Water on Hot Rocky Exoplanets

Edwin S. Kite; Laura Schaefer

doi:10.3847/2041-8213/abe7dc

paper

Water on Hot Rocky Exoplanets

arXiv:2103.07753 · doi:10.3847/2041-8213/abe7dc

Abstract

Data suggest that most rocky exoplanets with orbital period $p$ $<$ 100 d ("hot" rocky exoplanets) formed as gas-rich sub-Neptunes that subsequently lost most of their envelopes, but whether these rocky exoplanets still have atmospheres is unknown. We identify a pathway by which 1-1.7 $R_{E a r t h}$ (1-10 $M_{E a r t h}$ ) rocky exoplanets with orbital periods of 10-100 days can acquire long-lived 10-2000 bar atmospheres that are H $_{2}$ O-dominated, with mean molecular weight $>$ 10. These atmospheres form during the planets' evolution from sub-Neptunes into rocky exoplanets. H $_{2}$ O that is made by reduction of iron oxides in the silicate magma is highly soluble in the magma, forming a dissolved reservoir that is protected from loss so long as the H $_{2}$ -dominated atmosphere persists. The large size of the dissolved reservoir buffers the H $_{2}$ O atmosphere against loss after the H $_{2}$ has dispersed. Within our model, a long-lived, water-dominated atmosphere is a common outcome for efficient interaction between a nebula-derived atmosphere (peak atmosphere mass fraction 0.1-0.6 wt%) and oxidized magma ( $>$ 5 wt% FeO), followed by atmospheric loss. This idea predicts that most rocky planets that have orbital periods of 10-100 days and that have radii within 0.1-0.2 $R_{E a r t h}$ of the lower edge of the radius valley still retain H $_{2}$ O atmospheres. This prediction is imminently testable with JWST and has implications for the interpretation of data for transiting super-Earths.

Astrophysical Journal Letters, in press

Water on Hot Rocky Exoplanets

Abstract

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