Howard Chen

Among potentially habitable worlds, rocky planets orbiting M dwarfs offer the most favorable prospects for atmospheric characterization, yet their climates may differ substantially from those of Earth analogs. In the tidally locked limit, the nightside’s tendency to radiatively cool and potentially trap volatiles as permanent ice introduces a strong dependence of habitability on the planet’s surface and atmospheric boundary conditions. We perform a suite of synchronously rotating exoplanet climate experiments spanning a series of topographic and orographic realizations with different mean elevation and landmass distributions. Across a grid of p _N2 = 0.5–8 bar and F _⋆ = 1000–1700 W m ⁻² , we find that surface relief breaks the flow symmetry, replacing circumpolar vortices with mechanically forced stationary waves. For example, Steep orography produces coherent standing Rossby gyres that strengthen the cross-terminator jet and align vertical uplift with the day/night boundary. These new circulation regimes enhance moisture transport, increasing the infrared optical depth and promoting additional nightside cloud formation, which produces stronger cloud–greenhouse feedback, and ultimately lower the critical stellar flux required for global planetary deglaciation. Broad elevated plateaus drive a similarly fragmented but slightly weaker circulation, yielding less effective moisture transport. These results show that the relief and spatial distribution of landmasses, parameters that are unconstrained for most exoplanets, can exert strong controls on the climatic bifurcations of tidally locked M dwarf exoplanets.

Exoplanet sub-Neptunes are nearly as common as stars, but their nature remains uncertain. Sub-Neptunes orbit close to their host stars, and many are apparently shrouded in large and hot hydrogen-dominated atmospheres. Harsh irradiation conditions and the presence of greenhouse gases enhance the melting of the underlying silicate surface and promote the exchange of volatiles between the atmosphere and a magma ocean. Here we show that magma oceans can dissolve ≥2 wt% hydrogen at typical conditions relevant to the interface between the atmosphere and the condensed surface in such planets. The influx of hydrogen into the molten silicate lowers magma density, which alters the sub-Neptune radius-mass relationship (equation of state). Moreover, the hydrogen influx changes the redox state of the magma ocean (which becomes reduced), and generates a massive outflux of oxygen, which reacts with atmospheric hydrogen to form large amounts of water vapor. Thus, the release of oxygen will profoundly change the atmospheric chemistry and bear spectral signatures observable from space telescopes. These atmospheric-composition changes are a testable signature of hidden magma oceans in exoplanets. The uptake of volatiles by the magma implies that the interiors of sub-Neptunes are more volatile-rich than once thought. Moreover, our atomistic simulations suggest that the dissolution and evaporation process is chemically complex, forming hundreds of molecular species in the atmosphere. These magma-buffered atmospheric recipes will need to be accounted for in future studies addressing models of exoplanet atmosphere origins.

It is still unclear whether exoplanets in compact multiplanet systems such as TRAPPIST-1 are able to accrete large quantities of volatiles, grow to sufficient mass, and maintain robust atmospheres and hydrospheres. Previous estimates of water content in M-dwarf systems have largely relied on population synthesis or atmosphere–interior evolution models, often treating impacts and atmospheric loss in isolation. In this work, we combined impact delivery, impact erosion, and mantle-atmosphere exchange within a unified framework that tracks volatile evolution through stochastic collision histories. By explicitly including both planetesimal accretion and the prolonged luminous pre-main-sequence phase of M dwarfs, we find systematically lower water inventories for the inner TRAPPIST-1 analogs (b–e), spanning only 10⁻⁴ and 1% of Earth’s ocean mass across a wide range of disk structures and impact scenarios. By contrast, the outer planets (f–h analogs) frequently retain water inventories exceeding an Earth ocean mass. This volatile gradient provides a physically motivated explanation for JWST’s nondetections of atmospheres on TRAPPIST-1 b and c, suggesting an origin rooted in rocky planet formation. Our results suggest that many rocky planets in compact M-dwarf systems may form already depleted in volatile compounds, fundamentally limiting their capacity to sustain atmospheres or surface oceans. More broadly, our multistage framework for volatile tracking can help interpret future observations of compact system and set more realistic initial conditions for exoplanet interior compositions and atmospheric models.

