arXiv subjects

Tianjun Gan

Publications and source records attributed to Tianjun Gan.

At least 19 recordsLinked to original sources

arXiv2026

Stellar Multiplicity of M Dwarfs with Short-period Giant Planets, and the Characterization of TOI-5628Ab

Binary stars are ubiquitous, yet it remains unclear how wide-orbit stellar companions influence the formation of hot Jupiters, particularly around M dwarfs. Here, we first report the discovery of TOI-5628Ab, a giant planet transiting a mid-type M dwarf ($M_\ast=0.36\pm0.02\ M_\odot$) every 4.34 days, accompanied by an associated white dwarf TOI-5628B ($M_{\rm WD}=0.59\pm0.16\ M_\odot$) at a projected distance of about 2,500 AU. Using TESS, ground-based photometry and SPIRou RVs, we constrain the planet radius to $0.74\pm0.04\ R_J$ and mass to $0.09\pm0.04\ M_J$, with a $3\sigma$ upper limit of $0.22\ M_J$. Building on this system, we further conduct a homogeneous systematic search for co-moving stellar companions with projected semi-major axis between 100 and 10,000 AU around all M dwarfs with confirmed giant planets with periods smaller than 10 days and radii larger than 0.7 $R_J$, as well as a group of field M stars with stellar properties similar to the planet sample, based on the stellar kinematics from Gaia DR3. We measure a stellar multiplicity of $34.2\pm9.5\%$ for M dwarfs hosting short-period giant planets, which is substantially higher than the fraction of $5.3\pm3.7\%$ for the field M stars by approximately a factor of 6. Our results suggest that wide-orbit stellar companions tend to promote the formation of short-period giant planets around M stars with masses $0.21 \leq M_\ast\leq 0.64\ M_\odot$, and high-eccentricity migration may play an important role in producing such systems.

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arXiv2026

Discovery of an Inflated Hot Neptune and Its Formation from Jovian Mass Loss

The production of Neptune-like planets with orbital periods of 3--6 days is challenging for conventional models of high-eccentricity migration. We present the discovery and characterization of TOI-2195~A~b, an inflated hot Neptune ($P = 4.16$ days, $m_p= 1.46M_{\rm Nep},\,R_p = 0.79R_{\rm J}$) orbiting an early K-type star with a wide binary companion at $\sim 600$~au. Detection of the Rossiter-McLaughlin effect at $\sim2.6\sigma$ confidence with Magellan/PFS reveals the planet is likely on a near-polar orbit with a sky-projected stellar obliquity $\lambda = {109^{+35}_{-53}} ^{\circ}$. We perform coupled dynamical and structural modeling that reproduces the observed characteristics of the system. We show that the planet may have originated as a cold, Jovian planet that was excited to high eccentricities via the stellar Eccentric Kozai-Lidov (EKL) mechanism, where it lost up to $\sim90\%$ of its mass via Roche lobe overflow during close periastron passages, enabling rapid tidal migration and radius inflation due to tidal heating. TOI-2195 A b provides a test for planetary migration theories, and our simulations suggest that puffy hot Neptunes originated as more massive Jovians that underwent mass loss during high-eccentricity migration.

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arXiv2026

Measuring Asteroid Rotation Periods Using the KMTNet Bulge Survey Data

Since 2015, the Korea Microlensing Telescope Network (KMTNet) has conducted high-cadence, near-continuous observations of the Galactic bulge for nearly nine months each year from three sites in Chile, South Africa, and Australia, and its wide field of view provides a unique opportunity to extract asteroid lightcurves from archival survey data. In this work, we performed photometric measurements of bright asteroids ($V< 20$~mag) identified within a one-square-degree field during the 2018 KMTNet bulge season. We derived reliable rotation periods for 96 asteroids, including 84 objects without previously published lightcurves. The reliable spin-rates of the asteroids in our sample are broadly consistent with those reported in the Asteroid Lightcurve Database. This archival mining approach can be readily extended to a much larger KMTNet footprint, and the existing $\sim12$~deg$^2$ high-cadence KMTNet dataset has the potential to yield reliable rotation periods (U $\geq 2+$) for more than 5,500 asteroids, substantially expanding the current database of asteroid rotational properties.

