arXiv subjects

James S. Jenkins

Publications and source records attributed to James S. Jenkins.

At least 37 records · Page 2Linked to original sources

arXiv2025

Evidence of Titanate Clouds in the Day-side Atmosphere of the Ultra-Hot Jupiter WASP-121b

The day-side atmospheres of the hottest ultra-hot Jupiters (UHJs) have long been subject to speculation about cloud formation, often without direct observational evidence. Here, we present a detailed analysis of the panchromatic day-side emission spectrum of WASP-121b$\unicode{x2014}$one of the hottest known UHJs$\unicode{x2014}$covering a broad wavelength range of $\sim$0.6-5.1$\unicode{x00B5}$m, based on archival JWST observations from NIRISS and NIRSpec/G396H. We report statistically significant detections of several key molecular species, including H$_2$O (13.4 $σ$), CO (14.7 $σ$), SiO (4.9 $σ$), TiO (5.4 $σ$), and VO (6.6 $σ$), establishing WASP-121b as one of the most thoroughly characterized exoplanetary atmospheres to date. Additionally, we present the robust detection of Titanate (CaTiO$_3$) clouds at 6.7$σ$$\unicode{x2014}$the first such detection in any exoplanet atmosphere. Our analysis further reveals strong evidence of TiO depletion, likely due to sequestration into refractory condensates such as Titanate clouds. The precisely constrained molecular abundances yield a super-solar C/O ratio of 0.963$\pm$0.024, a sub-solar Si/O ratio of 0.034$\pm$0.024, and a metallicity of 4.7$_{-1.38}^{+1.99}$ $\times$solar. These findings offer a unique window into the atmospheric chemistry of an extreme UHJ, positioning WASP-121b as a key benchmark for next-generation models of atmospheric evolution and dynamics.

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arXiv2025

Resolving star spots on WASP-85 A using high-resolution transit spectroscopy

Stellar surface inhomogeneities such as spots and faculae introduce Doppler variations that challenge exoplanet detection via the radial velocity method. While their impact on disc-integrated spectra is well established, detailed studies of the underlying local line profiles have so far been limited to the Sun. We present an observational campaign targeting the active star WASP-85 A during transits of its hot Jupiter companion. The transits span two stellar rotation periods, allowing us to probe the evolution of active regions. From ground-based photometry we identify seven active regions, six containing dark spots. Using simultaneous ESPRESSO transit spectroscopy, we spatially resolve these regions on the stellar surface by using the planet as a probe. We detect significant bisector shape changes, line broadening, and net redshifts during spot occultations, with velocity shifts of 108-333 m/s (mean uncertainty 50 m/s). The observed broadening is consistent with the Zeeman effect, implying magnetic field strengths (Stokes $I$) $B$ = 2.7-4.4 kG (mean uncertainty 0.6 kG), comparable to solar umbrae. Combined with our photometric spot model, this yields lower limits to the disc-integrated field $Bf = 16 \pm 3$ G and $61 \pm 9$ G for the two hemispheres probed -- at least three times higher than Sun-as-a-star values. We also measure centre-to-limb variations in FWHM, line depth, equivalent width, and convective blueshift, which broadly agree with solar observations and 3D MHD models. This work demonstrates a new way to characterise the surfaces of exoplanet host stars, paving the way for future analyses incorporating synthetic line profiles from 3D MHD simulations.

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arXiv2025

NGTS-11 c: a transiting Neptune-mass planet interior to the warm Saturn NGTS-11 b

