Exoplanetology: Exoplanets & Exomoons

Water-rich sub-Neptunes and Rocky Super Earths Around Different Stars: Radii Shaped by Volatile Partitioning, Formation, and Evolution

By Keith Cowing

Status Report

astro-ph.EP

November 28, 2024

Filed under astro-ph.EP, astrochemistry, astrogeology, exoplanet, sub-Neptune, super- Earth

Water-rich sub-Neptunes and Rocky Super Earths Around Different Stars: Radii Shaped by Volatile Partitioning, Formation, and Evolution — Mass versus radius diagram of observed and synthetic planets. The four panels show the resulting planets using different model variations contrasted against the observational data from Parc et al. (2024). The synthetic data for all models is sampled according to the stellar mass distribution of the observations and an estimate of the detection and transit probability is applied. More saturated dots imply that the planet was sampled multiple times due to being more likely to be detected and their color is given by the planets’ water mass fraction. — astro-ph.EP

The nature of sub-Neptunes remains unknown due to degeneracies in interior structure solutions. However, a statistical set of small planets with measured masses and radii has been compiled. It can be used to test the prediction of large water reservoirs on sub-Neptunes by planet formation theory.

We want to find out whether this water reservoir is included in photoevaporative winds and how much of it can partition into the rocky and metallic interior. We couple the result of a planetary formation model to evolution models which assume perfect mixing of water with H/He in the envelope or complete segregation. For the mixed envelopes, we also include fractionation during photoevaporative mass-loss.

Further, the effect of equilibrium dissolution of water into an assumed magma ocean and into the metallic core is studied for the first time in coupled formation-evolution models. Out of the tested scenarios, the mass-radius relation of exoplanets is best matched under the mixed assumption without water sequestration to the interior.

We quantify the radius valley location and scaling with stellar mass. Fractionation is not found to significantly alter the composition of the planets for our initial conditions due to initially massive envelopes on all planets. In contrast, water sequestration has a profound effect on the radius evolution and compositional budget of the planets.

The model predicts the preservation of large quantities of water even if the gaseous envelope is lost. Planets with corresponding bulk densities are not observed in comparably large numbers. By combining formation and evolution model, we probe a parameter space favored by core accretion theory.

We conclude that the dissolution of different volatiles into the planetary interior and solidification of the magma ocean are natural next steps for comprehensive treatment of atmosphere-interior interaction. (abridged)

Remo Burn, Komal Bali, Caroline Dorn, Rafael Luque, Simon L. Grimm

Comments: 25 pages, 17 figures, submitted to A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2411.16879 [astro-ph.EP] (or arXiv:2411.16879v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2411.16879
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Submission history
From: Remo Burn
[v1] Mon, 25 Nov 2024 19:18:53 UTC (14,772 KB)
https://arxiv.org/abs/2411.16879
Astrobiology,

Keith Cowing

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) 🖖🏻

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