Frying the Na'vi

In the foreground here is a moon called Pandora:

(source)

I should preface everything else in this post by explaining that Pandora is, rather emphatically, not real. Those of you who enjoy poorly-written-but-visually-spectacular movies as much as I do will recognize it as the setting of James Cameron's Dances with Wolves remake Avatar. For those of you who don't, it is worth explaining that Pandora is the habitable moon of a Jupiter-sized gas giant, visible in the background above--it even has an analog of the Great Red Spot, though (like nearly everything else in the movie) it's blue.

Habitable moons pop up all over the place in science fiction, from Avatar to Star Wars to certain works of Robert Heinlein. However, the planet they're orbiting (which is almost invariably a gas giant) seldom exists as more than a dramatic backdrop to the immediate setting. This, as it turns out, is a bit problematic. A recent paper by René Heller and Rory Barnes at McMaster University and the University of Washington points out that, while past studies of exomoon habitability largely ignored the evolution of a given moon's orbit and of its parent planet, these factors are critically important over the first billion years after a planet-moon system's formation.

Part of the key to this study is that, as gas giants form, they shrink and cool. Thus, a gas giant in a distant solar system (the study used the example of a 13-Jupiter-mass planet and an Earth-sized moon) would be significantly larger in its early years. Among other things, this would translate to more light being reflected from the system's star onto any potentially habitable moons, which means more energy (and thus, heat) being received by the moon in the form of flux. Combined with this reflected flux is energy produced by the Kelvin-Helmholtz mechanism, where a planet cools and shrinks, then heats up as a result of compression and expels heat (Jupiter produces more energy via this mechanism than via reflected solar flux). Combining this with the tidal heating that all moons undergo and flux from the system's star leads to a lot more heat than past models have evidently accounted for. A possible result of this is that the water on a newly formed potentially habitable moon would begin to evaporate more quickly than it could precipitate. Water is a heck of a greenhouse gas, trapping heat much more effectively even than carbon dioxide or methane. As a result, as the exomoon's water evaporated, it would heat up even more, leading to more evaporation, and so on and so forth in a runaway greenhouse effect--this process is how our sister planet Venus became the scorched hellscape that it is today.

The surface of Venus, imaged by the Venera 13 lander in 1982. The lander lasted for just over two hours, enduring a temperature of 457 degrees C and pressure of 89 Earth atmospheres, before it ceased transmitting (source).

In order to escape Venus' fate, an exomoon must exist in a kind of secondary habitable zone around its parent gas giant (assuming it already exists within its star's habitable zone). The closer to the inner edge of the stellar habitable zone the planet and moon are, the farther from the planet this limit becomes--in the case of the 13-Jupiter-mass planet and Earth-sized moon, the moon can orbit no closer than 28 Jupiter radii (for reference, Callisto, the most distant of the Galilean Moons, orbits Jupiter at a distance of about 26 Jupiter radii). Moons inside of this orbital limit would be in danger of losing any water that might exist on them, along with any chance of being habitable, in the first few million years of their existence. Settling an exomoon is a cool idea, and one we'll no doubt see again in sci-fi movies. But next time you see one, see how close it looks to its parent planet. That should tell you whether the director did his homework.

One last note: over the course of researching this post, I came across this page from the Center for Astrophysics, for a project called the Hunt for Exomoons with Kepler (HEK), which uses the exact same still from Avatar that I did. So minus points on me for unoriginality, but bonus points to CfA researchers for doing some seriously cool research.