arXiv subjects

James G. Rogers

Publications and source records attributed to James G. Rogers.

At least 19 recordsLinked to original sources

arXiv2026

No statistically significant evidence for a correlation between stellar and planetary composition

The chemical compositions of planets and their hosts stars are intrinsically linked, having formed from the same protostellar material. Characterising this relationship provides key constraints on the processes governing planetary formation and evolution. Focusing on host stars with near-solar chemical abundances limits our understanding of how planetary composition varies across a broader chemical parameter space. Expanding the sample to include stars with compositions markedly different from the Sun is therefore essential for building a complete picture of star--planet compositional connections. Here, we focus specifically on iron-poor hosts since they are more likely to be alpha-enhanced and represent some of the most chemically distinct stars relative to the Sun (i.e., thick disc stars). We present a sample of 45 stars hosting 64 planets across the super-Earth and sub-Neptune regimes, for which we homogeneously obtain new stellar parameters including abundances, re-derive planetary mass and radius, and model the bulk interior compositions of their planetary companions. No statistically significant evidence for a correlation between stellar and planetary composition was found in our sample. We suggest that this null result is primarily driven by the large uncertainties inherent to both compositional proxies, which may obscure an underlying relationship. Quantifying these uncertainties is therefore a critical step that previous studies have not fully addressed. Furthermore, even with improved precision, uncovering such a relation may require a higher-dimensional treatment that accounts for additional parameters such as planetary equilibrium temperature.

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arXiv2026

Testing the prevalence of hydrogen-silicate miscibility in young sub-Neptunes

Hydrogen-silicate miscibility can significantly alter the interior structure and thermal evolution of sub-Neptunes. We consider the interplay between this miscibility and stellar-driven atmospheric escape. We find that, for the first $\sim 100$ Myrs, sub-Neptunes store most of their hydrogen content within their miscible interiors, protecting it from escape. As hydrogen is removed from the top of the atmosphere, more hydrogen is exsolved from the miscible interior, resupplying the envelope mass and delaying the planet's contraction when compared with models that do not account for miscibility. Regardless of miscibility, atmospheric escape reproduces the young planet observations from TESS, and we highlight the emergence of the primordial Neptune desert at short orbital periods. We construct a population-level test for the prevalence of miscible sub-Neptunes which exploits their slower radial contraction. We find that $\sim 70-100$ observed young sub-Neptunes with ages $\lesssim 100$ Myrs are required to answer this question.

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arXiv2026

The Barnard's Star Planetary System: Stability, Composition, and Evolution of Four Sub-Earth Exoplanets

Barnard's Star is the nearest single star to the Sun (1.8 pc), and hosts four recently-discovered planets. The star also has well-characterized stellar abundances of important rock-forming elements, including Fe, Mg, and Si. For refractory elements like these, the planets have likely inherited similar bulk elemental abundance ratios to the star, facilitating modelling of their interior structures. We present here an analysis of the Barnard's Star planetary system on several fronts. We perform a detailed stability analysis of the system, ascertaining that all four planets likely have masses between 0.19 and 0.84 $M_{\oplus}$, and are likely tidally locked, whereas a 4:3 mean-motion resonance chain for the inner three planets cannot be ruled out. Using atmospheric evolution models, we show that the prospect of extant primary atmospheres is highly unlikely on any of the planets. Barnard's Star's abnormally high Mg/Si ratio and low Th/Mg ratio imply planetary mantles which (a) are rich in (Mg,Fe)O ferropericlase; (b) have less than half the water capacity as Earth; (c) generate about half of the radiogenic heating as Earth; and (d) are cool and unlikely to have outgassed secondary atmospheres. Our analysis of this system presents an accessible set of first steps for the study of other nearby exoplanetary systems, as well as sub-Earth planets which will be increasingly discovered over the coming years.

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arXiv2026

Using observations of escaping H/He to constrain the atmospheric composition of sub-Neptunes

The internal composition of sub-Neptunes remains a prominent unresolved question in exoplanetary science. We present a technique to place constraints on envelope mean molecular weight that utilises observations of escaping hydrogen or helium exospheres. This method is based on a simple timescale argument, which states that sub-Neptunes require a sufficiently large hydrogen or helium reservoir to explain on-going escape at their observed rates. This then naturally leads to an upper limit on atmospheric mean molecular weight. We formalise this argument within a Bayesian inference model and apply it to the archetypal sub-Neptunes GJ-436 b, TOI-776 b and TOI-776 c, which have all been observed to be losing significant hydrogen content as well as relatively featureless transit spectra when observed with JWST. Combining constraints from atmospheric escape and transit spectroscopy in the case of TOI-776 c allows us to tentatively rule out the high mean molecular weight scenario, pointing towards a low mean molecular weight atmosphere with high-altitude aerosols muting spectral features in the infra-red. Finally, we reframe our analysis to the hycean candidate K2-18 b, which has also been shown to host a tentative escaping hydrogen exosphere. If such a detection is robust, we infer a hydrogen-rich envelope mass fraction of $\log_{10} f_\text{env} = -1.67\pm0.78$, which is inconsistent with the hycean scenario at the $\sim 4σ$ level. This latter result requires further observational follow-up to confirm.

