Planetary Climate Dynamics

Abstract:

Abstract Ultrashort period (USP) rocky planets are expected to be depleted of any substantial gaseous envelope due to the intense irradiation they receive from their host star, making the recent detection of an atmosphere around TOI-561 b particularly surprising. That finding was based on the planet’s bulk density and dayside emission spectrum, but full-orbit phase-curve observations offer a more comprehensive way to constrain the presence and characteristics of a planet’s atmosphere. In this paper, we map TOI-561 b’s 3–5  μ m emission using JWST/NIRSpec multiorbit spectroscopic phase curves, explicitly accounting for the curvature of the out-of-eclipse baseline and possible hotspot offsets. From a simple energy balance argument as well as comparing to general circulation models, we find the phase-curve observations are best explained with high Bond albedo and moderate global heat transfer. Our 37 hr continuous observation provides a long baseline that allows us to model stellar granulation, the planetary phase curve, and dayside emission simultaneously, effectively disentangling these signals and yielding a dayside emission spectrum that is more robust against stellar variability and consistent with previous eclipse-only fits. Using general circulation model outputs, we constrain where clouds can plausibly form and test candidate compositions, finding that silicate clouds, such as SiO 2 and MgSiO 3 , can form on the dayside near the terminator and explain the observed albedo. Our findings confirm that TOI-561 b appears to have a global reflective atmosphere, suggesting exchange of volatiles with the interior to maintain it.

Abstract:

Context. The earliest rocky planet atmospheres were shaped by the competition between initial volatile inventories and atmospheric escape driven by stellar irradiation. On young magma ocean planets, volatile exchange between the atmosphere and the interior through in- and outgassing competes with atmospheric escape, controlling volatile retention and atmospheric evolution. Aims. We investigate how atmospheric escape and replenishment via outgassing during magma ocean crystallization shape rocky exoplanet atmospheres. Methods. We extended a coupled interior-atmosphere model to simulate rocky planet evolution during the magma ocean era by incorporating an energy-limited atmospheric escape module. Comparing self-consistent radiative-convective and prescribed-convective atmospheres, we quantified how atmospheric energy transport affects escape. We explored a wide range of orbital separations, escape efficiencies, oxidation states, and initial volatile inventories to identify regimes where sustained magma-ocean outgassing or escape dominates. We estimated atmospheric mass loss and compositions for young rocky planets around Sun-like and M dwarf stars over geologic timescales. Results. Atmospheric escape shortens magma ocean lifetimes by weakening greenhouse insulation and accelerating cooling, with radiative-convective atmospheres reducing solidification timescales compared to purely convective cases. Volatile dissolution into the magma ocean interacts with the escape of atmospheric species to chemically fractionate the planetary volatile budget over time by retaining more soluble species. For Earth-mass planets, atmospheres survive if mass-loss rates remain moderate ( Ṁ EL ≈ 10 10 g s −1 ). When atmospheric escape is included, the mantle redox state remains a key control on atmospheric composition: high oxygen fugacity ( f O 2 ) yields heavier H 2 O- and CO 2 -rich atmospheres, while low f O 2 produces light H 2 - or CO-dominated atmospheres, consistent with previous studies. Orbital separation, initial volatile inventory, and stellar type produce diverse evolutionary pathways, from bare rocky planets to magma oceans with thick atmospheres, spanning compositions from H 2 - to SO 2 -dominated atmospheres.