Exomoons around migrant hot Jupiters could hold onto life-giving atmospheres and maintain surface oceans for billions and billions of years. This is the conclusion of a paper from the University of Washington modeling the impact of inward migrating gas giants on their frozen moons.

Exomoons are of increasing interest in the search for habitable worlds. The number and variety of moons visited by Voyager in our own solar system first prompted thoughts of the possibilities further out.

With some of Jupiter’s moons composed of 40% water there has been speculation that the numbers of habitable exomoons might match or even outnumber habitable exoplanets.

However, there is a problem that could hold back the development of more complex life on a moon.

Whilst gas giant moons like Europa might well have liquid water deep below its thick ice crust, it is very difficult to form a watery moon in the habitable zone where temperatures during the early planetary formation stage are too hot for water ice to exist.

If only there was a way to bring the water rich but frozen outer solar system moons in closer?

Step forward hot Jupiters. This class of giant gaseous exoplanet orbit far closer to their stars than Jupiter or Saturn.

Because of their size it is likely hot Jupiters form further out, where ice, as well as rock and gases were all available.

They would then migrate in, possibly due to interactions with the remnant planetary disk, or planetesimals yet to be aggregated or thrown out of the system.

“Lots of the gas giants we have found are in the habitable zone so it is not unreasonable to suspect that this sort of migration is common,” says Owen Lehmer who has been studying the impact of gas giant movements, and believes our observation history suggests this is where many end up.

“The time spent in migration is often brief compared to star/planet lifetimes so it is reasonable to expect that the gaseous planets we observe in the habitable zone are orbiting at that location.”

During hot Jupiter migration water ice found on the orbiting moons will sublimate, much like it does on comets approaching the Sun.

Whether this freed up H 2 O escapes like a comet’s tail or is retained to form an atmosphere is dependent on the moon’s size and therefore gravitational hold, and the radiation energy being received from the nearby star, which decreases with distance.

But how large does a moon have to be? Would any of our solar system moons be able to hold on to surface oceans and UV ray deflecting atmospheres for a biologically significant period of time?

With direct observation of these moon systems impossible, Lehmer and his colleagues turned to planetary model of atmospheric escape and retention, and applied it to two of Jupiter’s moons — Europa and Ganymede — to see what would happen if their parent planet began migrating towards our Sun.

Their results, published in the Astrophysical Journal, describe a delicate balancing act where a gradual thickening of the atmosphere can suddenly turn into a runaway greenhouse effect that removes all moon water if their planet strays too close to its star.

For the smaller Europa moon, at less than 1% the mass of the Earth, Lehmer found if it were to end up near to Earth orbit it would only be able to hold onto its atmosphere for a few million years.

“It would be a fleeting period of habitability. Just a flash in the pan in terms of planetary biology,” says Lehmer.

However for any larger, Ganymede-sized moons venturing into its solar system’s habitable zone, an atmosphere and surface water could be retained pretty much indefinitely.

“This is a very interesting finding for the field of exomoon habitability,” says Rene Heller from ESA’s PLATO mission to discover habitable zone planets.

“Our models for moon formation suggest the formation of even more massive moons than Ganymede is common around many of the super-Jovian exoplanets.”

To further verify his models Lehmer believes that despite a low chance of direct observation, signs of this melting could be detected with modern equipment.

He envisage the near future observation of a torus of escaped atmosphere, similar to the doughnut shaped formation around Jupiter created, not by lunar melting, but by the intense volcanism of the inner moon Io.

“This would be a very positive sign of migration and could be followed up on in the future with larger telescopes to see if there might also be a bigger moon, better at holding on to its newly created atmosphere,” says Lehmer.

_____

Owen R. Lehmer et al. 2017. The Longevity of Water Ice on Ganymedes and Europas around Migrated Giant Planets. ApJ 839, 32; doi: 10.3847/1538-4357/aa67ea