https://www.scientificamerican.com/article/what-prevents-plutos-ocean-from-freezing/

Buried oceans like the one thought to slosh beneath the icy surface of the dwarf planet Pluto may be incredibly common across the cosmos.

A gassy insulating layer probably keeps Pluto’s liquid-water ocean from freezing solid, a new study reports. And something similar could be happening under the surfaces of frigid worlds in other solar systems as well, study team members said.