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arXiv2026

The Pan-Pacific Planet Search -- IX. A menagerie of companions orbiting evolved stars

We present resolutions as to the nature of six speculative candidate companions proposed in the final data release of the Pan-Pacific Planet Search, a 6-year radial-velocity survey of 164 southern evolved stars using the now-decommissioned UCLES spectrograph on the 3.9m Anglo-Australian Telescope. New radial-velocity observations, TESS asteroseismology, and Hipparcos-Gaia astrometry are incorporated to refine the companion and host-star parameters. We confirm that HD 126105b is a giant planet ($P=524.0\pm$2.9 d, $m$ sin $i=1.67^{+0.19}_{-0.17}M_{Jup}$), and that HD 205577B is a massive, eccentric brown dwarf ($P\sim$11.2 yr, $m=77^{+11}_{-9}M_{Jup}$, $e=0.68$). HD 115066B and HD 121156B are low-mass stellar companions, while HD 114899 and HD 159743 are shown to be unadorned by any detectable companions whatsoever. This demonstrates the utility of astrometric information to help overcome the temporal limitations of incomplete radial-velocity data sets and elucidate the true nature of suspected companion bodies.

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arXiv2026

Exoplanets in ancient stellar populations: occurrence constraints and hot-Jupiter candidates in the Galactic halo

The Galactic halo preserves a record of the Milky Way's earliest assembly and contains both in-situ stars and stars accreted from dwarf galaxies. Possible planets around these stars, therefore, probe formation in ancient, metal-poor environments, including systems of extragalactic origin. We present a search for short-period transiting planets around kinematically selected halo dwarfs using Gaia DR3 and TESS, focusing on planets with periods of $1 < P < 10$ days. We identify two hot-Jupiter (HJ) candidates, one in the in-situ and one in the accreted halo, although the latter is highly grazing and excluded from the occurrence analysis. The accreted candidate, if confirmed, would orbit the most metal-poor HJ host known ([Fe/H] $\approx -1$). Using injection--recovery tests and automated vetting, we constrain occurrence in the full halo, in-situ, and accreted samples. In the HJ regime ($8\,R_\oplus < R_{\rm p} < 22\,R_\oplus$, $1\,\text{day}\ < P < 10$ days), the non-grazing candidate implies an overall halo occurrence rate of $0.13^{+0.12}_{-0.07}\%$ if planetary, while the absence of confirmed detections gives a corresponding $1\sigma$ upper limit of $<0.14\%$. For the in-situ halo, we infer $0.17^{+0.17}_{-0.10}\%$ (or $<0.19\%$ assuming no detections), while for the accreted halo we derive an upper limit of $<0.56\%$. These rates lie well below the corresponding short-period giant-planet occurrence measured in the Galactic disc. A forward model assuming Kepler-like occurrence also predicts $10 \pm 3$ detections compared with at most one observed. We find no significant occurrence difference between the in-situ and accreted halo populations, strengthening the evidence that close-in giant planets are rare across the old, metal-poor halo.

