Image: An artist’s rendition of a sunset view from the perspective of an imagined Earth-like moon orbiting the giant planet, PH2 b. The scene is spectacular, but how likely is it that gas giants would have moons beyond Mars size? The answer to the question awaits further work in exomoon detection. Credit: H. Giguere, M. Giguere/Yale University.

Gas Giants in the Habitable Zone


Because the sky is full of surprises, we can’t afford to be too doctrinaire about what tomorrow’s discovery might be. After all, ‘hot Jupiters’ were considered wildly unlikely by all but a few, and even here in the Solar System, probes like our Voyagers have turned up one startling thing after another — volcanoes on Io were predicted just before Voyager arrived, but who thought we’d actually see them in the act of erupting? So I don’t think we can rule out the idea of habitable moons around a gas giant in the habitable zone, but there are reasons to question how numerous they would be.

We’ve had this discussion before on Centauri Dreams, and while I love the idea of a huge ‘Jupiter’ hanging in the sky of a verdant, life-bearing planet, there are some factors that argue against it, as reader FrankH pointed out recently. One problem is that moons around a gas giant will probably be made largely of ice and rock, because the planet itself would have formed beyond the snow line and migrated into the habitable zone. A Mars-sized moon is going to melt and, given its low escape velocity, will gradually lose its atmosphere in these warmer regions.

We could imagine capture scenarios as a migrating gas giant moves into the warm inner system, but it’s hard to see that as a frequent occurrence. The key question for me would be what factors govern the formation of gas giant moons in the first place, and what is the likelihood of finding moons much larger than Mars? David Kipping’s continuing exomoon work has suggested we could detect a moon of approximately 0.2 Earth masses with existing technology, but this is far larger than Ganymede, and we have no analog in our own Solar System.

Read more: Gas Giants in the Habitable Zone — Centauri Dreams.

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