On the Stability of Low-mass Planets with Supercritical Hydrospheres

• Published in: The Astrophysical journal 2022-06, Vol.931 (2), p.143
• Main Authors: Vivien, H. G., Aguichine, A., Mousis, O., Deleuil, M., Marcq, E.
• Format: Article
• Language: English
• Subjects: Astrophysics; Atmosphere; Atmospheric models; Exoplanet astronomy; Exoplanet atmospheric composition; Exoplanet structure; Extrasolar planets; Hydrosphere; Irradiation; Moisture content; Planetary atmospheres; Planetary structure; Planets; Scale height; Sciences of the Universe; Terrestrial environments; Terrestrial planets; Water content
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac66e2

Short-period, low-mass water-rich planets are subject to strong irradiation from their host star, resulting in hydrospheres in a supercritical state. In this context, we explore the role of irradiation on small terrestrial planets that are moderately wet in the low-mass regime (0.2–1 M ⊕ ). We investigate their bulk properties for water content in the 0.01–5% range by making use of an internal structure model that is coupled to an atmosphere model. This coupling allows us to take into account both the compression of the interior due to the weight of the hydrosphere and the possibility of atmospheric instability in the low-mass regime. We show that, even for low masses and low water content, these planets display inflated atmospheres. For extremely low planetary masses and high irradiation temperatures, we find that steam atmospheres become gravitationally unstable when the ratio η of their scale height to planetary radius exceeds a critical value of ∼0.1. This result is supported by observational data, as all currently detected exoplanets exhibit values of η smaller than 0.013. Depending on their water content, our results show that highly irradiated, low-mass planets up to 0.9 M ⊕ with significative hydrospheres are not in a stable form and should lose their volatile envelope.