The session is organized into two 90-minute blocks, separated by a 20-minute coffee break.
Block 1 — Star–planet Connection and Interiors (90 min)
State-of-the-art overview of the star–planet chemical connection, with emphasis on precision and systematics for cool-star abundances and planetary composition constraints. Topics may include: abundance systematics in K/M dwarfs, disk fractionation and inheritance, interior-structure constraints from Fe/Mg/Si, and empirical star–planet composition offsets.
- Invited keynote: Dr. Vardan Adibekyan (25 min + 5 min Q&A)
The Compositional Mirror: Reflecting Stellar Abundances in Planetary Interiors
Stars and their planets emerge from the same primordial reservoir, tied together by a chemical heritage that persists throughout their evolution. While protoplanetary disks dissipate within a few million years, the host star’s atmosphere remains a high-fidelity, long-lived record of the system’s initial building blocks. In this talk, I will explore the "Star–Planet Connection," evaluating the extent to which stellar chemical abundances can—or cannot—serve as reliable constraints for modeling planetary interior structures. Bridging the gap between high-resolution stellar spectroscopy and the interior modeling of rocky worlds presents significant challenges. I will discuss the current state-of-the-art in cool-star abundances, highlighting the critical role of precision and the physical processes (such as atomic diffusion or planet engulfment) that can alter present-day stellar compositions relative to their birth material. Finally, I will address the ongoing debate regarding the direct compositional link between stars and their terrestrial planets, emphasizing how systematic uncertainties in both stellar and planetary data limit our ability to uniquely characterize rocky planet interiors. - Contributed talk: Yoshi Nike Emilia Eschen (10 min + 2 min)
Chemically Connecting Diverse Main Sequence Stars to their Planetary Interiors
Planets and stars form from the same proto-stellar material. Hence the stellar refractory elemental abundances are assumed to be strongly linked to rocky planet interiors. This is also found for the refractory elemental abundances of the Sun and Earth. For exoplanets, this compositional link has been recently suggested and explored in small demographic studies. However, the sample of rocky planets around metal-poor dwarf stars is limited. This makes it challenging to find and validate potentially vital chemical trends. We present a novel machine learning approach to identify planet hosting stars of interest to fill in the lack of well-characterised small planets around metal-poor stars. This algorithm leverages stellar abundances from the large stellar surveys of APOGEE and GALAH to classify host stars chemically followed by a search for planets around these golden targets. We present newly characterised systems containing Ultra-Short Period Super-Earths and Sub-Neptunes around compositionally-diverse main sequence stars. These planetary systems were observed with transit photometry and radial velocity allowing us to measure their radius and masses precisely. We modelled their interior structures for which we further developed tools to link stellar abundances to the core and mantle mass fractions of the planet. These give insights into the elemental abundance ratios of the planets. Hence, we directly study the connection between the planet and host star abundances. These new discoveries significantly add to the sample of rocky planets around thick disk stars and will be followed by further detections and characterisations from our machine learning algorithm. Placing our new and upcoming systems in demographic context, we are now able to further explore the link between stellar refractory elemental abundances and their impact on the interiors of small planets in a larger and more diverse sample. This enables insights into rocky planet formation across the galaxy. - Contributed talk: Dr. Thomas Wilson (10 min + 2 min)
How four small planets orbiting a thick disk M-dwarf provide evidence for gas-depleted formation