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arXiv2026

Preparing for the Early eVolution Explorer: Detecting the Primordial, Transiting Exoplanet Population

The close-in small planet population may be formed either with hydrogen/helium dominated envelopes or with water-rich interiors. Both scenarios reproduce the present day planet population in mass, radius, and periods, and are difficult to differentiate with the mature planet demographic. Hydrogen/Helium `gas-dwarfs' have low mean molecular weight atmospheres, while `water-worlds' have envelopes that are significantly heavier, and as such these two scenarios have different evolution tracks that diverge in the first ~50 Myr of their evolution. We show that a low Earth orbit multi-band photometric survey mission, within the scope of the NASA Small Explorers Program (SMEX), can determine the frequency of young close-in planets at the 5% level and definitively differentiate between the competing `gas-dwarf' and `water-world' hypotheses. We simulate a 2.5 year mission capable of simultaneous multi-band near-ultraviolet (NUV), optical, and near infrared (NIR) wide field photometry. Such a mission would perform a photometric survey of 30 different stare-fields selected to probe the young star population. The mission will yield ~100 transiting planets in young star clusters and associations with ages <50 Myr. In comparison, only 20 such planets are known from K2 and TESS today.

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arXiv2025

Redefining interiors and envelopes: hydrogen-silicate miscibility and its consequences for the structure and evolution of sub-Neptunes

We present the first evolving interior structure model for sub-Neptunes that accounts for the miscibility between silicate magma and hydrogen. Silicate and hydrogen are miscible above $\sim 4000$K at pressures relevant to sub-Neptune interiors. Using the H$_2$-MgSiO$_3$ phase diagram, we self-consistently couple physics and chemistry to determine the radial extent of the fully miscible interior. Above this region lies the envelope, where hydrogen and silicates are immiscible and exist in both gaseous and melt phases. The binodal surface, representing a phase transition, provides a physically/chemically informed boundary between a planet's "interior" and "envelope". We find that young sub-Neptunes can store several tens of per cent of their hydrogen mass within their interiors. As the planet cools, its radius and the binodal surface contract, and the temperature at the binodal drops from $\sim 4000$K to $\sim 3000$K. Since the planet's interior stores hydrogen, its density is lower than that of pure-silicate. Gravitational contraction and thermal evolution lead to hydrogen exsolving from the interior into the envelope. This process slows planetary contraction compared to models without miscibility, potentially producing observable signatures in young sub-Neptune populations. At early times ($\sim 10$-$100$Myr), the high temperature at the binodal surface results in more silicate vapour in the envelope, increasing its mean molecular weight and enabling convection inhibition. After $\sim$Gyr of evolution, most hydrogen has exsolved, and the radii of miscible and immiscible models converge. However, the internal distribution of hydrogen and silicates remains distinct, with some hydrogen retained in the interior.

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arXiv2025

A 16 Myr super-Neptune in Upper-Centaurus Lupus and a preliminary survey of transiting planets in Sco-Cen with TESS

Measuring the properties of planets younger than about 50 Myr helps to test different planetary formation and evolution models. NASA's Transiting Exoplanet Survey Satellite (TESS) has observed nearly the entire sky, including a wide range of star-forming regions and young stellar clusters, expanding our census of the newborn planet population. In this work, we present the discovery of the TIC 88785435 planetary system located in the Upper-Centaurus Lupus (UCL) region of the Scorpius-Centaurus OB association (Sco-Cen) and a preliminary survey of the planet population within Sco-Cen. TIC 88785435 is a pre-main sequence, K7V dwarf ($M_\star = 0.72M_\odot$, $R_\star = 0.91R_\odot$, $T_\mathrm{eff}$ = 3998K, V = 11.7 mag) located within the bounds of UCL. We investigate the distribution of rotation periods measured from the TESS long-cadence data and the Halpha and Li abundances from the spectra of TIC 88785435. TESS long-candence data reveal that TIC 88785435 hosts a transiting super-Neptune ($R_b = 5.03R_\oplus$, P = 10.51 days), TIC 88785435 b. Ground-based follow-up validates the planetary nature of TIC 88785435 b. Using the TESS data, we perform a preliminary survey to investigate how TIC 88785435 b compares to the population of newly born planets located within Sco-Cen.

