June 29, 2015

Exomoons are the wild card in the ongoing chase for habitable worlds.
Astronomers have discovered more than 1,900 exoplanets, a few of which might be friendly to life, but no moons circling these alien bodies have turned up as of yet. Astronomers do not know how common exomoons might be or, for that matter, what they might be like.
A new study fills in an important gap in the developing theoretical framework regarding exomoons. The paper looks at a special set of exomoons located in the habitable, or "Goldilocks zone" — the not-too-cold, not-too-hot band where water neither freezes nor boils off a planet's (or moon's) surface.
The research hearkens back to the holy grail of exoplanet hunts: an Earth-like clone. Astronomers have fervently sought a world the mass and size of our own, orbiting at a similar distance from a star similar to our sun.
To date, however, most worlds with known masses, and orbiting at Earth-like distances from sun-like stars, are "super-Jupiters." As their name implies, these behemoths are even-more-jumbo versions of Jupiter, our solar system's biggest planet. Such bloated, gaseous worlds, of course, would not themselves be habitable in any way.
But if these super-Jupiters orbiting in habitable zones could form large exomoons, these bodies might be excellent candidates for supporting life. So far, most exomoon formation models have been restricted to exoplanets with masses in the Saturn-Jupiter range, because scenarios similar to our solar system have primarily piqued astronomers. Given the fact that dozens of super-Jupiters are now known to occupy Earth-like real estate around sun-like stars, the study authors figured it was high time to broaden those models.
"We wanted to extend, for the first time, the calculations for the formation of water-rich moons around gas giants beyond the Jupiter mass regime," said lead author René Heller, a postdoctoral fellow in astrophysics at McMaster University in Ontario, Canada.
The work, accepted for publication in The Astrophysical Journal, is co-authored by Ralph Pudritz, director of the Origins Institute at McMaster University.
In short, Heller and Pudritz first confirmed earlier research showing that the bigger the planet, the bigger its moons can potentially be. Proportionally, super-Jupiters could therefore sport moons up to a few times more massive than Mars. But unlike any previous study, the authors further showed that these moons could be extremely water-rich and near the stellar Goldilocks zone.
Two moons of Jupiter — Callisto and Ganymede — are kinsmen of this new class of giant, water-rich moons, though unlike those potentially habitable worlds, the Galilean moons are frigid because of their remoteness from the sun.
Giant, water-rich, Goldilocks-zone moons might be ideal for the nurturing ofalien life. These exomoons could represent the most prevalent, life-friendly abodes in the universe, scientists say.
"If Mars-sized, water-rich moons around super-Jupiters are common, as our study suggests, then habitable moons could easily outnumber habitable planets," said Heller.

Image:
(Top) This artist's impressions depicts three Mars-mass moons, two of which have liquid surface water and one of which is dry, orbiting a giant planet with rings.
(Bottom) A protoplanetary disk contains the gassy and dusty material that gives rise to planets and their moons. A few planets have coalesced out of the disk in this artist's impression.