The drastic growth in the amount of exoplanets discovered around cool stars has facilitated paradigm shifts in planet formation and evolution. Observational demographic studies motivate theoretical models aiming to understand underlying physical processes. The M-dwarf radius valley can be described by two processes; evolution by thermally-driven mass loss or formation in a gas-depleted environment. Host star irradiation or internal planet heating would strip planetary atmospheres altering their atmospheres. Exoplanets might form in a reduced proto-planetary disk which would alter their rocky interior structures. Vitally, these models predict different physical planetary properties. Thus, by characterising the interiors of exoplanets orbiting M-dwarfs we can directly test formation and evolution theory. However there are few multi-planet M-dwarf systems to test predictions, severely restricting our understanding of the hidden histories of these worlds. I will report the characterisation of the first four-planet system spanning the radius valley around a Galactic thick disk M-dwarf. We find that this cool star hosts one super-Earth below and two planets above the radius valley, with a distant fourth world. The three inner planets follow the trend of decreasing densities as predicted by planet formation theory. However, the fourth planet breaks this trend as it is a smaller, denser, and essentially gas-devoid, rocky body. This landmark finding rejects thermally-driven mass-loss for the radius valley and supports gas-depleted planet formation around M-dwarfs at longer orbital periods for the first time. This result would strongly imply that the outer planet formed later than the inner worlds and thus provide an unprecedented view into planet formation timing. Intriguingly, the upcoming PLATO mission will directly test this formation pathway by expanding this work to the outer regions of Sun-like stars and revolutionise our understanding of the formation and evolution of planetary systems. - Contributed talk: Dr. Fábio Wanderley (10 min + 2 min)
Star–planet connection in M dwarf systems: metallicity and the planetary radius gap
Stars with higher metallicities are expected to host larger reservoirs of solids, increasing the likelihood of forming more massive planets. In this work, we investigate the star–planet connection in M dwarf systems and its impact on planetary radii. We analyze a sample of 48 M dwarfs using high-resolution near-infrared spectra from the SDSS/APOGEE survey. Stellar parameters—effective temperature, metallicity, surface gravity, oxygen abundance, and projected rotational velocity—are derived using the Turbospectrum code and MARCS atmosphere models. Planetary radii are determined for 65 exoplanets orbiting these stars. We find that planets with radii smaller than 3R⊕ are hosted by stars spanning a wide metallicity range (−0.6<[M/H]<+0.3), while larger planets are found exclusively around stars with positive metallicities. We applied a spectroscopic calibration to derive stellar radii for 188 M dwarfs and planetary radii for 246 exoplanets. Our results show that small planets have a range of orbital periods, while planets larger than sub-Neptune size (Rp>4) are confined to short orbital periods (2–5 days), suggesting efficient inward migration. We also investigated the planetary radii distribution for a sub-sample of 218 exoplanets with radii smaller than sub-Neptune sizes. We found a radius gap over Rp ∼ 1.6─2.0 R⊕, bordered by a super-Earth peak at Rp ∼ 1.2─1.6 R⊕, and a sub-neptune peak at 2.0─2.4 R⊕. The planetary radius – orbital period distribution exhibits a nearly flat slope (m=+0.01), which is in agreement with models incorporating photoevaporation and inward migration. This contrasts with the negative slope typically observed for solar-type stars, suggesting that different physical mechanisms govern the evolution of planetary systems around M dwarfs. - Contributed talk: Dr. Emily Calamari (10 min + 2 min)
Connecting the Atmospheres of the Coldest Worlds and Hottest Planets
In the era of JWST, it is now possible to look into substellar atmospheres in unparalleled detail and precision. With exquisite JWST NIRSpec or MIRI data in hand, atmospheric retrievals are a leading data-driven tool that can recover an extrasolar world’s fundamental parameters such as mass, radius, effective temperature, gravity, cloud structure and chemical abundances. However, there is one clear atmospheric phenomenon that confounds all modeling attempts: the overall impact of clouds on observational properties. To break model degeneracies largely brought on by issues with clouds, several works have shown the power that main sequence star–brown dwarf companion systems have when the host star can anchor metallicity, age, and abundance measurements for its substellar companion. In this work, we bring together insight from brown dwarf atmospheric theory and our own Solar system to recently published giant exoplanet analyses. We will show the results of a re-examination of the retrievals for HD 189733 b, WASP-107 b, WASP-17 b and WASP-39 b – four JWST-observed Hot Jupiters – using host star abundances for context. We ground our re-analyses using the respective host star chemistry to benchmark the retrieved chemistry and silicate cloud solutions (i.e. enstatite; MgSiO_3, forsterite; Mg_2SiO_4, and/or quartz; SiO_2). In applying analytical techniques used in companion brown dwarf studies, we find confirmation that giant exoplanets form and accrete in the outer protoplanetary disk, thus inheriting their host star Mg/Si ratio. This talk aims to discuss this work in detail and highlight the importance that brown dwarf literature plays in understanding giant exoplanet formation and evolution. - Contributed talk: Dr. David Coria (10 min + 2 min)
