Alien Life on Moons?

Maybe Not

Remember James Cameron’s movie Avatar? It took place on a moon called Pandora, which was a satellite of a gas giant world in the Alpha Centauri system, some 4.2 light-years away from the Sun. The idea was that Pandora somehow had a habitable environment friendly to certain forms of life. But, how true is this? Could you get a moon around a distant gas giant with flowing water and higher life forms?

For one answer, look at the moons of the gas giants in our own solar system. They’re in some pretty hostile environments. The Jovian moons circle their planet in about as hostile a radiation environment as you can imagine, and are embedded inside Jupiter’s magnetic field. Yet, we continue to think that perhaps Europa (at least) could harbor some forms of life.

Any exomoon (that is, a satellite around a distant Jupiter-like planet around other star) would have to contend with its planet’s magnetic field just as the Jovians do even if it did have water, warmth, and light enough to support a habitable zone.

The topics of gas giant magnetic fields and habitability of their moons is the focus of a study by two astronomers named Jorge I. Zuluaga and René Heller.  They concentrated on the environments of extrasolar gas giant planets that exist in the habitable zones of their stars. They wanted to understand the relationship between those planets’ magnetic fields and the “habitable edge” around them—that is, the minimum distance from the planet at which its possibly habitable moon would have to lie to avoid a runaway greenhouse effect in its atmosphere (like Venus’s).  Some of Zuluaga’s prior work has been in predicting the sizes of giant planet magnetospheres and what effect they would have on the habitability of moons “shielded” inside the fields. Heller’s work has focused on figuring out just how close a moon could come to its primary and still have a chance of supporting life (and not have its atmosphere go completely wonky).

So, can a habitable exomoon exist and support life around a gas giant planet with a huge magnetosphere?

The answer turns out to be: probably not. Exomoons orbiting planets similar to Neptune, Saturn or Jupiter in the habitable zone of a 0.7-solar-mass star (that is, a star that is 70 percent the mass of the Sun) and with orbital eccentricities typical for solar system moons will be shielded by their planet’s magnetic field, but only if they are closer than the runaway greenhouse edge.  That means that they are protected, but their atmospheres will run wild. Ultimately, it means an exomoon can be shielded inside its planet’s magnetosphere, or it can be habitable. But it can’t be both.

So, this is not so good news for future Pandorans  around gas giants similar to our own.  But, if you play with the variables and come up with a Neptune-like world that is very rocky, there’s a chance it  might have habitable worlds AND a giant magnetosphere to shield them. And then maybe there could be a chance for life-supporting environments. However, as Heller and Zuluaga point out, the inhabitants of such worlds would have a difficult time reading compasses on their moons, probably due to interference from the larger planetary magnetic field.