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 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 $\lesssim 10^{10}$[moleccm$^{-2}$s$^{-1}$],theoutgassedatmospherehasbasepressure$\lesssim$ 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.