Exoplanets & Exomoons

The Imprint of Escaping Hydrogen Atmospheres on Super-Earth Interiors

By Keith Cowing
Status Report
February 23, 2024
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The Imprint of Escaping Hydrogen Atmospheres on Super-Earth Interiors
Our super-Earth models are shown in dark-blue circles, compared with a sample of observed exoplanets from the NASA Exoplanet archive with masses and radii constrained to 20% and 5% respectively. In the left-hand panel, we show the mass-radius diagram, whereas, in the right-hand panel, we show normalised density as a function of planet mass, which is the density of each planet when scaled to be 1M⊕. Grey lines represent an Earth-like density of ∼ 5.5 g cm−1 . The blue-shaded region is the inferred 1σ range in interior density from the population-level evolution analysis from Rogers & Owen (2021). The orange-shaded region is the range of plausible sizes (or densities) of sub-Neptunes hosting significant hydrogen-dominated atmospheres from Rogers et al. (2023c), which accounts for thermal evolution, including cooling and mass loss. — astro-ph.EP

Small, close-in exoplanets are divided into two sub-populations: super-Earths and sub-Neptunes. Most super-Earths are thought to have lost their primordially accreted hydrogen-dominated atmospheres via thermally driven winds.

We consider the global chemical equilibrium of super-Earths and the lasting impacts of their fleeting hydrogen atmospheres. We find that hydrogen is efficiently sequestered into the interior, oxidising iron and endogenously producing ∼0.5−1.0% water by mass.

As the atmospheres of super-Earths are continuously sculpted by mass loss and chemical equilibration, they remain hydrogen-dominated by mole (number) fraction but become steam-dominated by mass, which may be observable with JWST for planets transitioning across the radius valley.

One of the main effects of efficient sequestration of hydrogen into the interior is to produce an under-dense bulk interior compared to that of Earth. We predict bulk densities of super-Earths to be ∼5.0 g cm−3 for a 1M⊕ planet, which is consistent with high-precision mass measurements and also population-level inference analyses from atmospheric escape models.

James G. Rogers, Hilke E. Schlichting, Edward E. Young

Comments: 14 pages, 4 figures. Submitted to ApJ. Comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2402.14072 [astro-ph.EP] (or arXiv:2402.14072v1 [astro-ph.EP] for this version)
Submission history
From: James Rogers
[v1] Wed, 21 Feb 2024 19:01:02 UTC (316 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) 🖖🏻