We report the discovery of NGTS-11 c, a transiting warm Neptune ($P \approx 12.8$ d; $M_{p} = 1.2^{+0.3}_{-0.2} M_{\mathrm{Nep}}$; $R_{p} = 1.24 \pm 0.03 R_{\mathrm{Nep}}$), in an orbit interior to the previously reported transiting warm Saturn NGTS-11 b ($P \approx 35.5$ d). We also find evidence of a third outer companion orbiting the K-dwarf NGTS-11. We first detected transits of NGTS-11 c in TESS light curves and confirmed them with follow-up transits from NGTS and many other ground-based facilities. Radial-velocity monitoring with the HARPS and FEROS spectrographs revealed the mass of NGTS-11 c and provides evidence for a long-period companion ($P > 2300$ d; $M_{p} \sin i > 3.6 M_{\mathrm{Jup}}$). Taking into account the two additional bodies in our expanded datasets, we find that the mass of NGTS-11 b ($M_{p} = 0.63 \pm 0.09 M_{\mathrm{Sat}}$; $R_{p} = 0.97 \pm 0.02 R_{\mathrm{Sat}}$) is lower than previously reported ($M_{p} = 1.2 \pm 0.3 M_{\mathrm{Sat}}$). Given their near-circular and compact orbits, NGTS-11 c and b are unlikely to have reached their present locations via high-eccentricity migration. Instead, they probably either formed in situ or formed farther out and then underwent disk migration. A comparison of NGTS-11 with the eight other known systems hosting multiple well-characterized warm giants shows that it is most similar to Kepler-56. Finally, we find that the commonly used 10-day boundary between hot and warm Jupiters is empirically well supported.

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arXiv2025

A Metal-Rich Atmosphere with a Super-Solar C/O Ratio for the Extreme Ultra-Hot Jupiter WASP-178b

The population of ultra-hot Jupiters (UHJs) provide unique opportunities to probe the extreme formation and evolutionary pathways in exoplanets. Owing to their very high temperatures and inflated atmospheres, UHJs are among the most favorable targets for both transmission and emission spectroscopy, enabling detailed characterization of their atmospheric properties. Here, we present a reanalysis of the JWST NIRSpec/G395H transmission spectra of the extreme ultra-hot Jupiter (EUHJ) WASP-178b, aimed at precisely characterizing its atmospheric composition. Our approach combines data reduction using two independent pipelines, lightcurve modeling with robust detrending techniques, and rigorous atmospheric retrievals. We report statistically significant detections of CO (7.24 $σ$) and CO$_2$ (7.22 $σ$), along with marginal evidence for C$_2$H$_2$ (1.34 $σ$), but no clear evidence for H$_2$O, suggesting depletion. From these retrieved abundances, we constrain the C/O ratio to a precise super-solar value of 0.954$\pm$0.033, consistent with an emerging trend in other UHJs. We also infer a very high atmospheric metallicity for a Jupiter-sized gas giant$\unicode{x2014}$11.44$_{-6.94}^{+12.54}$ $\times$solar$\unicode{x2014}$indicating unique atmospheric evolutions. These findings provide a critical benchmark for an extreme exoplanet atmosphere, offering a testbed for developing next-generation atmospheric evolution models and enabling comparative population-level studies across the UHJ population.

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arXiv2025

NGTS-EB-8: A double-lined eclipsing M+M binary discovered by citizen scientists

We report the identification and characterization of a new binary system composed of two near-equal mass M-dwarfs. The binary NGTS-EB-8 was identified as a planet candidate in data from the Next Generation Transit Survey (NGTS) by citizen scientists participating in the Planet Hunters NGTS project. High-resolution spectroscopic observations reveal the system to be a double-lined binary. By modeling the photometric and radial velocity observations, we determine an orbital period of 4.2 days and the masses and radii of both stars to be $M_A=0.250^{+0.005}_{-0.004}$ M$_{\odot}$, $M_B=0.208^{+0.005}_{-0.004}$ M$_{\odot}$, $R_A=0.255^{+0.004}_{-0.005}$ R$_{\odot}$, $R_B=0.233^{+0.006}_{-0.005}$ R$_{\odot}$. We detect Balmer line emission from at least one of the stars but no significant flare activity. We note that both components lie in the fully convective regime of low-mass stars ($<0.35$ M$_{\odot}$), therefore can be a valuable test for stellar evolutionary models. We demonstrate that the photometric observations, speckle imaging and initial radial velocity measurements were unable to identify the true nature of this system and highlight that high-resolution spectroscopic observations are crucial in determining whether systems such as this are in fact binaries.