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arXiv2026

RV and TTV Measurements of Two Transiting Long-Period Giants around TOI-4600

TOI-4600b and c, originally identified by the Transiting Exoplanet Survey Satellite (TESS) and reported by I. Mireles et al. (2023), are a rare pair of transiting long-period giant planets ($\rm P_b=82.7$ days, $\rm P_c=482.8$ days) orbiting an early K dwarf. In this work, we refine the orbital parameters of the TOI-4600 system by combining new TESS photometry, ground-based transit follow-up, and radial velocity (RV) observations from MAROON-X. We obtain improved constraints on planetary masses and eccentricities, and update other parameters, such as the stellar age. For TOI-4600b, we measure a mass of $M_p = 74.7^{+4.7}_{-4.4}\,M_{\oplus}$ and an eccentricity of $e=0.153^{+0.020}_{-0.018}$, and $M_p = 212.53^{+13.26}_{-13.03}\,M_{\oplus}$ and $e=0.219^{+0.015}_{-0.018}$ for TOI-4600c. We find significant transit timing variations (TTV) in both planets, with semi-amplitudes of approximately $1$\,hr. We derive Transit Spectroscopy Metric values of 16.87 for TOI-4600b and 10.09 for TOI-4600c, indicating that both planets are promising JWST targets for studying the atmospheres of temperate and cold Jupiters, a relatively poorly characterized sample thus far. These updated parameters and TTV ephemerides are important for planning and interpreting future photometric, spectroscopic, and dynamical studies of the TOI-4600 system.

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arXiv2026

An Aligned Very-Low-Mass Star Orbiting an M dwarf and Obliquity Patterns Across Giant Planets, Brown Dwarfs, and Binary Stars

Stellar obliquity serves as a key diagnostic for tracing the dynamical evolution of bound systems-from giant planets and brown dwarfs to stellar binaries-revealing whether these diverse populations share analogous histories. Here, we report the first obliquity measurement for a double M dwarf system, determined via the Rossiter-McLaughlin effect. The spin axis of the primary star, TOI-5375 ($M_\ast=0.62\pm0.02\,M_\odot$), is well aligned with the orbit of its low-mass stellar companion ($M_c=84.8\pm1.5\, M_J$, $\rm P=1.72\,days$) with a projected obliquity of $\lambda=-13.5_{-13.8}^{+12.4}\,^{\circ}$ and a true 3D obliquity of $\psi=37.5_{-13.4}^{+10.6}\,^{\circ}$. The result indicates that the system either formed with a primordially aligned configuration or has undergone tidal realignment. We further investigate obliquity patterns across giant planets, brown dwarfs and binary stars. It turns out that a few obliquity trends observed in giant planets also tentatively exhibit in the latter two higher-mass populations: 1) well-aligned orbits are preferentially found around cooler host stars ($T_{\rm eff}\leq 6250\,K$); 2) wide-orbit ($a/R_\ast\geq 10$) companions are predominantly aligned; 3) no significant correlation shows up between obliquity and orbital eccentricity in any of the companion classes. By modeling $|\lambda|$ with a two-component Gaussian distribution, we find that the low-$|\lambda|$ components of binary stars and brown dwarfs are more concentrated near zero than giant planets while the high-$|\lambda|$ components of brown dwarfs and binaries remain unclear due to the small sample size.

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arXiv2026

A Warm Massive Pair of Planets around TOI-1232 Revealed with Transit-timing Variations and Doppler Spectroscopy

TOI-1232 is a G-dwarf star with a mass of $1.06_{-0.06}^{+0.07} M_\odot$, a radius of $1.07\pm 0.05 R_\odot$, and slightly higher metallicity than solar of Fe/H = $0.18 \pm 0.05$. The star hosts a transiting warm Jovian-mass planet, TOI-1232 b, with an orbital period of $P_{b} = 14.256_{-0.001}^{+0.001}$ days, identified with data from multiple sectors of the $\textit{TESS}$ space telescope. The $\textit{TESS}$ light curve of TOI-1232 is complex, as it is contaminated by a background eclipsing binary with a period of $1.37$ days. The TOI-1232 b was firmly confirmed by ground-based transit follow-up campaigns from Las Cumbres, Hazelwood, Brierfield, and ASTEP observatories.Additionally, the $\textit{TESS}$ transits of TOI-1232 b exhibit strong transit timing variations (TTVs) with a super-period of $235.5 \pm 0.7$ days and a semi-amplitude of 27 minutes. Radial velocity (RV) follow-up with the FEROS spectrograph confirms the planetary nature of the transiting candidate, while a self-consistent $N$-body analysis of RVs and TTVs pinpoints the presence of a second outer Saturn-mass companion, TOI-1232 c with a period of $P_{c} = 30.356_{-0.012}^{+0.010}$ days. The TOI-1232 warm-giant system is particularly important due to the evidence of two massive planets that reside near the 2:1 commensurability but are not locked in a mean motion resonance (MMR). Thanks to $\textit{TESS}$, we have revealed a handful of these rare systems. Hence, TOI-1232 is an important addition to understanding the formation and dynamical evolution of such compact, massive, warm giant planets.