Exoplanet super-Earths and sub-Neptunes are likely more prevalent than stars, yet their nature remains uncertain. Notably, sub-Neptunes orbit in close proximity to their host stars, often enveloped in substantial and hot hydrogen-rich atmospheres. These conditions, characterized by intense irradiation and the presence of greenhouse gases, accelerate the melting of the underlying silicate surface and facilitate the exchange of volatiles with the atmosphere. In this study, we conduct extensive large-scale ab initio simulations to investigate the reaction between hydrogen gas and molten multi-component silicates. Our findings reveal that magma oceans possess the capability to dissolve up to 2 wt% hydrogen at typical conditions pertinent to the interface between the atmosphere and the condensed surface of such planets. The influx of hydrogen into the molten silicate results in a reduction in magma density, leading to the formation of a substantial buffer layer between the condensed interior and the outer atmosphere. This layer effectively suppresses convection and restricts chemical exchanges within these planets.Furthermore, the incorporation of hydrogen into the molten silicate alters the redox state of the magma ocean, causing it to become reduced. Consequently, a substantial outflux of oxygen is generated, which combines with atmospheric hydrogen to produce substantial amounts of water vapor. Consequently, the release of oxygen will profoundly alter the atmospheric chemistry and manifest spectral signatures detectable from space telescopes. These atmospheric composition modifications constitute a testable signature of concealed magma oceans on exoplanets. The uptake of volatiles by the magma implies that sub-Neptunes exhibit a higher volatility compared to previously assumed. Additionally, our atomistic simulations suggest that the dissolution and evaporation process is chemically intricate, resulting in the formation of hundreds of distinct molecular species within the atmosphere. These magma-buffered atmospheric recipes will necessitate consideration in future studies focused on the origins of exoplanet atmospheres.

Space weather events in exoplanetary environments sourced from transient host star emissions, including stellar flares, coronal mass ejections, and stellar proton events, can substantially influence a planet's habitability and atmospheric evolution history. These time-dependent events may also affect our ability to measure and interpret its properties by modulating reservoirs of key chemical compounds and changing the atmosphere’s brightness temperature. The majority of previous work focusing on photochemical effects, ground-level UV dosages, and consequences on observed spectra. Here, using three-dimensional general circulation models with interactive photochemistry, we simulate the climate and chemical impacts of stellar energetic particle events and periodic enhancements of UV photons. We use statistical methods to examine their effects on synchronously rotating TRAPPIST-1e-like planets on a range of spatiotemporal scales. We find that abrupt thermospheric cooling is associated with radiative cooling of NO and CO₂, and middle-to-lower atmospheric warming is associated with elevated infrared absorbers such as N₂O and H₂O. In certain regimes, in particular for climates around moderately active stars, atmospheric temperature changes are strongly affected by O₃ variability. Cumulative effects are largely determined by the flare frequency and the instantaneous effects are dependent on the flare’s spectral shape and energy. In addition to effects on planetary climate and atmospheric chemistry, we find that intense flares can energize the middle atmosphere, causing enhancements in wind velocities up to 40 m s⁻¹ in substellar nightsides between 30 and 50 km in altitude. Our results suggest that successive, more energetic eruptive events from younger stars may be a pivotal factor in determining the atmosphere dynamics of their planets.

White-dwarf (WD) systems are attractive candidates for planet detection and follow-up transmission spectroscopy campaigns. However, the climate and habitability of terrestrial planets around WDs still remain largely elusive, and a central question lies in the width and extent of their habitable zones (HZs) and how their climates may be shaped by their stellar environments. A recently published study, using a 3D general circulation model (GCM), found a novel atmospheric circulation pattern for ultrarapid rotators around WDs. This “bat-rotation” regime features mean equatorial subrotation and a displacement of the surface’s hottest regions from the equator toward the midlatitudes. Here, we used a GCM of reduced complexity to simulate a large range of landmass fraction (LMF) assumptions on these planets and examine the regimes in which these planets would exhibit the new dynamical regime. We find that as the dayside LMF approaches 80%, the bat-rotator features begin to diminish and the circulation behavior becomes more akin to the thermally driven direction circulation patterns observed around slowly rotating planets. Our results suggest that the introduction of different surface characteristics would drastically modify previous predictions of the atmospheres of ultrashort-period planets in the HZ of WD stars.