arXiv subjects

Thomas G. Wilson

Publications and source records attributed to Thomas G. Wilson.

At least 19 recordsLinked to original sources

arXiv2026

Connecting Granulation and Magnetic Activity in Radial Velocities: The Next Breakthrough for High-Precision Spectroscopy

Stellar variability has become the dominant limitation to achieving the radial velocity (RV) precision required for the detection and characterization of Earth-like exoplanets. While significant progress has been made in mitigating the effects of oscillations and magnetic activity, convective granulation and its interaction with stellar magnetic fields remain among the least understood sources of RV variability. To address these challenges, we organized the splinter session `Connecting Granulation and Magnetic Activity in Radial Velocities: The Next Breakthrough for High-Precision Spectroscopy' at Cool Stars 23. The session brought together researchers working on observations, numerical simulations, and data-driven techniques to discuss the current understanding of granulation-driven RV signals and identify the most promising directions for future progress. Through invited and contributed talks, followed by community discussions, participants emphasized the importance of combining physically motivated models with data-driven approaches, developing standardized benchmark datasets, and validating simulations against high-quality observations across a range of stellar types. The discussions also highlighted the need for coordinated observing strategies, improved characterization of individual spectral lines, and physically informed line-by-line analyses to disentangle convective and magnetic signals. This contribution summarises the scientific discussions and community perspectives that emerged during the session and outlines the key challenges that must be addressed to reach the sub-40 cm/s precision required for the next generation of RV planet searches.

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arXiv2026

Evidence for a giant companion orbiting Tabby's star

KIC 8462852, also known as Tabby's star, shows deep, irregular flux variations up to 20 per cent in its Kepler light curve. These unusual events show no apparent periodicity. Multi-wavelength observations show the dips are consistent with transiting dust, although its origin remains uncertain. The leading explanation is that dust may originate from a family of exocomets or planetesimal fragments. TESS data reveal a unique, symmetric transit event consistent with a planet or brown dwarf. This would support the theory that incoming exocometary bodies may result from orbital perturbations by a planet. The analysis of the TESS and, archival and newly obtained radial velocity (RV) data, suggests that a companion could orbit around Tabby's star with a period of at least ~1030 d. When combined with the constraints imposed by Kepler and other photometric surveys, we find only five orbital period windows are possible. We conduct observations to follow-up the first two windows, significantly reducing the allowed ranges. Fitting the TESS transit using exocometary models produces a poor fit or the need of physically unrealistic parameters, favouring the giant companion hypothesis. A 2.3 $\sigma$ detection of $M=9.4^{+4.9}_{-4.1}$ M$_{\mathrm{Jup}}$ (3$\sigma$ upper limit < 28.0 M$_{\mathrm{Jup}}$) at the transit orbital period provides tentative evidence of the companion, but more RV observations are required. Along with the radius $R=1.7\pm0.1$ R$_{\mathrm{Jup}}$, the companion would lie in the giant planet-brown dwarf regime. In the future Gaia DR4 astrometry will reveal the nature of this object.

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arXiv2026

CHEOPS observations of V1298 Tau: updated planetary densities and implications on the early evolution of the young system

The young (10-30 Myr) multi-planet system V1298 Tau presents a unique opportunity to probe the early formation and evolution of young systems. We present new CHEOPS observations of the three innermost planets, yielding high-precision planetary radii ($\sim$5-11 $R_\oplus$ ) and improving the radius ratios (Rp/Rs) by 30-71% compared to previous multiple TESS observations. Combined with refined period and mass determinations from transit-timing variation (TTV) measurements, we derive revised bulk densities (0.06-0.23 g/cm$^3$) for these planets. We find that the innermost planet c is denser compared to the outermost planet at the 3.4-$\sigma$ level, while the bulk densities of the three outermost planets are consistent within the reported uncertainties. These bulk densities suggest differing envelope mass fraction across the system, indicating differential atmospheric evolution in the young system. We further assess the early dynamical state of the V1298 Tau system and find that within the range of simulations performed we find no evidence for present-day mean-motion resonance trapping. As an independent diagnostic, we compute the forced eccentricities and low Normalized Angular Momentum Deficit (NAMD) exhibited by the system. Our simulations suggest that no past dynamical excitation is required to explain the present orbital architecture.