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arXiv2026

Confirmation of the hot super-Neptune TOI-672 b with NIRPS and HARPS and Insights into the Neptunian desert around M dwarfs

The Neptunian desert is a distinct lack of Neptune-sized planets at short orbital periods, purportedly carved by photoevaporation and tidal circularization following high-eccentricity migration. Constraining these processes and how they vary across different host-star spectral types requires the detailed characterization of planets in the desert and around its boundaries. In this study, we confirm the planetary nature of a massive super-Neptune identified by TESS around the M0 dwarf TOI-672. We analyse photometry from TESS and ExTrA and precise radial velocity measurements taken with the recently commissioned Near-InfraRed Planet Searcher (NIRPS) and HARPS spectrographs. We measure the planetary orbital period, radius, and mass of 3.634 days, 5.31 +0.24 -0.26 Rearth, and 50.9 +4.5 -4.4 Mearth, respectively. Our findings place TOI-672 b within the Neptunian ridge, a pile-up of planets from 3--5 days at the Neptunian desert boundary. We then use a novel approach to determine the desert boundaries in period-radius space and instellation-radius space, and, for the first time, compare the Neptunian desert boundaries for planets orbiting FGK versus M dwarf stars. We determine that the boundary ridge shifts slightly inward from 3.3 +- 1.4 days for FGK host stars to 2.2 +- 1.0 days for M dwarf host stars; these values do not statistically significantly differ from each other, and the shift to shorter periods for M dwarf planets is smaller than theoretical photoevaporation models predict. We also find that TOI-672 b is a single-planet system within the sensitivity limits of our RV and TTV datasets.

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arXiv2026

TESS planets in known radial velocity cold Jupiter systems: Hot super Earth occurrence is enhanced by cold Jupiters

The correlation between inner super-Earths (SEs) and outer cold Jupiters (CJs) provides an important constraint on the formation and dynamical evolution of planetary systems. Previous studies have suggested a positive connection between these two populations, particularly around metal-rich stars, and proposed that nearly all CJ-hosting stars may also harbor inner SEs. In this work, we use TESS transits to investigate the occurrence of hot super-Earths (HSE; $1$-$4R_\oplus$, $P<10\mathrm{d}$) in systems with known CJs detected by radial velocity. Out of a statistical sample of 132 CJ systems, we identify five transiting HSEs around four stars, including one new candidate (TOI-6965.01) around HD 50554. To enable statistical analysis, we first validate the two candidates around HD 50554 using TESS photometry, archival RV measurements, and Gaia astrometry. After accounting for detection sensitivity and geometric transit probability, we find that the presence of CJs enhances the occurrence rate of HSEs by a factor of $6.5^{+3.1}_{-2.3}$ relative to field stars, with the case of no enhancement being ruled out at 99.9% confidence level. Taking into account the average multiplicity of HSEs, we find that about 91% of CJ systems host at least one HSE. Our results provide strong supporting evidence for a positive HSE-CJ correlation. We also briefly explore the correlation around metal-poor hosts and for specific sub-populations (e.g., warm super Earths or cold super Jupiters).

