Alien moons could bake dry from young gas giants' hot glow

An Earthlike moon orbiting a gas giant host planet. 

Credit: NASA

When we think of where else life might exist in the universe, we tend to focus on planets but on a grander cosmic scale, moons could prove the more common life-friendly abode.

A single gas giant planet in the not-too-warm, not-too-cold habitable zone around its star, where Earth and Mars correspondingly reside, could host several livable moons.

At this early point in our hunt for exoplanets, most of the worlds we have found in the habitable zone are giants, not Earths.

It's possible that the first inhabited place we discover outside our Solar System will be a moon.

René Heller

It is this sort of consideration that inspires René Heller, a postdoctoral fellow in astronomy at McMaster University, in Ontario, Canada.

He studies how "exomoons" could form, what they might be like and how we might detect them with current or future astronomical instruments.

A major part of his work deals with gauging the habitability of exomoons, which is a bit trickier than planetary scenarios because they orbit another body besides their star.

Rory Barnes

A new paper by Heller and his colleague Rory Barnes, University of Washington and the NASA Virtual Planetary Laboratory examines how heat emanating from a freshly formed exoplanet, coupled with irradiation from the solar system's star, can roast the planet's moons.

Before the planet cools off sufficiently, its close-orbiting moons could lose all their water, leaving them bone-dry and barren.

"An exomoon's habitability is of course constrained by its location in the stellar habitable zone, but it also has a second heat source, its host planet, that has to be accounted for," said Heller, whose paper has been accepted for publication in the International Journal of Astrobiology.

"With regard to this second source, our study shows that at close range, the illumination from young and hot giant planets can render their moons uninhabitable."

Researchers believe moons could serve as suitable abodes for life just as well as planets.

Even moons far beyond the habitable zone, such as Jupiter's Europa and Saturn's Titan, offer tantalizing hints of potential habitability thanks to the subsurface ocean in the former and the intriguing organic chemistry of the latter.

Still, a moon around an exoplanet in the habitable zone stands as a far better bet for life than these frigid candidates.

Heller's findings suggest that we ought to exercise caution, however, before declaring that an Earth-sized, habitable-zone exomoon is a real-life Pandora, the lush moon of science fiction fame in "Avatar."

Before assuming an exomoon is habitable based on its host planet's locale, the moon's current and conjectured past orbital distances will need to be assessed.

"Earth-size exomoons that could soon be detected by our telescopes might have been desiccated shortly after formation and still be dry today," said Heller.

"In evaluating a moon's habitability, it is crucial to consider its history together with that of its host planet."

More Information: International Journal of Astrobiology / FirstView Article pp 1-9 Copyright©Cambridge University Press 2014 DOI:dx.doi.org/10.1017/S1473550413000463