Tracing Elemental and Isotopic CO Abundances in JWST Planet-Hosting K Dwarf Stars Using High-Resolution NIR Spectroscopy
In the era of the James Webb Space Telescope (JWST) and large ground-based telescopes, Jupiter-class exoplanet atmospheres are being characterized in unprecedented detail. Studies have revealed the presence of several carbon- and oxygen-bearing species including CO, CO2, and H2O. Volatile ratios like C/O and 12C/13C offer crucial diagnostics for a planet’s formation, migration and accretion history. Such diagnostics, however, require host star abundances for context which are exceedingly rare in the literature for K dwarf stars. To address this gap, I present an elemental and isotopic abundance analysis for six high-priority, solar to super-solar metallicity K dwarf exoplanet host stars. Each of these stars hosts a giant exoplanet companion targeted by JWST atmospheric characterization efforts. I use MARCS atmospheric models, the TurboSpectrum spectral synthesis code, and high-resolution, high S/N IGRINS spectra. The NIR coverage provides several well-resolved CO and OH lines used to derive novel [C/H], [O/H], 12C/13C abundance ratios via a chi-squared minimization routine. Elemental carbon and oxygen abundances for the sample are found to be solar to super-solar with C/O ratios ranging from 0.48 to 0.81. Isotopic 12C/13C are mostly sub-solar, indicating enrichment in the minor isotope 13C. Statistical uncertainties on these measurements are small and instead dominated by systematic uncertainties arising from the choice of fundamental stellar parameters including effective temperature, surface gravity, and metallicity. I will discuss the pros and cons of current techniques for determining these parameters and ages since they, alongside chemical abundances, provide crucial context for planetary growth and galactic chemical evolution models. Finally, I will also discuss how these host star abundances can now be used to constrain the atmospheres, interiors, and evolution histories of their companion exoplanets.
Break (20 min)
Block 2 — Host-Star Abundances and Exoplanetary Atmospheres: Connections and Interpretation (90 min)
How atmospheric properties (metallicity, C/O, disequilibrium chemistry) relate to host-star abundances and interior physics in the JWST era. Topics may include: JWST results and comparative trends, retrieval systematics tied to stellar abundances, and pathways to consistent chemical scales.
- Invited keynote: Dr. Luis Welbanks (20 min + 5 min Q&A)
The (Cool) Star-Planet Context for JWST Observations of Exoplanet Atmospheres
Interpreting a single exoplanet spectrum can resemble a Pointillist painting viewed too closely: individual features may be measurable, but their physical meaning can be ambiguous without context. Is a given “dot” part of the planet, a trace of the star, or a mirage created by simplified models? JWST has expanded both the number and quality of atmospheric spectra, yet connecting retrieved compositions to formation and evolution, especially in a way that is consistent with host-star abundances, remains limited by degeneracies, stellar contamination, and incomplete modeling frameworks. In this talk, I will frame progress and remaining obstacles around three themes. (1) Why single-spectrum interpretation is hard: low-resolution degeneracies among temperature structure, clouds/hazes, and abundances; stellar heterogeneity and activity that can imprint, dilute, or mimic planetary features; and modeling assumptions that can generate apparent chemical “detections” or mask true composition. (2) What is robust today, individually and at the population level: which inferences persist across retrieval approaches and observing modes, what the current sample suggests about atmospheric metallicities and key elemental ratios, and where comparisons to stellar [Fe/H] (and ratios such as C/O) succeed or fail—particularly for planets orbiting cool stars. (3) What we need next: retrieval and interpretation frameworks that more explicitly incorporate stellar properties and observational uncertainties, improved opacities and treatment of disequilibrium processes, and methods that leverage coordinated campaigns across facilities to break degeneracies. The goal is to turn JWST-era spectra from a collection of “dots” into physically anchored constraints on how planets assemble and evolve. - Contributed talk: Dr. Hannah Diamond-Lowe (10 min + 2 min)
