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

We examine the possibility that icy super-Earth mass planets, formed over long timescales (0.1-1 Gyr) at large distances (∼200-1000 au) from their host stars, will develop massive H-rich atmospheres. Within the interior of these planets, high pressure converts CH₄ into ethane, butane, or diamond and releases H₂. Using simplified models that capture the basic physics of the internal structure, we show that the physical properties of the atmosphere depend on the outflux of H₂ from the mantle. When this outflux is molec cm^-2 s^-1, the outgassed atmosphere has a base pressure of ≲1 bar. Larger outflows result in a substantial atmosphere where the base pressure may approach 10³-10⁴ 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.