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arXiv2026

Densities of small planets around the M dwarfs TOI-4336 A and TOI-4342 with ESPRESSO: Three sub-Neptunes, one super-Earth, and a Neptune-mass candidate

We present the characterization of two planetary systems orbiting the M dwarfs TOI-4336 A (M3.5V) and TOI-4342 (M0V), each hosting two transiting planets previously validated with TESS and ground-based observations. We refined the photometry of the TOI-4342 system using TESS and LCOGT data, and characterized the host stars with NIRPS and ESPRESSO spectroscopy. High-precision ESPRESSO radial velocities allowed us to constrain the planetary masses and investigate their potential compositions. The TOI-4336 A system is composed of a sub-Neptune with a period of 16.34 days, a radius of $2.14 \pm 0.08$ Re, and a mass of $3.33 \pm 0.36$ Me, along with an inner super-Earth on a 7.59-day orbit with a radius of $1.25 \pm 0.07$ Re and a mass of $1.55 \pm 0.13$ Me. The TOI-4342 system hosts two sub-Neptunes of similar sizes ($2.33 \pm 0.09$ Re and $2.35 \pm 0.09$ Re), with periods of 5.54 and 10.69 days. Their masses are measured to be $7.3 \pm 1.3$ Me and $4.8 \pm 1.4$ Me, respectively. The RVs also reveal a planet candidate around TOI-4342, likely non-transiting, with a period of 47.5 days and a minimum mass of $17.8 \pm 3.0$ Me. With precise radii and masses, we derived bulk densities and explored possible compositions. The TOI-4336 A sub-Neptune and super-Earth have densities of $1.87 \pm 0.30$ and $4.35 \pm 0.79$ g cm$^{-3}$, while the two similar-sized sub-Neptunes in TOI-4342 show distinct densities of $3.18 \pm 0.67$ and $2.01 \pm 0.63$ g cm$^{-3}$. All four planets are excellent targets for future atmospheric characterization with JWST, and their multi-planet nature makes them especially interesting for comparative planetology. Notably, TOI-4336 A b stands out as one of the best-known targets in its size and temperature regime, with a TSM of 138, comparable to benchmark planets such as K2-18 b and LHS 1140 b.

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arXiv2025

Revisiting the atmosphere of HAT-P-70b with CARMENES high-resolution transmission spectroscopy

Owing to hot and inflated envelopes that facilitate atmospheric studies, ultra-hot Jupiters (UHJs) have attracted much attention. Significant progress has been achieved, from enlarging the sample size to broadening the studies to encompass diverse stellar types and ages. Here, we present a transmission spectroscopy study of HAT-P-70b, an UHJ orbiting a young A-type star, through high-resolution observations with CARMENES at the 3.5m Calar Alto telescope. By using the line-by-line technique, we confirm the previous detections of Ha, Na I, and Ca II, report a new tentative detection of K I, and impose an upper limit on the He triplet absorption. Through cross-correlation analysis, we identify the Ca II and Fe I absorptions, both blue-shifted by approximately 5 km/s, indicating a day-to-night side atmospheric wind. Additionally, we find a new tentative detection of K I. We do not see any significant atmospheric molecular signal in the near-infrared data. Putting HAT-P-70b in the context of UHJs from the literature, it turns out that (1) Ha absorption is more common on gas giants orbiting stars younger than 1 Gyr, with a relative detection probability of $P_{\rm Age<1\,Gyr}({\rm Ha})/P_{\rm Age\geq1\,Gyr}({\rm Ha})\sim 3$; (2) any UHJ is likely to exhibit Fe I absorption if it has Ca II.

