	${ \mathcal J }$(1.00,100.00)
RV lin. trend [m s−1/day]	${0.316}_{-0.121}^{+0.254}$	0.395	${ \mathcal J }$(-5.00,5.00)
Transit offsetTESS−S21 [ppm]	${46}_{-8}^{+8}$	45	${ \mathcal N }$(0.0,1000.0)
Transit offsetLCO−SAAO [ppm]	${11}_{-108}^{+110}$	−5	${ \mathcal N }$(0.0,1000.0)
Transit offsetLCO−CTIO 1 [ppm]	${158}_{-127}^{+140}$	118	${ \mathcal N }$(0.0,1000.0)
Transit offsetLCO−CTIO 2 [ppm]	−1834${}_{-122}^{+132}$	−1811	${ \mathcal N }$(0.0,1000.0)
Transit jitterTESS−S21 [ppm]	${40}_{-23}^{+50}$	22	${ \mathcal J }$(0.0,1000.0)
Transit jitterLCO−SAAO [ppm]	${1143}_{-82}^{+94}$	1127	${ \mathcal J }$(0.0,1000.0)
Transit jitterLCO−CTIO 1 [ppm]	${1275}_{-85}^{+123}$	1277	${ \mathcal J }$(0.0,1000.0)
Transit jitterLCO−CTIO 2 [ppm]	${1638}_{-85}^{+120}$	1635	${ \mathcal J }$(0.0,1000.0)
Quad. limb-dark.TESS u1	${0.307}_{-0.175}^{+0.235}$	0.151	${ \mathcal U }$(0.00,1.00)
Quad. limb-dark.TESS u2	${0.586}_{-0.315}^{+0.255}$	0.718	${ \mathcal U }$(0.00,1.00)
Quad. limb-dark.LCO u1	${0.327}_{-0.190}^{+0.226}$	0.292	${ \mathcal U }$(0.00,1.00)
Quad. limb-dark.LCO u2	${0.481}_{-0.291}^{+0.293}$	0.508	${ \mathcal U }$(0.00,1.00)

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The transit modeling within the Exo-Striker is done by a wrapper of the BAsic Transit Model cAlculatioN package (batman; Kreidberg 2015). This package constructs the transit light-curve model by accepting planetary orbital parameters such as period P, eccentricity e, the argument of periastron, ω, and inclination i, which are common to the Keplerian RV model of the Exo-Striker. Additional transit parameters of batman are the time of inferior transit conjunction t₀, and the planet semimajor axis and radius in units relative to the stellar radius a/R_⋆ and R/R_⋆, respectively. The RV model component fits the RV signal semiamplitude K, which constrains the planetary mass.

In addition to the orbital parameters, our modeling scheme includes a number of nuisance parameters, which we optimize simultaneously. For each transit and RV data set we fit their respective relative mean offset and additional data "jitter" terms, the latter of which was simultaneously added in quadrature to the data error budget while evaluating the models' (negative) log-likelihood $-\mathrm{ln}{ \mathcal L }$ (Baluev 2009). In particular, we model the light curve of TESS obtained in Sector 21 and the LCOGT data of SAAO, CTIO 1, and CTIO 2, which adds eight additional parameters. We adopt separate quadratic limb-darkening transit models of TESS and LCOGT, for which we optimize separate quadratic limb-darkening coefficients, u1_TESS and u2_TESS, and u1_LCOGT and u2_LCOGT, respectively. For the RV data, we fit the precise RVs of Minerva obtained with telescopes 1, 4, and 5 and the NRES-CPT, which also adds eight nuisance parameters. We do not include the NRES-Wise RVs in our joint fit since we found that their scatter is too large, and overall, does not improve the outcome of our analysis. In addition, we find a marginally significant positive Doppler drift in the combined RV data, which is possibly induced by another companion orbiting at a greater distance. No companions are visible (Figure 2), and there is only one object within 1 arcmin, a faint background star at T = 17.595, separated by 28''. Since the nature of this signal cannot be determined based on the available data, we model it by applying an additional RV linear trend parameter.

We performed two sets of joint-fit nested sampling analysis: for a forced circular orbit of TOI-1842b (e = 0, for which ω is undefined), and with free e and ω. We found that the simpler circular model is moderately favored with a log evidence of $\mathrm{ln}Z=82096.06$, whereas the full-Keplerian model has a slightly poorer log-evidence of $\mathrm{ln}Z=82093.65$. From the full-Keplerian model, we find that the orbital eccentricity is consistent with 0, but with large uncertainties, from which we estimate an upper eccentricity limit of e < 0.25 at the 1σ level. Thus, as a final orbital solution of TOI-1842b, we adopted the simpler, circular orbit joint-fit model, which is presented in Table 3, as well as Figures 5 and 6.

