Planet Nine Climate Insights and Observational Constraints from Enhanced Atmospheric Modeling of Cold Sub-Neptunes

We present atmospheric simulations of an as-yet undiscovered planet which we refer to as Planet Nine (a.k.a., Planet X), a sub-Neptune hypothesized to be orbiting the Sun at a distance beyond 300 AU. We substantively improve our understanding of climate for this unusual object by (a) making use of model improvements which include the addition of a moist adiabat and methane cloud physics, and (b) incorporating updated constraints on Planet Nine orbital parameters. These advances allow us to provide new observational constraints to assist in the search for Planet Nine, including apparent magnitudes in a collection of relevant photometric filters. We describe how observations of Planet Nine, if discovered, may be used to constrain aspects of condensate processes and atmospheric structure such as fractional cloud coverage.