Rogue Moons: Warmth Without a Star ✎ express
Rogue planets, ejected from star systems, may harbor life on their moons. A moon of such a planet can preserve heat and liquid water for billions of years. The whole secret lies in a reliable "blanket" and an internal "stove."
But without a blanket, heat escapes. Carbon dioxide is a poor insulator: it freezes out too easily. A dense hydrogen envelope acts like a down comforter, trapping heat. This allows liquid water to exist for up to 4.3 billion years.
Add a bit of carbon, nitrogen, and ammonia to the atmosphere, and you get an alkaline environment, perfect for the chemistry of life. Even in the pitch darkness of interstellar space, warm worlds can bloom.
The hydrogen "fur coat" of such a moon can weigh as much as several Earth oceans, yet remain transparent — on the surface, eternal twilight, but warm.