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

32 pages, 11 figures, accepted to ApJ

References in corpus (15)

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• Making Planet Nine: A Scattered Giant in the Outer Solar System
• Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring
• Evolution and Magnitudes of Candidate Planet Nine
• Structure and Dynamics of Cold Water Super-Earths: The Case of Occluded CH4 and its Outgassing
• Blackbody Radiation from Isolated Neptunes
• Methane Planets and their Mass-Radius Relation

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• Growth Model Interpretation of Planet Size Distribution
• New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State
• Laboratory studies on the viability of life in H${}_2$-dominated exoplanet atmospheres
• So close, so different: characterization of the K2-36 planetary system with HARPS-N
• Chaotic Dynamics of Trans-Neptunian Objects Perturbed by Planet Nine
• Tetrahedral hydrocarbon nanoparticles in space: X-ray spectra