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arXiv2026

NGTS-39 b: A 58 d transiting warm Jupiter in an eccentric orbit

We report the discovery and characterisation of NGTS-39 b (TIC 453147896 b), a warm Jupiter transiting a Sun-like star on a 58.2 day, eccentric (e = 0.386 +/- 0.019) orbit. NGTS-39 b was first identified from a TESS single-transit event, and subsequently confirmed with NGTS photometry and radial-velocity measurements from CORALIE and HARPS. The host star is a bright (Tmag = 11.02) F9 dwarf with an effective temperature of Teff = 6053 +67/-30 K. NGTS-39 b is a Jupiter-sized gas giant with a radius of 1.088 +/- 0.012 RJ and a mass of 1.467 +/- 0.081 MJ. Its equilibrium temperature is 519 +6/-5 K, placing it between short-period hot Jupiters and cold, Jupiter-like giants. The high orbital eccentricity and intermediate equilibrium temperature of NGTS-39 b make it a valuable test case for formation and migration models, particularly in the poorly sampled regime of long-period gas giants. The RV data show a linear trend of gamma dot = -17.75 m s^-1 yr^-1, which indicates the presence of an outer companion. The discovery of NGTS-39 b contributes to the small but growing population of transiting warm Jupiters with P > 50 days orbiting bright stars.

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arXiv2026

Understanding eccentric temperate giants: an in-depth study of the architecture and stellar obliquity of the TOI-2134 system

We revisit the TOI-2134 planetary system with three new high-cadence TESS sectors and 98 more spectra. This new analysis confirms the two orbiting planets by simultaneously modelling a total of eight sectors of corrected TESS photometry and 280 HARPS-N and SOPHIE radial velocities: an inner mini-Neptune in a near-circular $9.229198\pm0.000003$ days orbit, and an outer temperate sub-Saturn orbiting with a $95.852840\pm0.000042$ days period and eccentricity of $0.31\pm0.01$. The masses and radii of the planets were computed to be $9.37\pm0.54$ Me and $2.735\pm0.068$ Re for planet b, and $58.3\pm1.9$ Me and $7.35\pm0.18$ Re for planet c. The new data not only improves the detection significance and precisions on the planetary orbits, but also breaks the original multimodality in the eccentricity solution for the outer planet. We also detect a long-term trend in the radial velocity data, which we attribute to a stellar magnetic cycle. We investigate the spin-orbit alignment of the system via observations of the Rossiter-McLaughlin effect for TOI-2134~b with EXPRES and TOI-2134~c with PARAS-2. No RM effect was detected for planet b, but we find a 4.7$\sigma$ detection of a $59\pm31^{\circ}$ obliquity for planet c. Finally, we examine the architecture of the system, assess its completeness, investigate the planetary interior, and their suitability for follow-up atmospheric analysis.

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arXiv2026

Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits Observed Contemporaneously with Magellan PFS, LCOGT, and CHEOPS

