Alien spacecraft have entered orbit around Earth. They have launched a few probes for closer observations, but made no attempt to land. Figuring out meaningful communication is an ongoing process, but observations of the ships indicate that they seem to keep the interiors at around -150C and 20 atmospheres, flooded with nitrogen and traces of liquid oxygen. If that's the environment they prefer, no wonder they haven't tried to land! Earth would be like Mercury to them--way too hot, and practically no air!

Nevertheless, it is obvious that these aliens are animal-like creatures with access to a decent amount of metabolic energy for thinking (after all, they figured out how to build an interstellar spacecraft) and moving themselves around.

Given what we know so far, what can we extrapolate about the nature of their homeworld? What circumstances could keep a planetary surface that cold, end yet maximize the energy flux available to power an ecosystem that can support complex animals?