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arXiv2025

Heat Reveals What Clouds Conceal: Global Carbon & Longitudinally Asymmetric Chemistry on LTT 9779 b

LTT-9779 b is an ultra-hot Neptune (Rp ~ 4.7 Re, Mp ~ 29 Me) orbiting its Sun-like host star in just 19 hours, placing it deep within the "hot Neptune desert," where Neptunian planets are seldom found. We present new JWST NIRSpec G395H phase-curve observations that probe its atmospheric composition in unprecedented detail. At near-infrared wavelengths, which penetrate the high-altitude clouds inferred from previous NIRISS/SOSS spectra, thermal emission reveals a carbon-rich atmosphere with opacity dominated by carbon monoxide (CO) and carbon dioxide (CO2). Both species are detected at all orbital phases, with retrieved mixing ratios of 10^-1 for CO and 10^-4 for CO2, indicating a globally well-mixed reservoir of carbon-bearing gases. We also moderately detect water vapor (H2O) and tentatively detect sulfur dioxide (SO2), providing insight into its chemistry and possible photochemical production under intense stellar irradiation. From these detections we infer a carbon-to-oxygen ratio near unity (C/O ~ 1) and a metallicity exceeding 500X Solar, consistent with equilibrium chemistry predictions for high-temperature atmospheres. This enrichment raises the mean molecular weight, reducing atmospheric escape, and likely helps LTT-9779 b retain a substantial atmosphere despite extreme irradiation. Our findings show that LTT-9779 b survives where few planets can, maintaining a carbon-rich atmosphere in a region where hot Neptune-class worlds are expected to evaporate. This makes LTT-9779 b a valuable laboratory for studying atmospheric escape and chemical processes under extreme conditions, offering new insight into the survival of planets in the hot Neptune desert.

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arXiv2025

Destruction of "Peas in a Pod?" A Candidate Multi-planet System Around the Nearby, Bright Star, HD208487

We re-investigate the HD208487 system to test the reality of the proposed HD208487c world. We also search for additional companions using applied Bayesian statistics and 15+ years of new RV data from the HARPS and the PFS instruments that were taken post-discovery of HD208487b. The RV data was analyzed with GLS Periodograms, followed by Bayesian analysis using the EMPEROR code. We scrutinised various stellar activity indices to search for any corresponding peaks in the power spectra, correlations with the RV measurements, or significant signals from a Bayesian analysis methodology. Finally, photometric data was checked to test for any transits or possible activity manifestations that could lead to possible false RV signals or excess noise. Our analysis points towards a candidate second planet in the system, positioned near the period of a previously proposed and subsequently challenged signal. This signal, HD208487c, would relate to a cool Saturn with an orbital period of 923.06 +2.02 -2.76 d and a minimum mass of Mj sini = 0.32 +/- 0.01Mj. Our analysis also gives rise to a newly discovered candidate planet, HD208487d, which would be the result of a cool super-Neptune/sub-Saturn with a period of 1380.13 +19.20 -8.25 d and a minimum mass of Mj sini = 0.15 +/- 0.01Mj. Neither stellar activity indices nor photometric data show signals statistically matching these periods. We have uncovered a candidate three planet system that would consist of an inner gas giant, a central Saturn and an outer super-Neptune/sub-Saturn. A dynamical analysis suggests that gravitational scattering of an initially ordered, equally-spaced system in a long resonant chain of six Neptunes can explain the current proposed architecture of HD208487. More RVs may also shed light on the reality of a fourth Doppler signal uncovered in the data that sits close to the 2:1 period-ratio with signal of HD208487c.