Young planets offer a unique window into the early stages of planetary evolution. AU Mic is one of the nearest (9.8 pc) pre-main sequence stars (~20 Myr), hosting two transiting Neptune-sized planets and a debris disk. Previous studies have shown that the rotation of the central star, the debris disk, and the inner planet b are all aligned, suggesting that the system has not undergone violent evolution. Here we report new Rossiter-McLaughlin (RM) measurements for both AU Mic b and c, which happened to transit back-to-back on Aug 24 and 25, 2024, using the Magellan Planet Finder Spectrograph (PFS), accompanioned with contanporaneous photometry from LCOGT and CHEOPS. We confirm the aligned orbit of AU Mic b ($\lambda_b=1{\deg} \pm 12{\deg}$) and finding two possible solutions for AU Mic c: we slightly favor an aligned solution ($\lambda_c=-10{\deg} \pm 16{\deg}$) but cannot rule out a polar solution ($\lambda_c=87{\deg}\ ^{+36{\deg}}_{-29{\deg}}$). Broader considerations, including dynamical stability and transit possibility, also support the mutually aligned scenario. An unexpected stellar signal during ingress and the poor TTV predictions of AU Mic c prevent a precise constraint on its obliquity, and various attempts using chromatic spectral analyses fail to outperform simple data exclusion in mitigating the stellar contamination. Our observation highlights the importance of understanding stellar activity across multiple timescales and channels when characterizing young, active systems. A robust solution for the AU Mic architecture will require either a better understanding of stellar activity or future observations fortuitously free from strong stellar contamination.

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arXiv2026

An Ultra-Short Period Super-Earth and a Sub-Neptune Orbiting the K dwarf TOI-4311

We report the discovery and characterisation of the multi-planetary system around TOI-4311, a K dwarf kinematically between the Galactic thick disk and Hercules stream. TOI-4311 hosts an ultra-short-period super-Earth (P$\sim$0.99 d, $1.376\substack{+0.077\\-0.080}$ R$_\oplus$) and a longer period sub-Neptune (P$\sim$15 d, $2.47\substack{+0.12\\-0.11}$ R$_\oplus$) that was first detected in the TESS photometry. Using follow-up observations with CHEOPS and HARPS, we refine the planetary radius of both planets, derive the mass of planet b ($4.5\substack{+1.5\\-1.4}$ M$_\oplus$) and confirm the planetary nature of planet c. Intriguingly, a third periodic signal is clearly detected in our HARPS RVs that we cannot link to stellar activity. This signal could be attributed to a third planet (P$\sim$38 d, Msin(i)=$26.4\substack{+6.3\\-6.8}$ M$_\oplus$) in the system, however with the current photometric dataset we do not find a transit. Our dynamical analysis highlights that this potential outer planet would remain stable. Using the precise radius and mass for TOI-4311 b we model its interior structure and find that it is very dense given the host star's galactic kinematics and chemistry. Hence this system could challenge current formation theories and provide insights into planet formation across the galaxy.

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arXiv2026

Constraining Small Planet Compositions for Future Missions

Accurate mass and radius measurements of small transiting exoplanets are essential for probing their compositions, formation histories, and potential habitability. We present a uniform analysis of six planetary systems (each hosting at least one small transiting planet): K2-79, K2-106, K2-111, K2-222, K2-263, and TOI-1634. Our study combines new CHEOPS transit observations with archival photometry from K2, TESS, and ground-based facilities, alongside new and archival radial velocity data from HARPS-N, HIRES, ESPRESSO, and others. For each system, we perform joint transit and RV modelling, achieving typical precisions better than 15% and 5% for mass and radius, respectively, and thus enabling precise bulk density determinations. These reveal a range of compositions, including rocky planets near the radius valley (e.g. K2-106 b, TOI-1634 b), intermediate-density planets requiring steam-rich or mixed volatile envelopes (e.g. K2-111 b, K2-263 b), and low-density regimes, consistent with gas dwarfs or water-worlds (e.g. K2-79 b, K2-222 b). Several systems show evidence of additional companions detectable via RVs but not seen in transit. The results highlight the value of coordinated CHEOPS and HARPS-N observations in delivering some of the most precise bulk densities for small planets to date and support the preparation for future atmospheric characterisation missions.

