🪐 Extraterrestrial life everywhere, and moving throughout the galaxy?

Life could emerge and travel in the total darkness of space, far from any star.

Recent work shows that moons accompanying rogue planets could maintain liquid oceans for billions of years. These isolated worlds thus represent potential niches for life, scattered and moving throughout the galaxy.

Ejected from their original stellar system following gravitational perturbations, these planets drift alone in the interstellar void. They can retain their natural satellites, whose orbits are often transformed after ejection to become highly elliptical.


This very elongated orbit causes constant changes in distance between the moon and its host planet. Such dynamics generate significant tidal forces, which periodically distort the satellite's interior. The friction resulting from this flexing produces enough internal heat to keep water liquid, even in the absence of stellar radiation and despite the intense cold of space. This mechanism, called tidal heating, can continue for durations comparable to our planet's history.

We therefore find ourselves in a situation where, counterintuitively, the ejection of the planet and its satellite prepares an environment suitable for the emergence of life. The concept of a habitable zone around a star is no longer relevant here: the system has created its own independent habitable zone, or habitable island.

To prevent this heat from being lost into space, however, the presence of a suitable atmosphere is necessary. Scientists have examined envelopes rich in hydrogen. Indeed, unlike carbon dioxide which would solidify under these frigid conditions, hydrogen remains gaseous. Under high pressure, its molecules can trap heat. This constitutes an effective insulating layer, allowing the surface of these moons to retain a temperature compatible with liquid water.

These configurations present similarities with early Earth. The tidal forces acting on these moons could also create cycles of evaporation and condensation, thus contributing to the formation of prebiotic molecules.

Rogue planets are thought to be very numerous in the Milky Way, with some estimates comparing their quantity to that of stars. If a fraction of them possess moons, these satellites offer stable shelters where habitable conditions could be maintained for billions of years.

This work, published in Monthly Notices of the Royal Astronomical Society, therefore broadens the field of potentially hospitable worlds beyond stellar systems, indicating that ecosystems could appear even in the darkest regions of the cosmos.