A Moon With No Sun Could Keep Water Liquid for 4.3 Billion Years, Scientists Say

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.

Heat without sunlight

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. You’d be tempted to think that’s the end of the story; a cold, dark planet wandering through the galaxy with no sunlight to warm it.

But these planets could have very interesting moons.

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.

Why hydrogen works better

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.

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The researchers modeled an Earth-sized moon orbiting a Jupiter-like rogue planet. They used HELIOS, a radiative transfer code that simulates how heat moves through an atmosphere, and GGchem, a chemistry model that tracks which gases stay aloft and which condense out. Combining these calculations with previous simulations of changing moon orbits allowed the team to estimate how long tidal heating could maintain temperatures suitable for surface liquid water.

Their results show a striking jump in survival time for habitable conditions. In the thickest atmospheres they tested (up to 100 bars, or about 100 times Earth’s surface pressure) some moons could keep surface temperatures suitable for liquid water for as long as 4.3 billion years.

Among the 6,945 simulated Earth-mass moons that survived the host planet’s ejection, 43 percent reached temperatures compatible with liquid water at some point in their evolution under one particular 100-bar atmospheric scenario. Importantly, this is a result from a simulated population, not an estimate that 43 percent of real rogue exomoons are habitable.

The maximum duration also exceeded the roughly 1.6 billion years estimated in earlier carbon-dioxide-based models.

“Such an exomoon could have surface temperatures sufficient to keep water liquid without a nearby star,” David Dahlbüdding, the study’s lead author, explained to Phys.org.

What makes these moons interesting for life

Liquid water alone does not make a world alive. Still, the paper goes a step further and asks whether these moons might also support some of the chemistry linked to life’s beginnings.

The authors connect their atmospheric results to ideas about early Earth, especially the role of wet-dry cycles in helping simple molecules assemble into longer RNA chains. On a rogue exomoon, strong tides could drive repeated flooding and drying in shallow environments, even without sunlight or seasons from a star.

The atmosphere could help in another way. When the team added nitrogen to their hydrogen-rich models, much of it became ammonia. Ammonia dissolved in water can make conditions more alkaline, and the paper argues that alkaline settings may support key prebiotic reactions, including RNA polymerization and templated ligation — processes that help build and join RNA strands.

But this is just a hypothesis. It’s not a real demonstration that RNA could form on such a moon. Basically, the researchers modeled atmospheric conditions, not the complete process of life’s emergence.

Nevertheless, the combination is intriguing. Tidal forces could provide heat and changing water levels, while the atmosphere could help retain that heat and supply chemicals involved in prebiotic reactions. All of this could theoretically happen without a sunrise, sunset, or nearby star.

A bold idea with real limits

The new work widens the map of possible habitable worlds, but it does not settle the case.

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The models still simplify several parts of the atmosphere. They account for the presence of water vapor but do not fully model moist convection, cloud formation, or the effects of condensation on atmospheric temperature profiles. The chemistry code also calculates each atmospheric layer separately, rather than tracking all the mixing that may happen between layers.

There is also a more basic unknown: whether rocky moons can form with, or hold onto, such thick hydrogen atmospheres in the first place. The paper argues that once the host planet has been ejected, these atmospheres can remain stable against thermal escape on timescales much longer than the age of the universe. Even so, formation history remains a major open question.

Observing any of this will be difficult. The study suggests these moons might eventually be detected through transits or microlensing, and volcanic hotspots might even hint at whether a moon lacks a thick atmosphere.

But confirming the composition of an exomoon atmosphere sits beyond the reach of current instruments. As Dahlbüdding put it in Phys.org, “But although such moons could even be detected in the near future, the confirmation and analysis of any atmosphere may well be impossible for a long time.”

That challenge does not make the idea less important. It makes it more provocative. For decades, astronomers have searched for life by looking for worlds bathed in starlight at just the right distance from their suns. This study argues that some of the best hiding places may lie far outside that script — on moons circling planets that have no sun at all.