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We present a population-level view of volatile gas species (H2, He, H2O, O2, CO, CO2, CH4) distribution during the sub-Neptune to rocky planet transition, revealing in detail the dynamic nature of small planet atmospheric compositions. Our novel model couples the atmospheric escape model IsoFATE with the magma ocean-atmosphere equilibrium chemistry model Atmodeller to simulate interior-atmosphere evolution over time for sub-Neptunes around G, K, and M stars. Chiefly, our simulations reveal that atmospheric mass fractionation driven by escape and interior-atmosphere exchange conspire to create a distinct oxidation gradient straddling the small-planet radius valley. We discover a key mechanism in shaping the oxidation landscape is the dissolution of water into the molten mantle, which shields oxygen from early escape, buffers the escape rate, and leads to oxidized secondary atmospheres following mantle outgassing. Our simulations reproduce a prominent population of He-rich worlds along the upper edge of the radius valley, revealing that they are stable on shorter timescales than previously predicted. Our simulations also robustly predict a broad population of O2-dominated atmospheres on close-in planets around low-mass stars, posing a potential source of false positive biosignature detection and marking a high-priority opportunity for the first-ever atmospheric O2 detection. We motivate future atmospheric characterization surveys by providing a target list of planet candidates predicted to have O2-, He-, and deuterium-rich atmospheres.

The NASA K2 mission obtained high-precision time-series photometry for four young clusters, including the near-twin 600–800 Myr old Praesepe and Hyades clusters. Hot sub-Neptunes are highly prone to mass-loss mechanisms, given their proximity to the host star and the weakly bound gaseous envelopes, and analyzing this population at young ages can provide strong constraints on planetary evolution models. Using our automated transit detection pipeline, we recover 15 planet candidates across the two clusters, including 10 previously confirmed planets. We find a hot sub-Neptune occurrence rate of 79%–107% for GKM stars in the Praesepe cluster. This is 2.5–3.5σ higher than the occurrence rate of 16.54−0.98+1.00 % for the same planets orbiting the ∼3–9 Gyr old GKM field stars observed by K2, even after accounting for the slightly supersolar metallicity ([Fe/H] ∼ 0.2 dex) of the Praesepe cluster. We examine the effect of adding ∼100 targets from the Hyades cluster and extending the planet parameter space under examination, and we find similarly high occurrence rates in both cases. The high occurrence rate of young, hot sub-Neptunes could indicate either that these planets are undergoing atmospheric evolution as they age, or that planetary systems that formed when the Galaxy was much younger are substantially different than from today. Under the assumption of the atmospheric mass-loss scenario, a significantly higher occurrence rate of these planets at the intermediate ages of Praesepe and Hyades appears more consistent with the core-powered mass-loss scenario for the origin of the planet radius valley, compared to the photoevaporation scenario.

Planet formation models suggest that the small exoplanets that migrate from beyond the snowline of the protoplanetary disk likely contain water-ice-rich cores (∼50% by mass), also known as water worlds. While the observed radius valley of the Kepler planets is well explained by the atmospheric dichotomy of the rocky planets, precise measurements of the mass and radius of the transiting planets hint at the existence of these water worlds. However, observations cannot confirm the core compositions of those planets, owing to the degeneracy between the density of a bare water-ice-rich planet and the bulk density of a rocky planet with a thin atmosphere. We combine different formation models from the Genesis library with atmospheric escape models, such as photoevaporation and impact stripping, to simulate planetary systems consistent with the observed radius valley. We then explore the possibility of water worlds being present in the currently observed sample by comparing them with simulated planets in the mass–radius–orbital period space. We find that the migration models suggest ≳10% and ≳20% of the bare planets, i.e., planets without primordial H/He atmospheres, to be water-ice-rich around G- and M-type host stars, respectively, consistent with the mass–radius distributions of the observed planets. However, most of the water worlds are predicted to be outside a period of 10 days. A unique identification of water worlds through radial velocity and transmission spectroscopy is likely to be more successful when targeting such planets with longer orbital periods.

