  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 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 H2 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 ~3-5 microns are ~20 orders of
magnitude larger than blackbody expectations. We report a search of the AllWISE
Source Catalog for Planet Nine, but find no detection.
