Moons orbiting wandering exoplanets could be habitable—with one catch

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Provided they host thick, hydrogen-dominated atmospheres, moons orbiting free-floating exoplanets could retain much of the heat generated deep within their interiors by tidal forces. Led by David Dahlbüdding at the Max Planck Institute for Extraterrestrial Physics and Giulia Roccetti at the European Space Agency, a new study predicts that hydrogen could act as a potent greenhouse gas—potentially providing habitable conditions for billions of years after their host planets are first ejected from their stellar systems. The work has been published in Monthly Notices of the Royal Astronomical Society.

Heat-absorbing hydrogen

Astronomers have now discovered hundreds of exoplanets drifting through interstellar space, most of them likely flung from their parent systems by violent gravitational encounters in the distant past. After ejection, these rogue worlds would likely have become extremely cold and dark—according to some astronomers, their moons may have faced more interesting fates.

During the chaos of ejection, a moon's orbit can become highly elongated, causing it to be repeatedly stretched and squeezed by its host planet's gravity. Much like Europa and Enceladus in our own solar system, these tidal forces could generate vast amounts of internal heat.

If such a moon's atmosphere were unstable enough for gases to condense into liquid form, most of this tidal heat would simply radiate into space. But the situation could be very different for high-pressure atmospheres dominated by hydrogen.

In Earth's present-day atmosphere, hydrogen molecules (simple pairs of bonded hydrogen atoms) have little warming effect—but under high pressures, they can absorb heat through a process known as "collision-induced absorption" (CIA). During fleeting collisions, hydrogen molecules form supramolecular complexes: temporary assemblies held together by weak, non-covalent bonds.

These complexes are far better at absorbing infrared radiation than the bonds within isolated hydrogen molecules and can rival the absorption of potent greenhouse gases like carbon dioxide and methane.

As a result, some previous studies have considered how much of the energy generated inside a moon, or even newly formed planets, could be trapped efficiently in a thick hydrogen atmosphere. If this were possible, these atmospheres could heat up without the large-scale condensation that plagued earlier carbon dioxide–dominated models.

"Such an exomoon could have surface temperatures sufficient to keep water liquid without a nearby star, significantly expanding the possibilities for life to emerge in the universe," Dahlbüdding explains. "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."

Combining calculations

For now, the best way to explore these exotic environments is through modeling. As Dahlbüdding explains, these simulations allow researchers to track how a moon's atmosphere and orbit evolve over billions of years following its planet's ejection.

"We combined accurate calculations of atmospheric temperatures with feedback on the chemical composition, mainly through condensation," he says. "This results in the most realistic—albeit still approximate—simulations of such moons to date."

On top of this, the researchers incorporated the latest theoretical insights into how exomoon orbits change over time. "In 2023, a study led by Giulia Roccetti modeled how orbital circularization leads to a decrease in the available tidal heat over time," Dahlbüdding continues. "Together with these previous results, we can calculate the maximum time spent in the habitable zone."

Retaining liquid water

The team's calculations reveal that in the thickest hydrogen-dominated atmospheres considered (reaching 100 times Earth's surface pressure), the effect of collision-induced absorption would make conditions both warm and stable enough to sustain liquid water. In some cases, these habitable conditions could persist for up to 4.3 billion years after the host planet's ejection—comparable to the current age of Earth.

"The hydrogen not only acts as a potent greenhouse gas but also as a stable background where more or less condensing species like methane, ammonia and water vapor can further contribute to retaining the internal heat," Dahlbüdding says.

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Parallels with early Earth

Beyond modeling distant exomoons, the researchers suggest their findings may also shed light on Earth's own past. Before life emerged, our planet's atmosphere may have been far richer in hydrogen than it is today, and periodically pressurized by frequent asteroid impacts—conditions that could have enhanced collision-induced absorption.

Such environments may have favored the formation and replication of RNA molecules, ultimately helping to kickstart the process of evolution.

"Through ongoing discussions, we are connecting our research to the latest advances in the search for the emergence of life on Earth," Dahlbüdding says. "And with our paper, we hope to build this bridge between bio- and astrophysics for other scientists as well."

Written for you by our author Sam Jarman, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

David Dahlbüdding et al, Habitability of Tidally Heated H2-Dominated Exomoons around Free-Floating Planets, Monthly Notices of the Royal Astronomical Society (2026). DOI: 10.1093/mnras/stag243. On arXiv: DOI: 10.48550/arxiv.2602.05378

Journal information: Monthly Notices of the Royal Astronomical Society , arXiv

Key concepts

Natural satellites (Extrasolar)Free floating planetsHabitable zone

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