Short-period sub-Neptunes are common in extrasolar systems. These sub-Neptunes are generally thought to have primary atmospheres of protoplanetary-disk gas origin. However, atmospheric escape followed by degassing from their interiors can lead to the transition to secondary atmospheres depleted in gases less soluble in magma, such as helium. These primary and secondary atmospheres can potentially be distinguished from observations of escaping hydrogen and helium. This study aims to elucidate the impact of the primary-secondary transition on the atmospheric compositions of short-period sub-Neptunes. We simulate their evolution with atmospheric escape driven by stellar X-ray and extreme ultraviolet irradiation and degassing of hydrogen, helium, and water from their rocky interiors, with a 1D structure model. We show that the transition takes place for low-mass, close-in planets that experience extensive atmospheric escape. These planets show the depletion of helium and enrichment of water in their atmospheres because of their low and high abundances in the planetary interiors, respectively. A compilation of our parameter survey (the orbital period, planetary mass, envelope mass, and mantle FeO content) shows a correlation between the planet radius and the helium escape rate. We suggest that the transition from primary to secondary atmospheres may serve as an explanation for helium non-detection for relatively small (≲2.5 R⊕) exoplanets.

The Kepler mission enabled us to look at the intrinsic population of exoplanets within our galaxy. In period-radius space, the distribution of the intrinsic population of planets contains structure that can trace planet formation and evolution history. The most distinctive feature in period-radius space is the radius cliff, a steep drop-off in occurrence between 2.5 and 4R ⊕ across all period ranges, separating the sub-Neptune population from the rarer Neptunes orbiting within 1 au. Following our earlier work to measure the occurrence rate of the Kepler population, we characterize the shape of the radius cliff as a function of orbital period (10–300 days) as well as insolation flux (9500S ⊕–10S ⊕). The shape of the cliff flattens at longer orbital periods, tracking the rising population of Neptune-sized planets. In insolation, however, the radius cliff is both less dramatic and the slope is more uniform. The difference in this feature between period space and insolation space can be linked to the effect of EUV/X-ray versus bolometric flux in the planet’s evolution. Models of atmospheric mass loss processes that predict the location and shape of the radius valley also predict the radius cliff. We compare our measured occurrence rate distribution to population synthesis models of photoevaporation and core-powered mass loss in order to constrain formation and evolution pathways. We find that the models do not statistically agree with our occurrence distributions of the radius cliff in period space or insolation space. Atmospheric mass loss that shapes the radius valley cannot fully explain the shape of the radius cliff.

The evolution of exoplanetary atmospheres is strongly influenced by atmospheric escape, particularly for close-in planets. Fractionation during atmospheric loss can preferentially remove lighter elements such as hydrogen, while retaining heavier species like oxygen. In this study, we investigate how and under what conditions hydrodynamic escape and chemical fractionation jointly shape the mass and composition of exoplanet atmospheres, especially for mixed H2+H2O atmospheres. We develop BOREAS, a self-consistent mass-loss model coupling a one-dimensional Parker wind formulation with a mass-dependent fractionation scheme, which we apply across a range of planet masses, radii, equilibrium temperatures, and incident X-ray and ultraviolet (XUV) fluxes, allowing us to track hydrogen and oxygen escape rates at different snapshots in time. We find that oxygen is efficiently retained over most of the parameter space. Significant oxygen loss occurs under high incident XUV fluxes, while at intermediate fluxes oxygen loss is largely confined to low-gravity planets. Where oxygen is retained, irradiation is too weak to drive significant escape of hydrogen, thus limiting atmospheric enrichment. By contrast, our model predicts that sub-Neptunes undergo substantial atmospheric enrichment over ∼200 Myr when hydrogen escape is efficient and accompanied by partial oxygen entrainment. Notably, our results imply that sub-Neptunes near the radius valley can evolve into water-rich planets, in agreement with GJ 9827d. Present-day water-rich atmospheres may have originated from water-poor envelopes under some conditions, highlighting the need to include chemical fractionation in evolution models. BOREAS is publicly available.