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arXiv2025

The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends

Sub-Neptune exoplanets are the most abundant type of planet known today. As they do not have a Solar System counterpart, many open questions exist about their composition and formation. Previous spectroscopic studies rule out aerosol-free hydrogen-helium-dominated atmospheres for many characterized sub-Neptunes but are inconclusive about their exact atmospheric compositions. Here we characterize the hot (Teq=1311K) sub-Neptune HD 86226 c, which orbits its G-type host star. Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We use HST data taken with WFC3 and STIS from the Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared 1.1-1.7micrometer transmission spectroscopy and UV characterization of the host star. We report a featureless transmission spectrum that is consistent within 0.4 sigma with a constant transit depth of 418+-14ppm. The amplitude of this spectrum is only 0.01 scale heights for a H/He-dominated atmosphere, excluding a cloud-free solar-metallicity atmosphere on HD 86226 c with a confidence of 6.5 sigma. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to a metal enrichment [M/H] above 2.3 (3 sigma confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO3), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we perform an investigation of cloud formation in high-metallicity, high-temperature atmospheres. Our results highlight that HD 86226c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the JWST could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species otherwise atypical for sub-Neptunes.

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arXiv2025

A High Geometric Albedo for LTT9779b Points Towards a Metal-rich Atmosphere and Silicate Clouds

Aims: In this work, we aim to confirm the high albedo of the benchmark ultrahot Neptune LTT9779b using 20 secondary eclipse measurements of the planet observed with CHEOPS. In addition, we perform a search for variability in the reflected light intensity of the planet as a function of time. Methods: First, we used the TESS follow-up data of LTT9779b from three sectors (2, 29, and 69) to remodel the transit signature and estimate an updated set of transit and ephemeris parameters, which were directly used in the modeling of the secondary eclipse lightcurves. This involved a critical noise-treatment algorithm, including sophisticated techniques such as wavelet denoising and Gaussian Process (GP) regression, to constrain noise levels from various sources. In addition to using the officially released reduced aperture photometry data from CHEOPS DRP, we also reduced the raw data using an independent PSF photometry pipeline, known as PIPE, to verify the robustness of our analysis. The extracted secondary eclipse lightcurves were modeled using the PYCHEOPS package, where we have detrended the background noise correlated with the spacecraft roll angle, originating from the inhomogeneous and asymmetric shape of the CHEOPS point spread function, using an N-order glint function. Results: Our independent lightcurve analyses have resulted in consistent estimations of the eclipse depths, with values of 89.9$\pm$13.7 ppm for the DRP analysis and 85.2$\pm$13.1 ppm from PIPE, indicating a high degree of statistical agreement. Adopting the DRP value yields a highly constrained geometric albedo of 0.73$\pm$0.11. No significant eclipse depth variability is detected down to a level of $\sim$37 ppm. Conclusions: Our results confirm that LTT9779b exhibits a strikingly high optical albedo, which substantially reduces the internal energy budget of the planet compared to more opaque...

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arXiv2025

Lessons learned from the detection of wide companions by radial velocity and astrometry

The detection and constraint of the orbits of long-period giant planets is essential for enabling their further study through direct imaging. Recently, it has been highlighted that there are discrepancies between different orbital fitting solutions. We address these concerns by reanalyzing the data for HD 28185, GJ 229, HD 62364, HD 38529, 14 Her, eps Ind A, HD 211847, HD 111031, and GJ 680, offering explanations for these discrepancies. Based on the comparison between our direct modeling of the astrometric catalog data and the orvara code, we find the discrepancies are primarily data-related rather than methodology-related. Our re-analysis of HD 28185 highlights many of the data-related issues and particularly the importance of parallax modeling for year-long companions. The case of eps Ind A b is instructive to emphasize the value of an extended RV baseline for accurately determining orbits of long period companions. Our orbital solutions highlight other causes for discrepancies between solutions including the combination of absolute and relative astrometry, clear definitions of conventions, and efficient posterior sampling for the detection of wide-orbit giant planets.