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arXiv2025

TOI-3288 b and TOI-4666 b: two gas giants transiting low-mass stars characterised by NIRPS

Gas giant planets orbiting low-mass stars are uncommon outcomes of planet formation. Increasing the sample of well-characterised giants around early M dwarfs will enable population-level studies of their properties, offering valuable insights into their formation and evolutionary histories. We aim to characterise giant exoplanets transiting M dwarfs identified by TESS. High-resolution spectroscopic data are obtained in the optical and nIR, combining HARPS and NIRPS. We derive RVs via the cross-correlation function and implement a novel post-processing procedure to further mitigate telluric contamination in the nIR. The resulting RVs are jointly fit with TESS and ground-based photometry to derive the orbital and physical parameters of the systems. We confirm two gas giants transiting the low-mass stars TOI-3288 A (K9V) and TOI-4666 (M2.5V). TOI-3288 A hosts a Hot Jupiter with a mass of $2.11\pm0.08~M_{\rm Jup}$ and a radius of $1.00 \pm 0.03~R_{\rm Jup}$, with an orbital period of 1.43 days ($T_{\rm eq} = 1059 \pm 20~{\rm K}$). TOI-4666 hosts a $0.70_{-0.06}^{+0.05}~M_{\rm Jup}$ warm Jupiter ($T_{\rm eq} = 713 \pm 14~{\rm K}$) with a radius of $1.11 \pm 0.04~R_{\rm Jup}$, and an orbital period of 2.91 days. We identify a decrease in planetary mass with spectral type, where late M dwarfs host less massive giant planets than early M dwarfs. More massive gas giants that deviate from this trend are preferentially hosted by more metal-rich stars. Furthermore, we find an increased binarity fraction among low-mass stars hosting gas giants, which may play a role in enhancing giant planet formation around low-mass stars. The observed population trends agree with theoretical expectations, where higher metallicity can compensate for lower disk masses, and wide binary systems may influence planet formation and migration through Kozai-Lidov cycles or disk instabilities.

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arXiv2025

Dynamical Instability of Multi-planet Systems and Free-floating Planets

The ejection of planets by the instability of planetary systems is a potential source of free-floating planets. We numerically simulate multi-planet systems to study the evolution process, the properties of surviving systems, and the statistics of ejected planets. For systems with only super-Earth planets, we find that the time (in units of the orbital period $P_{1}$ of the innermost planet) for the system to lose the first planet by collision or ejection increases with the semimajor axis of the innermost planet. In contrast, the time (in units of $P_{1}$) for the first close encounter between two planets is identical. These two timescales also depend differently on the orbital spacing between the planets. Most systems with only super-Earths do not have planets ejected. In systems with super-Earths and a cold Jupiter, we discover that a cold Jupiter significantly increases the probability of ejection of the super-Earths by close encounters. Of 38\% of ejected super-Earths, most velocities relative to their parent stars are smaller than $6\ \mathrm{km\ s^{-1}}$. We conservatively estimate that more than 86\% of the surviving two-planet systems in the super-Earths plus cold Jupiter sample are long-term stable by using empirical criteria. Most super-Earths in the remaining two-planet systems are on highly elliptical but stable orbits and have migrated inwards compared with their initial states.

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arXiv2025

A New Brown Dwarf Orbiting an M star and An Investigation on the Eccentricity Distribution of Transiting Long-Period Brown Dwarfs

The orbital eccentricities of brown dwarfs encode valuable information of their formation and evolution history, providing insights into whether they resemble giant planets or stellar binaries. Here, we report the discovery of TOI-5575b, a long-period, massive brown dwarf orbiting a low-mass M5V star ($\rm 0.21\pm0.02\,M_\odot$) delivered by the TESS mission. The companion has a mass and radius of $\rm 72.4\pm4.1\,M_J$ and $\rm 0.84\pm0.07\,R_J$ on a 32-day moderately eccentric orbit ($e=0.187\pm0.002$), making it the third highest-mass-ratio transiting brown dwarf system known to date. Building on this discovery, we investigate the eccentricity distributions of a sample of transiting long-period ($10\leq P\lesssim 1000$ days, $\sim$0.1-1.5 AU) giant planets, brown dwarfs and low-mass stars. We find that brown dwarfs exhibit an eccentricity behavior nearly identical to that of giant planets: a preference for circular orbits with a long tail toward high eccentricities. Such a trend contrasts sharply with direct imaging findings, where cold (5-100 AU) brown dwarfs and giant planets display distinct eccentricity distributions. Our results suggest that transiting long-period brown dwarfs and giant planets probably 1) form in different routes at exterior orbits but undergo analogous dynamical evolution processes and migrate inwards; or 2) both contain two sub-groups, one with widely spread eccentricities while the other has circular orbits, that jointly sculpt the eccentricity distributions. The low-mass-star systems appear to be a distinctive population, showing a peak eccentricity at about 0.3, akin to more massive stellar binaries.