The M dwarf side of the rocky planet atmosphere equation: motivation, opportunities, and results from the Rocky Worlds Director’s Discretionary Time Program
The Rocky Worlds DDT program is using approximately 500 hours of JWST time to measure the thermal emission of M dwarf rocky planets at 15 μm and 250 orbits of HST to capture flux, from the far ultraviolet (FUV) to the blue-optical, of the M dwarf hosts themselves. Whether or not the target planets are airless bodies, how they got that way from their formation in the protoplanetary disc, through atmospheric outgassing and evolution, and to their present state today depends heavily on their host M dwarfs. The formation environment defines the volatile inventory of a rocky planet, and ultimately what kind of atmosphere it can outgas, but given that M dwarfs are not simply scaled-down G stars, it may follow that the formation mechanisms within M dwarf discs are unlike those of their FGK counterparts. Radius and mass measurements of nearby M dwarf rocky planets already tell us that if they have atmospheres, they must be composed of high mean molecular weight gases. Any rocky planet atmosphere detection therefore stands to constrain the volatile inventory of (and even rate of delivery to) the inner disc, complementing observational detections of hydrocarbons in the discs of low-mass stars with JWST. High mean molecular weight atmospheres also mean a more complex set of photochemical reactions and interior-exterior exchanges are taking place than we are used to thinking about for planets with hydrogen-dominated atmospheres (e.g., hot Jupiters). It is the high-energy part of the stellar SED, and especially the FUV/NUV fraction that dictates which molecules are destroyed and which can form. Complementary programs from the community can support Rocky Worlds DDT in measuring X-ray outputs and estimating the unobservable EUV through differential emission measures. Part of the Rocky Worlds DDT program will also be monitoring the host M dwarfs for flares in the FUV, whose cumulative effect on a rocky world’s high mean molecular weight atmosphere is an unsolved problem. Though the HST observations of the Rocky Worlds DDT amount to a brief snapshot in time for an individual M dwarf’s UV output, taken as an ensemble the program is providing the community with a homogenous high-fidelity data set of 9 new M dwarf spectra with flare monitoring that can act as empirical inputs to developing models of planet formation, atmosphere outgassing, and atmosphere loss, not just for rocky planets, but also for M dwarf sub-Neptune worlds that may be more susceptible to escaping outflows. In this talk I will present the current status and early results of the Rocky Worlds DDT with a focus on the HST side of the program. At the time of writing we have taken all HST observations of the first target, GJ 3929, which include reconstructing the Ly-alpha profile from measurement of the blue and red wings, as well as flare detections in the FUV time series. All data, data products, and analysis codes are made publicly available for use by the community. - Contributed talk: Giannina Guzman Caloca (10 min + 2 min)
Hidden GEMS: The atmospheres of Giant Exoplanets around M-dwarf Stars
Despite their rarity, Giant Exoplanets around M-dwarf Stars (GEMS) are easily detectable and characterizable given their advantageous radius and mass ratios compared to their small, cool host stars. Recently, the population of GEMS discovered has grown, yet their existence continues to challenge our current frameworks of planet formation models. Understanding the nature of this unique population of planets motivated our JWST GEMS program, the largest JWST Cycle 2 GO exoplanet program, to observe seven of these mysterious objects with NIRSpec/PRISM to constrain their atmospheric compositions and provide insight into their formation histories. Our JWST observations reveal these planets often show strong stellar contamination signals, but due to their size ratios against their host stars also still have strongly detectable atmospheric features, allowing us to understand both planet and host better. We find atmospheric compositions that are markedly different from those of many giant exoplanets orbiting other stellar types, with extremely low metallicities and a diverse set of molecular features. Here, I will present results from our entire sample, highlighting the star-planet connection, the uniqueness of these atmospheres in comparison to planets orbiting other stellar types, and lessons learned. - Contributed talk: Barry O’Donovan (10 min + 2 min)