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arXiv2025

Exploring the Neptunian Desert: Insights from a Homogeneous Planetary Sample

In this paper, we present a homogeneous analysis of close-in Neptune planets. To do this, we compile a sample of TESS-observed planets using a ranking criterion which takes into account the planet's period, radius, and the visual magnitude of its host star. We use archival and new HARPS data to ensure every target in this sample has precise radial velocities. This yields a total of 64 targets, 46 of which are confirmed planets and 18 of which show no significant radial velocity signal. We explore the mass-radius distribution, planetary density, stellar host metallicity, and stellar and planetary companions of our targets. We find 26$\%$ of our sample are in multi-planet systems, which are typically seen for planets located near the lower edge of the Neptunian desert. We define a 'gold' subset of our sample consisting of 33 confirmed planets with planetary radii between 2$R_{\oplus}$ and 10$R_{\oplus}$. With these targets, we calculate envelope mass fractions (EMF) using the GAS gianT modeL for Interiors (GASTLI). We find a clear split in EMF between planets with equilibrium temperatures below and above 1300~K, equivalent to an orbital period of $\sim$3.5~days. Below this period, EMFs are consistent with zero, while above they typically range from 20$\%$ to 40$\%$, scaling linearly with the planetary mass. The orbital period separating these two populations coincides with the transition between the Neptunian desert and the recently identified Neptunian ridge, further suggesting that different formation and/or evolution mechanisms are at play for Neptune planets across different close-in orbital regions.

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arXiv2025

Constraining the Scattered Light properties of LTT 9779 b Using HST/WFC3 UVIS

A planet's albedo is a fundamental property that sets its energy budget by dictating the fraction of incident radiation absorbed versus reflected back to space. Generally, optical eclipse observations have revealed the majority of hot, giant planets to have low albedos, indicating dayside atmospheres dominated by absorption instead of reflection. However, there are several exceptions to this rule, including the ultra-hot-Neptune LTT 9779b, which have been found to have high geometric albedos. We observed four eclipses of LTT 9779b with the G280 grism of the Hubble Space Telescope's WFC3 UVIS mode; targeting the scattering signatures of the cloud condensate species causing the planet's elevated reflectivity. However, we do not definitively detect the planet's eclipse in our observations, with injection-recovery tests yielding a 3-$σ$ upper limit of 113 ppm on the eclipse depth of LTT 9779b in the 0.2-0.8$μ$m waveband. We create reflectance spectrum grids for LTT 9779b's dayside using VIRGA/PICASO and compare to our UVIS limit, as well as previously published CHEOPS and TESS eclipse photometry. We find that silicate condensates are best able to explain LTT 9779b's highly-reflective dayside. Our forward model grids only enable weak constraints on vertical mixing efficiency, and suggest that, regardless of their particular composition, the clouds are likely composed of smaller and more reflective particles. Our work facilitates a deeper understanding of the reflectance properties of LTT 9779b as well as the UVIS spectroscopic mode itself, which will remain the community's primary access to UV wavelengths until next-generation telescopes like the Habitable Worlds Observatory.

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arXiv2025

A closer look at LTT 9779b: The ESPRESSO endeavour to pierce the atmospheric veil

The proliferation of exoplanet discoveries in exotic environments like the Neptune desert challenges our understanding of planetary atmospheres under intense irradiation. The unexpected discovery of LTT9779 b, an ultra-hot Neptune within this desert, offers a prime opportunity for atmospheric studies. We build on prior observations of LTT9779 b from TESS, Spitzer, and CHEOPS, incorporating new VLT/ESPRESSO data to probe its atmospheric dynamics. Preliminary analyses suggest a metal-rich atmosphere and a high day-side geometric albedo, possibly indicating silicate clouds. Minimal atmospheric escape is observed, contrasting existing models of planetary evolution under extreme irradiation. We obtained the transmission spectrum of LTT9779 b between 0.4 and 0.78 microns with ESPRESSO, addressing systematics across three transits. Our analysis focused on the sodium doublet and H-alpha, using cross-correlation with models containing Na, K, FeH, TiO, and VO. No significant atmospheric signal was detected, with metallicity limits set at [Fe/H] $\geq$ 2.25 ($\geq$ 180 times solar). The non-detection aligns with a high-metallicity, cloud-free model, implying a high mean molecular weight and reduced atmospheric scale height. We interpret this as evidence for a metal-rich atmosphere with suppressed spectral features, possibly due to high-altitude clouds or hazes. These findings are consistent with JWST observations, supporting the hypothesis of metal-rich atmospheres obscured by aerosols in extreme environments.