The values of three most representative parameters from our best fit are $P={9.5739}_{-0.0001}^{+0.0002}$ days, ${t}_{0}={2459402.4575}_{-0.0024}^{+0.0026}$ days, and $K={15.9}_{-2.80}^{+2.9}$ m s⁻¹. Given the parameter posteriors, and our estimates of the stellar mass, radius, and their uncertainties, we derive a dynamical planetary mass of ${m}_{\mathrm{pl}.}={0.214}_{-0.038}^{+0.040}$ M_Jup, semimajor axis of $a={0.1001}_{-0.0007}^{+0.0007}$ au, and a planetary radius of ${R}_{\mathrm{pl}.}={1.04}_{-0.05}^{+0.06}{R}_{\mathrm{Jup}}$, which makes it one of the lowest-density planets discovered to date (ρ = 0.252 ± 0.091 g cm⁻³).

We further inspected the TESS residuals with TLS and the RV residuals with GLS to check for evidence of additional planets, but none was found.

We have confirmed the planetary nature of the warm Saturn TOI-1842b (P = 9.5737 ± 0.0015 days), and we have measured its mass to be ${0.214}_{-0.038}^{+0.040}$ M_J, with a radius of ${1.04}_{-0.05}^{+0.06}$ R_J. This planet, moving on a circular orbit with a = 0.10 au about its subgiant host star, joins a small cohort of 56 confirmed planets orbiting within 0.5 au of evolved stars. With a period of nearly 10 days, this planet's orbital properties are similar to TOI-481b (Brahm et al. 2020), another warm giant (T_eq = 1210 ± 29 K) orbiting a slightly evolved metal-rich star. While short-period giant planets orbiting stars more massive than the Sun remain rare, such planets appear to be preferentially found around supersolar metallicity stars, as evidenced by the positive planet-metallicity correlation found for evolved stars by several studies (e.g., Reffert et al. 2015; Jones et al. 2016; Wittenmyer et al. 2020; Wolthoff 2021). Petigura et al. (2018) also found that warm Saturns in particular are intrinsically less common than close-in Neptunes or Jupiters, and tend to be hosted by metal-rich stars. The properties of TOI-1842b are thus consistent with this class of rare low-mass gas giants.

TOI-1842b is remarkable due to its low density (ρ = 0.252 ± 0.091 g cm⁻³), which ranks it among the least-dense 9.3% of known planets. Figure 7 places this planet in context of the cohort of planets with measured bulk densities. From the derived equilibrium temperature (1210 K) and surface gravity of the planet and assuming a H/He atmosphere with a mean molecular mass of μ = 2.3 amu, the calculated atmospheric scale height (from H_b = kT_eq/(μ g_b)) is H_b = 893 km. ³⁵ The large scale height of TOI-1842b (h/R_p = 1.2% ) is reminiscent of KELT-11b (Figure 8). While KELT-11b has a larger scale height (Pepper et al. 2017), its primary transit time of ∼8 hr would require multiple full night observations to fully observe. Alternatively, TOI-1842b has a transit duration of ∼4 hr, making it an ideal candidate for single night observation follow up. The transmission spectroscopy metric (TSM; Kempton et al. 2018), used to assess the suitability of transmission spectroscopy observations with the James Webb Space Telescope (JWST), for TOI-1842b is 141. Planets with TSM values greater than ∼96 (for Jovians and sub-Jovians), such as for TOI-1842b, are considered suitable for these observations with JWST.

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The measured mass and radius of TOI-1842b place it within the category of "inflated sub-Saturns." From the relationship given in Weiss et al. (2013), TOI-1842b has a predicted radius of 9 R_⊕; its measured radius of 11.65 R_⊕ is thus ∼30% larger than a noninflated planet with its mass and incident flux. Inflated sub-Saturn planets occupy a relatively unexplored parameter space that is seen as a key transitional population group between inflated super-Earths and Jovian worlds (Lee & Chiang 2016; Colón et al. 2020). Conversely, the low density of some planets in longer period orbits, like TOI-1842b, might be indicative of their formation pathways. Lee & Chiang (2016) proposed that low density sub-Saturns formed in situ. The chemical composition of these planets should reflect the composition of the gas disk from which they were formed (Öberg et al. 2011). Furthermore, TOI-1842b is a rare close-in planet orbiting an evolved star. By observing and identifying the molecular species and physical dynamics present within the atmosphere of TOI-1842b, we can obtain a better understanding on how these inflated gas giants interact with their host star over time and better determine the formation process for this population.