Heavy-Element-Enriched Atmospheres and Where They Are Born
The heavy element content of giant exoplanets, inferred from structure models based on their radius and mass, often exceeds predictions based on classical core accretion, whereby a fixed mass core of heavy elements accretes an envelope of parent star composition material. Pebble drift, coupled with volatile evaporation, has been proposed as a possible remedy to this with the level of heavy element enrichment a planet can accrete, as well as its atmospheric composition, being strongly dependent on where in the disc it is forming. We consider 10 giant planets more massive than Saturn, as we expect that their heavy element content is dominated by an enrichment of their atmospheric envelope rather than by their core, for which both bulk heavy element content constraints and host star elemental abundance measurements are available. We use a planet formation model, Chemcomp, to simulate the growth and migration of planetary embryos in protoplanetary discs whose initial chemical compositions are matched to the host stars of the planets that we aim to reproduce; this provided a more realistic model of their growth than previous studies. As the planetary embryo migrates through a chemically evolving disc it accretes material of varying chemical composition and heavy element enrichment, dictated by the timescales of the accretion as well as the stellar chemistry of the host. By matching the simulated heavy element content to observational constraints we provide insights into where in the disc these planets could have originated in order to accrete their observed heavy element mass and what this means for their atmospheric composition (C/O ratio, volatile/refractory ratio, etc.). Our simulations predict formation in the inner disc regions, where the majority of the volatiles have already evaporated and can thus be accreted onto the planet via the gas. As the majority of the planetary heavy element content originates from water vapour accretion, our simulations predict a high atmospheric O/H ratio in combination with a low atmospheric C/O ratio, which is in general agreement with observations. For certain planets, namely WASP-84 b, a warm Jupiter with a relatively well constrained heavy element content orbiting a K type star, these properties will be observed with JWST NIRSpec in the near future, offering a test of our modelled constraints and a method of linking stellar chemistry to exoplanet atmospheric composition. - Contributed talk: Dr. Hajime Kawahara (10 min + 2 min)
Differentiable Modeling and Retrieval of Substellar Atmospheric Spectra
We present a differentiable framework for substellar atmospheric spectroscopy built in JAX. End-to-end differentiability opens the way to gradient-based Bayesian inference with Hamiltonian Monte Carlo and the No-U-Turn Sampler (HMC-NUTS), offering a robust route to posterior inference in atmospheric retrievals, particularly in high-dimensional settings. This capability is becoming increasingly important as spectroscopic data from JWST and high-dispersion observations continue to improve in precision and complexity. Our framework, ExoJAX, spans the full modeling chain from molecular spectroscopic data to opacity calculations, radiative transfer, and synthetic spectra for direct comparison with observations. It has already been applied to brown-dwarf emission spectra from high-dispersion spectroscopy, including Gl 229B and Luhman 16A/B, to JWST transmission spectroscopy of the hot Saturn WASP-39b, and to laboratory gas experiments relevant to hot substellar atmospheres. To extend this framework further, we are developing ExoGibbs, a differentiable thermochemical equilibrium model. Together, ExoJAX and ExoGibbs make it possible to perform chemically informed atmospheric retrievals within a gradient-based sampling framework. This talk will present recent applications of differentiable atmospheric modeling and outline future directions for physically consistent retrievals of next-generation substellar spectra. - Discussion panel (15 min)
Moderated audience discussion to identify key uncertainties, priorities for homogeneous analyses, and opportunities for cross-community collaboration.
Note: The detailed speaker list and schedule will be posted here once the submission process has finalized. We expect mid-April.