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arXiv2025

NGTS-EB-7, an eccentric, long-period, low-mass eclipsing binary

Despite being the most common types of stars in the Galaxy, the physical properties of late M dwarfs are often poorly constrained. A trend of radius inflation compared to evolutionary models has been observed for earlier type M dwarfs in eclipsing binaries, possibly caused by magnetic activity. It is currently unclear whether this trend also extends to later type M dwarfs below the convective boundary. This makes the discovery of lower-mass, fully convective, M dwarfs in eclipsing binaries valuable for testing evolutionary models especially in longer-period binaries where tidal interaction between the primary and secondary is negligible. With this context, we present the discovery of the NGTS-EB-7 AB system, an eclipsing binary containing a late M dwarf secondary and an evolved G-type primary star. The secondary star has a radius of $0.125 \pm 0.006 R_\odot$ , a mass of $0.096 \pm 0.004 M_\odot$ and follows a highly eccentric $(e=0.71436 \pm 0.00085)$ orbit every $193.35875 \pm 0.00034$ days. This makes NGTS-EB-7 AB the third longest-period eclipsing binary system with a secondary smaller than $200 M_J$ with the mass and radius constrained to better than $5 \%$. In addition, NGTS-EB-7 is situated near the centre of the proposed LOPS2 southern field of the upcoming PLATO mission, allowing for detection of the secondary eclipse and measurement of the companion`s temperature. With its long-period and well-constrained physical properties - NGTS-EB-7 B will make a valuable addition to the sample of M dwarfs in eclipsing binaries and help in determining accurate empirical mass/radius relations for later M dwarf stars.

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arXiv2024

NGTS-33b: A Young Super-Jupiter Hosted by a Fast Rotating Massive Hot Star

In the last few decades planet search surveys have been focusing on solar type stars, and only recently the high-mass regimes. This is mostly due to challenges arising from the lack of instrumental precision, and more importantly, the inherent active nature of fast rotating massive stars. Here we report NGTS-33b (TOI-6442b), a super-Jupiter planet with mass, radius and orbital period of 3.6 $\pm$ 0.3 M$_{\rm jup}$, 1.64 $\pm$ 0.07 R$_{\rm jup}$ and $2.827972 \pm 0.000001$ days, respectively. The host is a fast rotating ($0.6654 \pm 0.0006$ day) and hot (T$_{\rm eff}$ = 7437 $\pm$ 72 K) A9V type star, with a mass and radius of 1.60 $\pm$ 0.11 M$_{\odot}$ and 1.47 $\pm$ 0.06 R$_{\odot}$, respectively. Planet structure and Gyrochronology models shows that NGTS-33 is also very young with age limits of 10-50 Myr. In addition, membership analysis points towards the star being part of the Vela OB2 association, which has an age of $\sim$ 20-35 Myr, thus providing further evidences about the young nature of NGTS-33. Its low bulk density of 0.19$\pm$0.03 g cm$^{-3}$ is 13$\%$ smaller than expected when compared to transiting hot Jupiters with similar masses. Such cannot be solely explained by its age, where an up to 15$\%$ inflated atmosphere is expected from planet structure models. Finally, we found that its emission spectroscopy metric is similar to JWST community targets, making the planet an interesting target for atmospheric follow-up. Therefore, NGTS-33b's discovery will not only add to the scarce population of young, massive and hot Jupiters, but will also help place further strong constraints on current formation and evolution models for such planetary systems.