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arXiv2026

TIC-65910228 b / NGTS-38 b, a 180 day transiting warm super-Jupiter

We present the discovery of TIC-65910228 b / NGTS-38 b, a giant exoplanet with a radius of $1.081\pm0.047$ R$_\text{J}$ and a mass of $4.78_{-0.37}^{+0.39}$ M$_\text{J}$ on a long-period ($180.52797\pm0.00036$ day), moderately eccentric ($e=0.3086\pm0.010$) orbit transiting a bright (V=$10.230\pm0.020$ mag) metal rich ([Fe/H]=$0.33\pm0.09$, 'dex') F6V-F7V type host star. The planet was initially detected from a single transit in TESS Sector 33. A photometric monitoring campaign of 228 nights with NGTS detected a transit egress of the planet, which together with spectroscopic radial velocity monitoring with CORALIE and HARPS identified an orbital period of ~180.5,d. These radial velocity measurements also showed the mass of the companion to be planetary. Additional transit observations coordinated by the TESS follow-up observing program allowed further confirmation and refinement of this period. With its relatively cool equilibrium temperature of $457\pm11$ K, NGTS-38 b joins a small but growing population of well characterised transiting warm-Jupiters and has one of the longest periods of any discovered to date. The target is situated in the LOPS2 field of the upcoming PLATO mission which will allow for greater refinement of the system parameters and potential for the discovery of additional companions too small and/or too long-period to be seen by TESS or NGTS. NGTS-38 b's bright host star and wide orbital separation make it an attractive target for further study, including potential measurement of its spin-orbit alignment or targeted exomoon/ring searches.

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arXiv2026

Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903

Small exoplanet radii show two populations, referred to as super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This may be produced by the removal of atmospheres due to stellar or internal heating, or lack of an initial envelope. We us transit photometry and radial velocity measurements to detect and characterize four planets orbiting LHS 1903, a red dwarf (M-dwarf) star in the Milky Way's thick disk. The planets have orbital periods between 2.2 and 29.3 days, and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, while LHS 1903 c and LHS 1903 d have extended atmospheres. Although the most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating it formed from gas-depleted material.

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arXiv2026

Detached eclipsing binary star science in the 2040s

Detached eclipsing binary stars (DEBS) are currently the best source of accurate and precise fundamental stellar parameters. This makes DEBS crucial targets for constraining the impact of various physical processes on stellar structure and evolution. Long-period binaries are particularly interesting because their separation minimises interactions between the components. This makes long-period binaries more comparable to single stars. However, the current sample of DEBS with high precision stellar parameters are dominated by short-period systems (e.g. ~90% of the Gaia DR3 eclipsing binaries have periods < 5 days). Facilities capable of performing detailed studies of long-period DEBS will be essential to further improve our understanding of stellar structure and evolution. Such facilities would need to be able to obtain spectroscopic observations of more distant objects at high resolution and cadence. 2-8m class telescopes with echelle spectrographs and an ability to monitor a large sample of stars would be required.

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arXiv2026

Mass estimates of the young TOI-451 transiting planets: Multidimensional Gaussian Process on stellar spectroscopic and photometric signals

The young TOI-451 planetary system, aged 125 Myr, provides a unique opportunity to test theories of planetary internal structures and atmospheric mass loss through examination of its three transiting planets. We present an exhaustive photometric and spectroscopic follow-up to determine the orbital and physical properties of the system. We perform multidimensional Gaussian Process regression with the code pyaneti on spectroscopic time-series and NGTS/LCO light curves to disentangle the stellar and planetary signal in ESPRESSO radial velocities. We show how contemporaneous photometry serves as an activity indicator to inform RV modelling within a multidimensional Gaussian Processes framework. We argue that this can be exploited when spectroscopic observations are adversely affected by low signal-to-noise and/or poor sampling. We estimate the Doppler semi-amplitudes of Kb = 2.6(+1.1,-1.2) m/s, Kc = 1.2(+1.0,-0.8) m/s and Kd = 2.7 +/- 1.2 m/s. This translates into 2-sigma mass estimates for TOI-451 b and d of Mb = 4.7(+2.1,-2.2) Earth masses and Md = 10.2(+4.6,-4.5) Earth masses, as well as a mass upper limit for TOI-451 c of Mc < 11.5 Earth masses. The derived planetary properties suggest that planets c and d contain significant hydrogen-rich envelopes. The inferred parameters of TOI-451 b are consistent with either a rocky world that still retains a small hydrogen envelope or a water world. These insights make the TOI-451 system an ideal laboratory for future follow-up studies aimed at measuring atmospheric compositions, detecting atmospheric mass-loss signatures, and further exploring planetary formation and evolution processes.

