# H-Atmospheres of Icy Super-Earths Formed in situ in the Outer Solar System: An Application to a Possible Planet Nine

## TL;DR

This study explores the formation of thick hydrogen-rich atmospheres on icy super-Earths formed in situ in the outer solar system, with implications for identifying Planet Nine's atmospheric origin.

## Contribution

It models the internal processes and atmospheric development of distant icy super-Earths, highlighting how outgassing influences atmospheric pressure and composition.

## Key findings

- Atmospheric base pressure varies with H2 outflux, from less than 1 bar to 10^3-10^4 bar.
- Planets have higher mean density (2.4-3 g/cm^3) than Uranus and Neptune.
- Observations can distinguish between primordial and outgassed atmospheres of Planet Nine.

## Abstract

We examine the possibility that icy super-Earth mass planets, formed over long time scales (0.1--1~Gyr) at large distances ($\sim$ 200--1000~AU) from their host stars, will develop massive H-rich atmospheres. Within the interior of these planets, high pressure converts CH$_4$ into ethane, butane, or diamond and releases H$_2$. Using simplified models which capture the basic physics of the internal structure, we show that the physical properties of the atmosphere depend on the outflux of H$_2$ from the mantle. When this outflux is $\lesssim 10^{10}$ [molec cm$^{-2}$ s$^{-1}$], the outgassed atmosphere has base pressure $\lesssim$ 1 bar. Larger outflows result in a substantial atmosphere where the base pressure may approach $10^3 - 10^4$ bar. For any pressure, the mean density of these planets, 2.4--3 [g cm$^{-3}$], is much larger than the mean density of Uranus and Neptune, 1.3--1.6 [g cm$^{-3}$]. Thus, observations can distinguish between a Planet Nine with a primordial H/He-rich atmosphere accreted from the protosolar nebula and one with an atmosphere outgassed from the core.