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arXiv2025

A transiting multi-planet system in the 61 million year old association Theia 116

Observing and characterizing young planetary systems can aid in unveiling the evolutionary mechanisms that sculpt the mature exoplanet population. As an all-sky survey, NASA's Transiting Exoplanet Survey Satellite (TESS) has expanded the known young planet population as it has observed young comoving stellar populations. This work presents the discovery of a multiplanet system orbiting the 61 Myr old G4V star TIC 434398831 (M = 0.99 Msun, R = 0.91 Rsun, Teff = 5638 K, Tmag = 11.31) located in the Theia 116 comoving population. We estimate the population's age based on rotation periods measured from the TESS light curves, isochrone fitting, and measurements of lithium equivalent widths in the spectra of Theia 116 members. The TESS FFI light curves reveal a mini-Neptune (Rb = 3.51 Rearth, Pb = 3.69 days) and super-Neptune (Rc = 5.63 Rearth, Pc = 6.21 days) with an orbital period ratio slightly larger than 5:3. Follow-up observations from CHEOPS and ground-based telescopes confirm the transits of TIC 434398831 b and c, and constrain their transit times. We explore the potential mass-loss histories of the two planets in order to probe possible initial conditions of the planets immediately after formation.

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arXiv2025

On the road to the radius valley: distinguishing between gas dwarfs and water worlds with young transiting exoplanets

The detection of young transiting exoplanets represents a new frontier in our understanding of planet formation and evolution. For the population of observed close-in sub-Neptunes, two proposed formation pathways can reproduce their observed masses and radii at $\sim$Gyr ages: the "gas dwarf" hypothesis and the "water world" hypothesis. We show that a sub-Neptune's size at early ages $\lesssim 100$ Myrs is strongly dependent on the bulk mean molecular weight within its envelope. As a result, gas dwarfs and water worlds should diverge in size at early ages since the mean molecular weight of gas dwarf envelopes is predicted to be smaller than that of water worlds. We construct population models under both scenarios that reproduce Kepler demographics in the age range $\sim1-10$ Gyrs. We find tentative evidence that the gas dwarf model is more consistent with the small population of young exoplanets $< 40$ Myrs from TESS. We show that planet radius is relatively insensitive to planet mass for young, puffy sub-Neptunes, meaning that well-characterised masses are not necessarily required to exploit the effects of mean molecular weight at the population level. We confirm the predicted difference in planet size between the models is also true under mixed-envelope scenarios, in which envelopes consist of mixtures of hydrogen and steam. We highlight that transit surveys of young exoplanets should target the youngest observable stellar clusters to exploit the effects of mean molecular weight.

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arXiv2025

Signatures of Atmospheric Mass Loss and Planet Migration in the Time Evolution of Short-Period Transiting Exoplanets

Comparative studies of young and old exoplanet populations offer a glimpse into how planets may form and evolve with time. We present an occurrence rate study of short-period ($<$12 days) planets between 1.8--10 Rearth around 1374 FGK stars in nearby (200 pc) young clusters ($<$1 Gyr), utilizing data from the Transiting Exoplanet Survey Satellite (TESS) mission. These planets represent a population closer to their primordial state. We find that the occurrence rate of young planets is higher ($64^{+32}_{-22}$%) compared to the Gyr-old population observed by \kepler ($7.98^{+0.37}_{-0.35}$%). Dividing our sample into bins of young (10--100 Myr) and intermediate (100\,Myr--1 Gyr) ages, we also find that the occurrence distribution in orbital period remains unchanged while the distribution in planet radius changes with time. Specifically, the radius distribution steepens with age, indicative of a larger planet population shrinking due to the atmospheric thermal cooling and mass loss. We also find evidence for an increase (1.9$σ$) in occurrence after 100 Myr, possibly due to tidal migration driving planets inside of 12 days. While evidence suggests post-disk migration and atmospheric mass loss shape the population of short-period planets, more detections of young planets are needed to improve statistical comparisons with older planets. Detecting long-period young planets and planets $<$1.8 Rearth will help us understand these processes better. Additionally, studying young planetary atmospheres provides insights into planet formation and the efficiency of atmospheric mass loss mechanisms on the evolution of planetary systems.