The radius and mass of TOI-1842b, at its orbital distance of 0.1 au, are consistent with a coreless planet (see Table 4 in Fortney et al. 2007). We note that the host star is metal-rich with nearly twice Solar [M/H], making the presence of a coreless giant planet most intriguing. One would expect metal-rich protoplanetary disks to form planets with larger solid cores due to a higher surface density (Pollack et al. 1996). For example, HD 149026 with a similar metal content hosts a planet with a ∼67 M_⊕ core (Sato et al. 2005). TOI-1842b thus presents a fascinating conundrum; a relatively rare class of planet for which detailed atmospheric studies will be eminently feasible and illuminating.

TOI-1842b joins a growing list of low density close-in gas giants orbiting stars evolving off the main sequence (e.g., Wang et al. 2019; Bryant et al. 2020; Sha et al. 2021), and provides a suitable test to models that explain the higher-than-expected radii of these gas giants via reinflation mechanisms (e.g., Lopez & Fortney 2016; Komacek et al. 2020; Thorngren et al. 2021). Adopting the stellar parameters from Table 2 and evolution tracks from the MIST models (Dotter 2016), the current irradiation received by TOI-1842b is 30% more than it originally received at zero-age main sequence (Figure 9). A number of similarly inflated warm Jovian planets have recently been found around post-main-sequence stars receiving many times their main-sequence incident irradiation (e.g., Grunblatt et al. 2016; Hartman et al. 2016; Grunblatt et al. 2017; Pepper et al. 2017).

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Figure 10 places TOI-1842b in context with other well-characterized sub-Jovian planets with masses and radii measured to precisions of better than 20% and 10%, respectively. The radius of TOI-1842b is near the upper envelope of similar planets in this mass regime. TOI-1842 is beginning to leave the main sequence and, as shown in Figure 9, will irradiate and further inflate TOI-1842b over the next tens of Myr before eventually consuming the planet (Villaver et al. 2014). As the star evolves off the main sequence, the strengthening incident irradiation and star-planet tidal interactions may induce increased heating of the planet's core, potentially reinflating the low density gas giants. While TOI-1842b is not at present immensely inflated by comparison to planets of similar mass, it stands out in its suitability for atmospheric characterization. In addition to the brightness of the host star (see Figure 8), TOI-1842b has a high ratio of atmospheric scale height to planetary radius. Figure 11 shows the ratio of atmospheric scale height to planet radius (H/R_p ) as a function of mass for the cohort of well-characterized sub-Jovian planets. TOI-1842b lies in the best 10% of these planets in terms of H/R_p . The two highest points in Figure 11 with H/R_p > 0.02 are KELT-11b (Pepper et al. 2017) and WASP-127b (Lam et al. 2017). Those two extremely inflated planets both orbit subgiant stars, and represent possible futures for TOI-1842b as its host star inexorably ascends the subgiant branch.

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We respectfully acknowledge the traditional custodians of all lands throughout Australia, and recognize their continued cultural and spiritual connection to the land, waterways, cosmos, and community. We pay our deepest respects to all Elders, ancestors, and descendants of the Giabal, Jarowair, and Kambuwal nations, upon whose lands the Minerva-Australis facility at Mt Kent is situated.

Minerva-Australis is supported by Australian Research Council LIEF Grant LE160100001, Discovery Grant DP180100972, Mount Cuba Astronomical Foundation, and institutional partners University of Southern Queensland, UNSW Sydney, MIT, Nanjing University, George Mason University, University of Louisville, University of California Riverside, University of Florida, and The University of Texas at Austin.

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 North (and/or 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).

This work makes use of observations from the LCOGT network.

B.A. is supported by Australian Research Council Discovery Grant DP180100972.

Funding for the TESS mission is provided by NASA's Science Mission directorate. We acknowledge the use of public TESS Alert 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. 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. This paper includes data collected by the TESS mission, which are publicly available from the Mikulski Archive for Space Telescopes (MAST). This paper is partially based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias.

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.

Facilities: TESS - , Minerva-Australis - , NOT/FIES - , LCOGT/NRES - , Exoplanet Archive. -

Software: AstroImageJ (Collins et al. 2017), isochrones (Morton 2015), ispec (Blanco-Cuaresma et al. 2014; Blanco-Cuaresma 2019), Tapir (Jensen 2013), BANZAI (McCully et al. 2018), Wotan (Hippke et al. 2019), SPOC pipeline (Jenkins et al. 2016), batman (Kreidberg 2015), Exo-Striker (Trifonov 2019).