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 T _ { eff } , from \sim 35-50 K for masses of 5-20 M _ { \oplus } , and we also explore lower T _ { eff } values .
Possible planetary radii could readily span from 2.7 to 6 R _ { \oplus } depending on the mass fraction of any H/He envelope .
Given its cold atmospheric temperatures , 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 value of the methane mixing ratio above the methane cloud .
The second is the wavelength at which cloud scattering can be seen , which yields the cloud-top pressure .
Surface reflection may be seen if the atmosphere is thin .
Due to collision-induced opacity of H _ { 2 } in the infrared , the planet would be extremely blue ( instead of red ) in the shortest wavelength WISE colors if methane is depleted , and would , in some cases , exist on the verge of detectability by WISE .
For a range of models , thermal fluxes from \sim 3 - 5 \mu m are \sim 20 orders of magnitude larger than blackbody expectations .
We report a search of the AllWISE Source Catalog for Planet Nine , but find no detection .