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Abstract:

Ultrashort-period (USP) exoplanets—with Rp ≤ 2R⊕ and periods ≤1 day—are expected to be stripped of volatile atmospheres by intense host star irradiation, which is corroborated by their nominal bulk densities and previous eclipse observations, consistent with bare-rock surfaces. However, a few USP planets appear anomalously underdense relative to an Earth-like composition, suggesting an exotic interior structure (e.g., coreless) or a volatile-rich secondary atmosphere increasing their apparent radius. Here, we present the first dayside emission spectrum of the low-density (4.3 ± 0.4 g cm−3) USP planet TOI-561 b, which orbits an iron-poor, alpha-rich, ∼10 Gyr old thick-disk star. Our 3–5 μm JWST/NIRSpec observations demonstrate the dayside of TOI-561 b is inconsistent with a bare-rock surface at high statistical significance, suggesting instead a thick volatile envelope that is cooling the dayside to well below the ∼3000 K expected in the bare-rock or thin-atmosphere case. These results reject the popular hypothesis of complete atmospheric desiccation for highly irradiated exoplanets and support predictions that planetary-scale magma oceans can retain substantial reservoirs of volatiles, opening up the geophysical study of ultrahot super-Earths through the lenses of their atmospheres.

Abstract:

Atmospheric convection behaves differently in hydrogen-rich atmospheres compared to higher mean molecular weight atmospheres due to compositional gradients of tracers. Previous 1D studies predict that when a condensable tracer exceeds a critical mixing ratio in H2-rich atmospheres, convection is inhibited, leading to the formation of radiative layers where the temperature decreases faster with height than in convective profiles. We use 3D convection-resolving simulations to test whether convection is inhibited in H2-rich atmospheres when the tracer mixing ratio exceeds the critical threshold, while including processes neglected in 1D, e.g., turbulent mixing and evaporation. We run two sets of simulations. First, we perform simulations initialized on saturated isothermal states and find that compositional gradients can destabilize isothermal atmospheres. Second, we perform simulations initialized on adiabatic profiles, which show distinct, stable inhibition layers form when the condensable tracer exceeds the critical threshold. Within the inhibition layer, only a small amount of energy is carried by latent heat flux, and turbulent mixing transports a small amount of tracer upward, but both are generally too weak to sustain substantial tracer or heat transport. The thermal profile gradually relaxes to a steep radiative state, but radiative relaxation timescales are long. Our results suggest stable layers driven by condensation-induced convective inhibition form in H2-rich atmospheres, including those of sub-Neptune exoplanets.

Abstract:

This dataset provides horizontal flow fields (velocity, relative vorticity, divergence, and shear strain rate) from nine rotating turbulence experiments carried out at TurLab at the University of Turin between November 2016 and February 2017 by a team led by Oxford scientists in collaboration with others at the time from Università degli Studi di Torino, University of South Florida, Università del Piemonte Orientale and Sapienza Università di Roma. The experimental device was a rotating cylinder, 5 m in diameter filled with fresh water to a depth of 56 cm. The tank had a conical bottom, sloping upwards towards the centre from the outer edge over a radius of 2.25 m at an angle of 11.1◦. The flow was mechanically forced by a moving comb of vertical paddles partially immersed in the water, which moved backwards and forwards along a near-radial line over 100–150 s. The resulting flow was visualised using a laser sheet and microscopic particles, imaged using two cameras, and then processed into horizontal velocity fields and derived quantities using UVMAT/CIV (http://servforge.legi. grenoble-inp.fr/projects/soft-uvmat). The main quantity varied between the experiments included in this dataset is the rotation period of the tank. There were minor differences in the comb configuration at each rotation period. †