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arXiv2025

The TESS Grand Unified Hot Jupiter Survey. III. Thirty More Giant Planets

We present the discovery of 30 transiting giant planets that were initially detected using data from NASA's Transiting Exoplanet Survey Satellite (TESS) mission. These new planets orbit relatively bright ($G \leq 12.5$) FGK host stars with orbital periods between 1.6 and 8.2 days, and have radii between 0.9 and 1.7 Jupiter radii. We performed follow-up ground-based photometry, high angular-resolution imaging, high-resolution spectroscopy and radial velocity monitoring for each of these objects to confirm that they are planets and determine their masses and other system parameters. The planets' masses span more than an order of magnitude ($0.17\,M_J < M_p < 3.3\,M_J$). For two planets, TOI-3593 b and TOI-4961 b, we measured significant non-zero eccentricities of $0.11^{+0.05}_{-0.03}$ and $0.18^{+0.04}_{-0.05}$ respectively, while for the other planets, the data typically provide a 1-$\sigma$ upper bound of 0.15 on the eccentricity. These discoveries represent a major step toward assembling a complete, magnitude-limited sample of transiting hot Jupiters around FGK stars.

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arXiv2025

A transiting giant planet in orbit around a 0.2-solar-mass host star

Planet formation models suggest that the formation of giant planets is significantly harder around low-mass stars, due to the scaling of protoplanetary disc masses with stellar mass. The discovery of giant planets orbiting such low-mass stars thus imposes strong constraints on giant planet formation processes. Here, we report the discovery of a transiting giant planet orbiting a $0.207 \pm 0.011 M_{\odot}$ star. The planet, TOI-6894 b, has a mass and radius of $M_P = 0.168 \pm 0.022 M_J (53.4 \pm 7.1 M_{\oplus})$ and $R_P = 0.855 \pm 0.022 R_J$, and likely includes $12 \pm 2 M_{\oplus}$ of metals. The discovery of TOI-6894 b highlights the need for a better understanding of giant planet formation mechanisms and the protoplanetary disc environments in which they occur. The extremely deep transits (17% depth) make TOI-6894 b one of the most accessible exoplanetary giants for atmospheric characterisation observations, which will be key for fully interpreting the formation history of this remarkable system and for the study of atmospheric methane chemistry.

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arXiv2025

The Eccentricity Distribution of Warm Sub-Saturns in TESS

We present the eccentricity distribution of warm sub-Saturns (4-8 Re, 8-200 day periods) as derived from an analysis of transit light curves from NASA's Transiting Exoplanet Survey Satellite (TESS) mission. We use the "photoeccentric" effect to constrain the eccentricities of 76 planets, comprising 60 and 16 from single- and multi-transiting systems, respectively. We employ Hierarchical Bayesian Modelling to infer the eccentricity distribution of the population, testing both a Beta and Mixture Beta distribution. We identify a few highly eccentric (e ~ 0.7-0.8) warm sub-Saturns with eccentricities that appear too high to be explained by disk migration or planet-planet scattering alone, suggesting high-eccentricity migration may play a role in their formation. The majority of the population have a mean eccentricity of e = 0.103+0.047-0.045, consistent with both planet-disk and planet-planet interactions. Notably, we find that the highly eccentric sub-Saturns occur in single-transiting systems. This study presents the first evidence at the population level that the eccentricities of sub-Saturns may be sculpted by dynamical processes.

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