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arXiv2024

Most Super-Earths Have Less Than 3% Water

Super-Earths are highly irradiated, small planets with bulk densities approximately consistent with Earth. We construct combined interior-atmosphere models of super-Earths that trace the partitioning of water throughout a planet, including an iron-rich core, silicate-rich mantle, and steam atmosphere. We compare these models with exoplanet observations to infer a $1σ$ upper limit on total water mass fraction of $\lesssim 3\%$ at the population level. We consider end-member scenarios that may change this value, including the efficiency of mantle outgassing, escape of high mean-molecular weight atmospheres, and increased iron core mass fractions. Although our constraints are agnostic as to the origin of water, we show that our upper limits are consistent with its production via chemical reactions of primordial hydrogen-dominated atmospheres with magma oceans. This mechanism has also been hypothesised to explain Earth's water content, possibly pointing to a unified channel for the origins of water on small terrestrial planets.

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arXiv2024

Phase equilibria of sub-Neptunes and super-Earths

We investigate the consequences of non-ideal chemical interaction between silicate and overlying hydrogen-rich envelopes for rocky planets using basic tenets of phase equilibria. Based on our current understanding of the temperature and pressure conditions for complete miscibility of silicate and hydrogen, we find that the silicate-hydrogen binary solvus will dictate the nature of atmospheres and internal layering in rocky planets that garnered H$_2$-rich primary atmospheres. The temperatures at the surfaces of supercritical magma oceans will correspond to the silicate-hydrogen solvus. As a result, the radial positions of supercritical magma ocean-atmosphere interfaces, rather than their temperatures and pressures, should reflect the thermal states of these planets. The conditions prescribed by the solvus influence the structure of the atmosphere, and thus the transit radii of sub-Neptunes. Separation of iron-rich metal to form metal cores in sub-Neptunes and super-Earths is not assured due to prospects for neutral buoyancy of metal in silicate melt induced by dissolution of H, Si, and O in the metal at high temperatures.

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arXiv2024

Searching for Neutral Hydrogen Escape from the 120 Myr Old Sub-Neptune HIP94235b using HST

HIP94235 b, a 120 Myr old sub-Neptune, provides us the unique opportunity to study mass loss at a pivotal stage of the system's evolution: the end of a 100 million year (Myr) old phase of intense XUV irradiation. We present two observations of HIP94235 b using the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS) in the Ly-alpha wavelength region. We do not observe discernible differences across either the blue and red wings of the Ly-alpha line profile in and out of transit, and report no significant detection of outflowing neutral hydrogen around the planet. We constrain the rate of neutral hydrogen escaping HIP94235 b to an upper limit of 10^13 g/s, which remains consistent with energy-limited model predictions of 10^11 g/s. The Ly-alpha non-detection is likely due to the extremely short photoionization timescale of the neutral hydrogen escaping the planet's atmosphere. This timescale, approximately 15 minutes, is significantly shorter than that of any other planets with STIS observations. Through energy-limited mass loss models, we anticipate that HIP94235 b will transition into a super-Earth within a timescale of 1 Gyr.

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arXiv2024

Shallower radius valley around low-mass hosts: Evidence for icy planets, collisions or high-energy radiation scatter

The radius valley, i.e., a dearth of planets with radii between 1.5 and 2 Earth radii, provides insights into planetary formation and evolution. Using homogenously revised planetary parameters from Kepler 1-minute short cadence light curves, we remodel transits of 72 small planets mostly orbiting low-mass stars, improving the precision and accuracy of planet parameters. By combining this sample with a similar sample of planets around higher-mass stars, we determine the depth of the radius valley as a function of stellar mass. We find that the radius valley is shallower for low-mass stars compared to their higher mass counterparts. Upon comparison, we find that theoretical models of photoevaporation under-predict the number of planets observed inside the radius valley for low-mass stars: with decreasing stellar mass, the predicted fraction of planets inside the valley remains approximately constant whereas the observed fraction increases. We argue that this provides evidence for the presence of icy planets around low-mass stars. Alternatively, planets orbiting low-mass stars undergo more frequent collisions and scatter in the stars' high-energy output may also cause planets to fill the valley. We predict that more precise mass measurements for planets orbiting low mass stars would be able to distinguish between these scenarios.

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arXiv2024

Fleeting but not Forgotten: the Imprint of Escaping Hydrogen Atmospheres on Super-Earth Interiors

Small, close-in exoplanets are divided into two sub-populations: super-Earths and sub-Neptunes. Most super-Earths are thought to have lost their primordially accreted hydrogen-dominated atmospheres via thermally driven winds. We consider the global chemical equilibrium of super-Earths and the lasting impacts of their fleeting hydrogen atmospheres. We find that hydrogen is efficiently sequestered into the interior, oxidising iron and endogenously producing $\sim0.5-1.0\%$ water by mass. As the atmospheres of super-Earths are continuously sculpted by mass loss and chemical equilibration, they remain hydrogen-dominated by mole (number) fraction but become steam-dominated by mass, which may be observable with JWST for planets transitioning across the radius valley. One of the main effects of efficient sequestration of hydrogen into the interior is to produce an under-dense bulk interior compared to that of Earth. We predict bulk densities of super-Earths to be $\sim 5.0 \text{ g cm}^{-3}$ for a $1M_\oplus$ planet, which is consistent with high-precision mass measurements and also population-level inference analyses from atmospheric escape models.