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arXiv2025

Searching for Exoplanets Born Outside the Milky Way: VOYAGERS Survey Design

Observations over the past few decades have found that planets are common around nearby stars in our Galaxy, but little is known about planets that formed outside the Milky Way. We describe the design and early implementation of a survey to test whether planets also exist orbiting the remnant stars of ancient dwarf galaxies that merged with the Milky Way, and if so, how they differ from their Milky Way counterparts. VOYAGERS (Views Of Yore - Ancient Gaia-enceladus Exoplanet Revealing Survey) is a radial velocity (RV) search using precision spectrographs to discover exoplanets orbiting very low metallicity ($-2.8 < [\mathrm{Fe/H}] \leq -0.8$) stars born in the dwarf galaxy Enceladus, which merged with the Milky Way galaxy about 10 Gyr ago. A sample of 22 candidates have been screened from a catalog of Gaia-Enceladus-Sausage (GES) members using a combination of stellar properties and reconnaissance observations from the TRES spectrograph. Precision RV measurements have been initiated using the NEID, HARPS-N, and CARMENES spectrographs. We plan to focus most upcoming observations on 10 main sequence targets. Data collection is well underway, with 778 observations on 22 candidates (385 of which are 10 focus targets), but far from complete. This survey is designed to be sensitive to sub-Neptune mass planets with periods up to hundreds of days. We note that the RV analysis gives mass multiplied by $\sin (inclination)$ or the minimum mass for exoplanets. The expected survey yield is three planets, assuming that occurrence rates are similar to those in the Milky Way and taking into account the degeneracy with inclination in our yield models. Our survey is designed to detect at least one exoplanet if occurrence rates are similar to known Milky Way exoplanets or, if no exoplanets are discovered, to rule out a Milky Way-like planet population in GES with 95% confidence level.

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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

An Ultra-Short Period Super-Earth and Sub-Neptune Spanning the Radius Valley Orbiting the Kinematic Thick Disk Star TOI-2345

A crucial chemical link between stars and their orbiting exoplanets is thought to exist. If universal, this connection could affect the formation and evolution of all planets. Therefore, this potential vital link needs testing by characterising exoplanets around chemically-diverse stars. We present the discovery of two planets orbiting the metal-poor, kinematic thick-disk K-dwarf TOI-2345. TOI-2345 b is a super-Earth with a period of 1.05 days and TOI-2345 c is a sub-Neptune with a period of 21 days. In addition to the target being observed in 4 TESS sectors, we obtained 5 CHEOPS visits and 26 radial velocities from HARPS. By conducting a joint analysis of all the data, we find TOI-2345 b to have a radius of $1.504\substack{+0.047\\-0.044}$ R$_\oplus$ and a mass of $3.49\pm0.85$ M$_\oplus$; and TOI-2345 c to have a radius of $2.451\substack{+0.045\\-0.046}$ R$_\oplus$ and a mass of $7.27\substack{+2.27\\-2.45}$ M$_\oplus$. To explore chemical links between these planets and their host star, we model their interior structures newly accounting for devolatised stellar abundances. TOI-2345 adds to the limited sample of well characterised planetary systems around thick disk stars. This system challenges theories of formation and populations of planets around thick disk stars with its Ultra-Short Period super-Earth and the wide period distribution of these two planets spanning the radius valley.

