The Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability
Jonathan J. Fortney, Mark S. Marley, Gregory Laughlin, Nadine, Nettelmann, Caroline V. Morley, Roxana E. Lupu, Channon Visscher, Pavle, Jeremic, Wade G. Khadder, Mason Hargrave

TL;DR
This study models the atmosphere, spectra, and detectability of proposed Planet Nine, revealing its potential temperature, size, atmospheric composition, and the challenges in observing it with current infrared surveys.
Contribution
It provides the first detailed atmospheric and spectral models of Planet Nine, highlighting its potential observability and unique atmospheric features based on plausible interior structures.
Findings
Planet Nine's intrinsic temperature likely between 35-50 K.
Atmosphere may be Rayleigh scattering with high albedo (~0.75).
Thermal fluxes at 3-5 microns are vastly higher than blackbody predictions.
Abstract
We investigate the physical characteristics of the Solar System's proposed Planet Nine using modeling tools with a heritage in studying Uranus and Neptune. For a range of plausible masses and interior structures, we find upper limits on the intrinsic Teff, from ~35-50 K for masses of 5-20 M_Earth, and we also explore lower Teff values. Possible planetary radii could readily span from 3 to 6 R_Earth depending on the mass fraction of any H/He envelope. Given its cold temperature, the planet encounters significant methane condensation, which dramatically alters the atmosphere away from simple Neptune-like expectations. We find the atmosphere is strongly depleted in molecular absorption at visible wavelengths, suggesting a Rayleigh scattering atmosphere with a high geometric albedo approaching 0.75. We highlight two diagnostics for the atmosphere's temperature structure, the first being the…
Peer Reviews
No public reviews on file for this paper yet. If you reviewed it on a platform where reviews are public (OpenReview, ICLR, NeurIPS, ICML), you can paste yours below so the community can read it here.
Videos
No videos yet. Explain this paper in a talk, walkthrough, or lecture? Add one.
Taxonomy
TopicsStellar, planetary, and galactic studies · Astronomy and Astrophysical Research