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arXiv2024

Under the light of a new star: evolution of planetary atmospheres through protoplanetary disc dispersal and boil-off

The atmospheres of small, close-in exoplanets are vulnerable to rapid mass-loss during protoplanetary disc dispersal via a process referred to as `boil-off', in which confining pressure from the local gas disc reduces, inducing atmospheric loss and contraction. We construct self-consistent models of planet evolution during gaseous core accretion and boil-off. As the surrounding disc gas dissipates, we find that planets lose mass via subsonic breeze outflows which allow causal contact to exist between disc and planet. Planets initially accrete of order $\sim 10\%$ in atmospheric mass, however, boil-off can remove $\gtrsim 90\%$ of this mass during disc dispersal. We show that a planet's final atmospheric mass fraction is strongly dictated by the ratio of cooling timescale to disc dispersal timescale, as well as the planet's core mass and equilibrium temperature. With contributions from core cooling and radioactivity, we show that core luminosity eventually leads to the transition from boil-off to core-powered mass-loss. We find that smaller mass planets closest to their host star may have their atmospheres completely stripped through a combination of boil-off and core-powered mass-loss during disc dispersal, implying the existence of a population-level radius gap emerging as the disc disperses. We additionally consider the transition from boil-off/core-powered mass-loss to X-ray/EUV (XUV) photoevaporation by considering the penetration of stellar XUV photons below the planet's sonic surface. Finally, we show that planets may open gaps in their protoplanetary discs during the late stages of boil-off, which may enhance mass-loss rates.

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arXiv2023

Conclusive evidence for a population of water-worlds around M-dwarfs remains elusive

The population of small, close-in exoplanets is bifurcated into super-Earths and sub-Neptunes. We calculate physically motivated mass-radius relations for sub-Neptunes, with rocky cores and H/He dominated atmospheres, accounting for their thermal evolution, irradiation and mass-loss. For planets $\lesssim 10~$M$_\oplus$, we find that sub-Neptunes retain atmospheric mass fractions that scale with planet mass and show that the resulting mass-radius relations are degenerate with results for `water-worlds' consisting of a 1:1 silicate-to-ice composition ratio. We further demonstrate that our derived mass-radius relation is in excellent agreement with the observed exoplanet population orbiting M-dwarfs and that planet mass and radii alone are insufficient to determine the composition of some sub-Neptunes. Finally, we highlight that current exoplanet demographics show an increase in the ratio of super-Earths to sub-Neptunes with both stellar mass (and therefore luminosity) and age, which are both indicative of thermally driven atmospheric escape processes. Therefore, such processes should not be ignored when making compositional inferences in the mass-radius diagram.

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arXiv2023

Exoplanet atmosphere evolution: emulation with neural networks

Atmospheric mass-loss is known to play a leading role in sculpting the demographics of small, close-in exoplanets. Knowledge of how such planets evolve allows one to ``rewind the clock'' to infer the conditions in which they formed. Here, we explore the relationship between a planet's core mass and their atmospheric mass after protoplanetary disc dispersal by exploiting XUV photoevaporation as an evolutionary process. Historically, this style of inference problem would be computationally infeasible due to the large number of planet models required; however, we make use of a novel atmospheric evolution emulator which utilises neural networks to provide three orders of magnitude in speedup. First, we provide proof-of-concept for this emulator on a real problem, by inferring the initial atmospheric conditions to the TOI-270 multi-planet system. Using the emulator we find near-indistinguishable results when compared to original model. We then apply the emulator to the more complex inference problem, which aims to find the initial conditions for a sample of \textit{Kepler}, \textit{K2} and \textit{TESS} planets with well-constrained masses and radii. We demonstrate there is a relationship between core masses and the atmospheric mass that they retain after disc dispersal, and this trend is consistent with the `boil-off' scenario, in which close-in planets undergo dramatic atmospheric escape during disc dispersal. Thus, it appears the exoplanet population is consistent with the idea that close-in exoplanets initially acquired large massive atmospheres, the majority of which is lost during disc dispersal; before the final population is sculpted by atmospheric loss over 100~Myr to Gyr timescales.

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