Water on Hot Rocky Exoplanets

Abstract

Data suggest that most rocky exoplanets with orbital period p < 100 days (“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 ⊕ (1–10 M ⊕) rocky exoplanets with orbital periods of 10–100 days can acquire long-lived 10–2000 bar atmospheres that are H2O-dominated, with mean molecular weight >10. These atmospheres form during the planets’ evolution from sub-Neptunes into rocky exoplanets. H2O 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 H2-dominated atmosphere persists. The large size of the dissolved reservoir buffers the H2O atmosphere against loss after the H2 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 ⊕ of the lower edge of the radius valley still retain H2O atmospheres. This prediction is imminently testable with James Webb Space Telescope and has implications for the interpretation of data for transiting super-Earths.