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arXiv2024

TOI-2490b- The most eccentric brown dwarf transiting in the brown dwarf desert

We report the discovery of the most eccentric transiting brown dwarf in the brown dwarf desert, TOI02490b. The brown dwarf desert is the lack of brown dwarfs around main sequence stars within $\sim3$~AU and is thought to be caused by differences in formation mechanisms between a star and planet. To date, only $\sim40$ transiting brown dwarfs have been confirmed. \systemt is a $73.6\pm2.4$ \mjupnospace, $1.00\pm0.02$ \rjup brown dwarf orbiting a $1.004_{-0.022}^{+0.031}$ \msunnospace, $1.105_{-0.012}^{+0.012}$ \rsun sun-like star on a 60.33~d orbit with an eccentricity of $0.77989\pm0.00049$. The discovery was detected within \tess sectors 5 (30 minute cadence) and 32 (2 minute and 20 second cadence). It was then confirmed with 31 radial velocity measurements with \feros by the WINE collaboration and photometric observations with the Next Generation Transit Survey. Stellar modelling of the host star estimates an age of $\sim8$~Gyr, which is supported by estimations from kinematics likely placing the object within the thin disc. However, this is not consistent with model brown dwarf isochrones for the system age suggesting an inflated radius. Only one other transiting brown dwarf with an eccentricity higher than 0.6 is currently known in the brown dwarf desert. Demographic studies of brown dwarfs have suggested such high eccentricity is indicative of stellar formation mechanisms.

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arXiv2024

Surviving in the Hot Neptune Desert: The Discovery of the Ultra-Hot Neptune TOI-3261b

The recent discoveries of Neptune-sized ultra-short period planets (USPs) challenge existing planet formation theories. It is unclear whether these residents of the Hot Neptune Desert have similar origins to smaller, rocky USPs, or if this discrete population is evidence of a different formation pathway altogether. We report the discovery of TOI-3261b, an ultra-hot Neptune with an orbital period $P$ = 0.88 days. The host star is a $V = 13.2$ magnitude, slightly super-solar metallicity ([Fe/H] $\simeq$ 0.15), inactive K1.5 main sequence star at $d = 300$ pc. Using data from the Transiting Exoplanet Survey Satellite and the Las Cumbres Observatory Global Telescope, we find that TOI-3261b has a radius of $3.82_{-0.35}^{+0.42}$ $R_{\oplus}$. Moreover, radial velocities from ESPRESSO and HARPS reveal a mass of $30.3_{-2.4}^{+2.2}$ $M_{\oplus}$, more than twice the median mass of Neptune-sized planets on longer orbits. We investigate multiple mechanisms of mass loss that can reproduce the current-day properties of TOI-3261b, simulating the evolution of the planet via tidal stripping and photoevaporation. Thermal evolution models suggest that TOI-3261b should retain an envelope potentially enriched with volatiles constituting $\sim$5% of its total mass. This is the second highest envelope mass fraction among ultra-hot Neptunes discovered to date, making TOI-3261b an ideal candidate for atmospheric follow-up observations.

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arXiv2024

TOI-2447 b / NGTS-29 b: a 69-day Saturn around a Solar analogue

Discovering transiting exoplanets with relatively long orbital periods ($>$10 days) is crucial to facilitate the study of cool exoplanet atmospheres ($T_{\rm eq} < 700 K$) and to understand exoplanet formation and inward migration further out than typical transiting exoplanets. In order to discover these longer period transiting exoplanets, long-term photometric and radial velocity campaigns are required. We report the discovery of TOI-2447 b ($=$ NGTS-29b), a Saturn-mass transiting exoplanet orbiting a bright (T=10.0) Solar-type star (T$_{\rm eff}$=5730 K). TOI-2447 b was identified as a transiting exoplanet candidate from a single transit event of 1.3% depth and 7.29 h duration in $TESS$ Sector 31 and a prior transit event from 2017 in NGTS data. Four further transit events were observed with NGTS photometry which revealed an orbital period of P=69.34 days. The transit events establish a radius for TOI-2447 b of $0.865 \pm 0.010\rm R_{\rm J}$, while radial velocity measurements give a mass of $0.386 \pm 0.025 \rm M_{\rm J}$. The equilibrium temperature of the planet is $414$ K, making it much cooler than the majority of $TESS$ planet discoveries. We also detect a transit signal in NGTS data not caused by TOI-2447 b, along with transit timing variations and evidence for a $\sim$150 day signal in radial velocity measurements. It is likely that the system hosts additional planets, but further photometry and radial velocity campaigns will be needed to determine their parameters with confidence. TOI-2447 b/NGTS-29b joins a small but growing population of cool giants that will provide crucial insights into giant planet composition and formation mechanisms.

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