Moons of rogue planets may sustain liquid oceans for billions of years

A new study suggests that moons orbiting rogue planets—worlds that drift through space without a parent star—could remain warm enough to support liquid water for billions of years, potentially offering unexpected havens for life. Researchers at Ludwig Maximilian University of Munich calculated that such moons, heated only by the planet's internal energy and tidal forces, might sustain subsurface oceans for up to 4.3 billion years, roughly the age of Earth. This finding challenges the long-held assumption that life requires a star's warmth and light.

The study, published in 2026, focuses on moons orbiting gas giants that have been ejected from their solar systems. Without a sun, these planets would cool over time, but their moons could be kept warm by the planet's gravitational pull, which generates heat through tidal flexing—the same process that drives volcanic activity on Jupiter's moon Io. The researchers modeled how long a moon could maintain a liquid ocean beneath an icy crust, factoring in the planet's mass, the moon's orbit, and the gradual decay of tidal heating.

If such moons exist, they could be among the most common habitats in the galaxy. Rogue planets are thought to outnumber stars, and many may have formed moons before being flung into interstellar space. The team's calculations show that a moon with a thick ice layer and a rocky core could retain enough internal heat to keep water liquid for billions of years, even without starlight. This timescale is comparable to the time it took for life to emerge on Earth.

The next step, the researchers say, is to search for signs of such moons. Future telescopes might detect the faint infrared glow of a rogue planet's moon, or the plumes of water vapor erupting from its surface. If found, these worlds could become prime targets in the hunt for extraterrestrial life, proving that life might not need a sun after all.