We explore the evolution of sub-Neptune (radii between ∼1.5 and 4 R⊕) exoplanet interior structures using our upgraded evolution code, APPLE, which self-consistently couples the thermal and compositional evolution of the whole structure. We incorporate stably stratified regions with convective mixing and, for the first time, ab initio results on the phase separation of silicate-hydrogen mixtures to model silicate rain in sub-Neptune envelopes. We demonstrate that inefficient mantle cooling can retain sufficient heat to Gyr ages: inefficient heat transport from mantle to envelope alone keeps radii ∼10% larger than predicted by adiabatic models at late times. Silicate rain can contribute an additional ∼5% to the radius, depending on envelope mass and initial metal abundance. The silicate-hydrogen immiscibility region may lie in the middle or even upper envelope, far above the envelope–mantle boundary layer, and bifurcates the envelope into two an upper, hydrogen-rich region and a lower, metal-rich region above the mantle. If silicate rain occurs, atmospheres should appear depleted of silicates while radii remain inflated at late ages. To demonstrate the effects of inefficient mantle cooling, we present interior evolution models for GJ 1214 b, K2-18 b, TOI-270 d, and TOI-1801 b, showing that hot, liquid silicate mantles with thin envelopes reproduce their radii and mean densities, providing an alternative to water-world interpretations. These results imply that bulk compositions inferred from mean density must account for the mantle thermal state and the envelope mixing/phase-separation history; such thermal “memories” may constrain formation entropies and temperatures when metallicities are more precisely measured.

The dayside atmospheres of the hottest ultrahot 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 dayside emission spectrum of WASP-121b—one of the hottest known UHJs—covering a broad wavelength range of ∼0.6–5.1 μm, based on archival JWST observations from NIRISS and NIRSpec/G396H. We report statistically significant detections of several key molecular species, including H2O (13.4σ), CO (14.7σ), SiO (4.9σ), TiO (5.4σ), and VO (6.6σ), establishing WASP-121b as one of the most thoroughly characterized exoplanetary atmospheres to date. Additionally, we present the robust detection of titanate (CaTiO3) clouds at 6.7σ—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 ± 0.024, a subsolar Si/O ratio of 0.034 ± 0.024, and a metallicity of 4.7 −1.38+1.99 ×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.

Planet formation impacts exoplanet atmospheres by accreting metals in solid form, leading to atmospheric carbon-to-oxygen ratios (C/O) and sulfur-to-nitrogen ratios (S/N) that deviate from those of their host stars. Recent observations indicate differing metal abundances in planetary atmospheres compared to their stellar companions. However, these observations are biased toward mature planets, raising questions about whether these abundances result from formation or evolved over time. Another way to alter an atmosphere is through the escape of particles due to thermal heating. This study examines how billions of years of particle escape affect metal abundances. Using an adjusted stellar evolution code incorporating hydrodynamic escape, we model a warm (Teq ≈ 1000 K) super-Neptune-type planet (Mini = 26M⊕) orbiting a solar-type star. Our results show increased metal-to-hydrogen abundances of ∼50–70 × initial enrichment after 10 Gyr. We also see a 0.88 × decrease in C/O abundance and a 1.27 × increase in S/N abundance, which can affect the interpretation of planet formation parameters. We also simulate the evolving atmosphere using chemical kinetics and radiative transfer codes, finding substantial increases in SO2, CO2, and H2O abundances and a decrease in CH4 abundance. These changes are easily observable in the IR wave band transmission spectrum. Our findings demonstrate that extreme escape of lighter particles significantly influences the evolution of warm Neptunes and complicates the interpretation of their observational data. This highlights the need to consider long-term atmospheric evolution in understanding exoplanet compositions.

Terrestrial planets in the habitable zone (HZ) of Sun-like stars are priority targets for detection and observation by the next generation of space telescopes. Earth's long-term habitability may have been tied to the geological carbon cycle, a process critically facilitated by plate tectonics. In the modern Earth, plate motion corresponds to a mantle convection regime called mobile lid. The alternate, stagnant-lid regime is found on Mars and Venus, which may have lacked strong enough weathering feedback to sustain surface liquid water over geological timescales if initially present. Constraining observational strategies able to infer the most common regime in terrestrial exoplanets requires quantitative predictions of the atmospheric composition of planets in either regime. We use end-member models of volcanic outgassing and crust weathering for the stagnant- and mobile-lid convection regimes, which we couple to models of atmospheric chemistry and climate and ocean chemistry to simulate the atmospheric evolution of these worlds in the HZ. In our simulations under the two alternate regimes, we find that the fraction of planets possessing climates consistent with surface liquid water is virtually the same. Despite this unexpected similarity, we predict that a mission capable of detecting atmospheric CO2 abundance above 0.1 bar in 25 terrestrial exoplanets is extremely likely (≥95% of samples) to infer the dominant interior convection regime in that sample with strong evidence (10:1 odds). This offers guidance for the specifications of the Habitable Worlds Observatory NASA concept mission and other future missions capable of probing samples of habitable exoplanets.