"The cradle of life does not necessarily require a sun."

The Hydrogen-rich exomoon atmospheres could hold liquid water for billions of years.

A moon lost in interstellar darkness may still be able to hold on to liquid water, no star required. A modeling study suggests that some exomoons orbiting rogue planets could stay warm for billions of years, even after their host worlds are thrown out of their star systems and into the cold of deep space.

The finding challenges the familiar idea that potentially habitable worlds must receive light from a nearby star. Instead, the study points to a different engine for habitability — heat generated inside a moon by tidal stress, then trapped by a thick hydrogen atmosphere.

Rogue planets drift through space without orbiting a star. Astronomers think many of them were thrown out of their original planetary systems during violent gravitational encounters.

When a giant planet gets ejected, the orbits of its moons can become stretched into more elongated shapes. As the moon moves closer to and farther from its host planet, the planet’s gravity repeatedly squeezes and flexes the moon’s interior.

That process, called tidal heating, can generate a steady supply of internal heat. Jupiter’s moon Io offers a familiar example: tidal forces make it the most volcanically active body in the solar system. Europa and Enceladus also show how internal heat can keep water-rich worlds active far from the sun.

Earlier studies had already suggested that rogue-planet moons might stay warm enough for liquid water if they built up thick carbon dioxide atmospheres.

But those models ran into a serious problem. In the deep cold expected around free-floating planets, carbon dioxide can condense out of the air. Once that happens, the atmosphere can collapse, and much of the moon’s heat escapes into space.

The new study tackles that problem by swapping carbon dioxide for hydrogen. Hydrogen is the lightest element, and in Earth’s thin atmosphere it does very little to trap heat.

Under very high pressure, though, hydrogen behaves differently. When hydrogen molecules crowd together and collide, they briefly form pairs and clusters that can absorb infrared radiation.

Scientists call this process collision-induced absorption. In plain terms, a dense hydrogen atmosphere can act like a surprisingly strong heat blanket.