Exoplanets & Exomoons

On The Stability Of Low-mass Planets With Supercritical Hydrospheres

By Keith Cowing
April 17, 2022
Filed under
On The Stability Of Low-mass Planets With Supercritical Hydrospheres
Computed planetary radii Rp at the transiting depth Ptr = 20mbar as a function of planetary mass Mp and irradiation temperature Tirr. Contours show lines of constant planetary radii (in units of Earth radius) to improve the readability of each panel. Left, center and right columns correspond to CMF values that represent pure mantle (0.0), Earth-like (0.325) and Mercury-like (0.7), respectively. Top, middle and bottom rows correspond to WMFs of 5%, 1% and 0.01%, respectively. The missing data correspond to cases excluded from our calculations, due to hydrostatic instability.

Short-period and low-mass water-rich planets are subject to strong irradiation from their host star, resulting in hydrospheres in supercritical state. In this context, we explore the role of irradiation on small terrestrial planets that are moderately wet in the low-mass regime (0.2–1M⊕).

We investigate their bulk properties for water contents 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 contents, 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 and low-mass planets up to 0.9M⊕ with significative hydrospheres are not in stable form and should loose their volatile envelope.

Hugo Vivien, Artyom Aguichine, Olivier Mousis, Magali Deleuil, Emmanuel Marcq

Comments: Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2204.07451 [astro-ph.EP] (or arXiv:2204.07451v1 [astro-ph.EP] for this version)
Submission history
From: Hugo Vivien
[v1] Fri, 15 Apr 2022 13:08:48 UTC (358 KB)


Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