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arXiv2025

The HD 60779 Planetary System: A Transiting Sub-Neptune on a 30-day Orbit and a More Massive Outer World

We present the discovery of the planetary system orbiting the bright (V = 7.2), nearby (35 pc), Sun-like star HD 60779, which has a mass of 1.050 +/- 0.044 solar masses and a radius of 1.129 +/- 0.013 solar radii. We report two TESS transits and a subsequent CHEOPS transit of HD 60779 b, a sub-Neptune with a radius of 3.250 (+0.100 / -0.098) Earth radii on a 29.986175 (+0.000030 / -0.000033) day orbit. Additionally, 286 HARPS-N radial velocity measurements reveal the mass of planet b (14.7 +1.1 / -1.0 Earth masses) and the presence of an outer planet, HD 60779 c, with an orbital period of 104.25 (+0.30 / -0.29) days and a minimum mass (m sin i) of 27.7 +/- 1.6 Earth masses. Both planets' orbits are consistent with being circular, suggesting that they have a dynamically quiet history. The data are not sufficient to determine whether planet c transits. HD 60779's uniquely high systemic radial velocity (129.75 +/- 0.12 km/s) allows its Lyman-alpha emission to avoid absorption by the interstellar medium, making it a prime candidate for probing atmospheric escape from HD 60779 b. HD 60779 is also the third-brightest host of a sub-Neptune with orbital period greater than 25 days and with both mass and radius measured, distinguishing it in terms of accessibility to spectroscopic characterization.

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arXiv2025

Activity in White Dwarf Debris Disks I: Spitzer Legacy Reveals Variability Incompatible with the Canonical Model

This study presents all available, multi-epoch 3.6 and 4.5 $μ$m photometry from Spitzer Space Telescope observations of white dwarf debris disks, including weekly cadence observations of 16 relatively bright systems, and 5 h staring-mode observations for five of these. Significant variability is detected in 85 per cent of disks and across all timescales probed, from minutes to weeks to years, where the largest flux changes correlate with the longest time baselines, and the infrared excesses persist utterly. While each source is idiosyncratic, the overall results indicate the most variable disks correlate with those that are the brightest (dustiest), and also among those with detected gas, demonstrating both dust and gas are produced via ongoing collisions. There is a correlation between flux and colour changes, where disks tend to appear redder when dimmer and bluer when brighter, consistent with an excess of small dust grains produced in collisions, followed by a gradual return to equilibrium. The overall results are a drastic departure from the predictions of the canonical - geometrically thin, optically thick - disk in both flux and colour, but are broadly consistent with collisional evolution based on a simple model. The data presented herein constitute a legacy resource that can inform time-series studies of polluted and dusty white dwarfs, and importantly serve as a basis for future disk modelling, beyond the pioneering canonical framework.

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arXiv2025

High-resolution Ultraviolet-to-nearinfrared Characterization of Exoplanet Atmospheres

The Habitable Worlds Observatory (HWO) offers a unique opportunity to revolutionize our understanding of planetary formation and evolution. The goal of this Science Case Development Document (SCDD) is to investigate the physical and chemical processes that shape the composition and atmospheric mass loss in exoplanets. We review the key observables currently known as diagnostics of mass loss via transit observations, i.e., absorption lines of escaping hydrogen (Lyman-alpha), helium, and metals (Fe, Mg, C, O). We also explore the challenges to infer planetary formation processes based on atmospheric composition characterization. HWO could enable a broad, continuous coverage from far-ultraviolet to near-infrared spectroscopy (~100--1600 nm) at high resolution (R > 60, 000), which is essential to make these measurements, disentangle their planetary origin from stellar activity, and ultimately, contextualize the escape rates by simultaneously characterizing the composition, cloud predominance, and thermal structure of exoplanet atmospheres.

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