Potential for habitation? NASA/JPL-Caltech

Icy exomoons dragged towards their star could hold onto life-giving atmospheres and liquid oceans for billions of years – if they’re big enough.

Many of the planets we’ve spotted outside our solar system are Jupiter-like gas giants, unsuited to life as we know it. But if they host rocky moons, those moons could make for liveable habitats.

However, Earth’s own dry companion shows how difficult it is for watery moons to form in the habitable zone. Such moons are common further out in the solar system, but are usually encased in ice, like Jupiter’s Europa and Saturn’s Enceladus.


If only you could bring those water-rich moons in closer.

Step forward hot Jupiters. These common gas giant exoplanets have often been found orbiting in their star’s habitable zone. Yet their large size puts them at odds with the amount of planetary material thought to have been available at their orbital distance. There’s an explanation for this discrepancy: if they formed further out, where there was more material available, they could have migrated inwards later to end up where we see them.

During such a trip, the ice of any watery moon that orbited the planet may turn to gas and this could be retained, depending on the moon’s size and therefore its gravitational hold.

Keeping an atmosphere

To find out how large a melting moon would need to be to keep a habitable atmosphere, Owen Lehmer at the University of Washington in Seattle applied equations of atmospheric escape to the moons of our own solar system as he simulated them travelling towards the sun.

His calculations suggest that a body the size of Jupiter’s Ganymede, the solar system’s largest moon at about two-thirds the size of Mars, could hold on to a liveable watery atmosphere for hundreds of billions of years. Much smaller, though, and runaway greenhouse effects would render the moon uninhabitable.

“Lots of the gas giants we have found are in the habitable zone, so it is not unreasonable to suspect this sort of migration is common,” says Lehmer.

“This is a very interesting finding for exomoon habitability,” says René Heller of the European Space Agency’s PLATO mission, which aims to discover planets in the habitable zone of stars. “Our models suggest the formation of even more massive moons than Ganymede is common.”

Journal reference: Astrophysical Journal, DOI: 10.3847/1538-4357/aa67ea