We imagine Planet Nine as a distant icy world with hidden oceans. At 300 to 800 AU from the Sun the surface would be near 30 to 50 K and nitrogen would be frozen. Beneath an ice crust tens to hundreds of kilometres thick a liquid ocean could persist if internal heating balances conductive heat loss through the ice.
The most powerful internal heat source in this setting combines two effects working together: a Binary Planet configuration that maintains strong mutual tides and Distant Orbital Resonances that keep the binary orbits slightly eccentric over gigayear timescales.
# The Subsurface Ocean With continued heating the thermal structure stabilises into layers. A cold brittle lid at the top. A convecting warm ice layer below that. Then a liquid ocean tens to more than one hundred kilometres thick. Under the ocean a stack of high pressure ice polymorphs sits on a rocky core.
At the rock water interface hydrothermal systems produce hydrogen and methane. Redox gradients between vent effluents and oxidants cycled through the ice shell can power chemosynthetic ecosystems.
Periodic cracking of the surface opens conduits that vent vapour plumes. Those plumes provide samples of the interior without drilling.
# Calculation: how large and how close? We provide an order of magnitude calculation for the tidal heat required to keep water liquid beneath an ice shell. The target is to meet or exceed the conductive heat loss through the ice.
# Implications for Exploration Probes would navigate a complex gravity field parentheses binary plus distant perturbers. Power would come from compact nuclear sources.
Reconnaissance would search for warm lineaments, fresh fractures and plume fallout. A follow on mission would fly through plumes to collect salts, organics and isotopic tracers.
In a narrative this world reads as a remote but viable habitat where life powered by geochemistry, oceans warmed by the slow dance of a binary orbit, and the orbit itself kept alive by faint tugs from the deep outer system.
# See
- Enceladus and Icy Moons - Under the Seas and the Encaladus Moon Base - Moon Power