In a recent paper, Owen Lehmer of the University of Washington in Seattle and colleagues examined what would happen if a large, icy moon “wandered” out of its orbit around a giant gas planet to end up in the inner region of its solar system. The idea is based on observations that 1) Jupiter-size exoplanets tend to move toward their central star, and 2) all the gas giants orbiting our sun have icy moons. In our own solar system, Jupiter and the other gas giants migrated inward, but didn’t make it all the way to the inner solar system.
In the likely scenario that an icy moon did wander closer to its star, the ice would melt to create an ocean world. But how long would the liquid water last? Lehmer and colleagues studied just that problem, and found that the closer an icy moon comes to its star, the more likely it is to undergo a runaway greenhouse effect of the kind that turned Venus into a hellish inferno.
The researchers even identified at what distance this would happen, which depends on the type of star and the size of the moon. For a body the size of Jupiter’s moon Europa, with a diameter of about 3,100 kilometers and 1/125th the mass of Earth, water will persist for more than a billion years at an orbital distance between 0.74 and 1.49 astronomical units (the average distance from the Sun to the Earth). For an even bigger moon—say, the size of Jupiter’s Ganymede (5,250 km wide and 1/40th the mass of Earth)—surface water would last indefinitely as long as the moon didn’t come too close to the star. Thus, the size of the moon is critical to determining how long such a water world could last.
It has long been recognized that exo-moons—and not just planets—may be habitable. Icy moons migrating into the inner part of another solar system may therefore be promising candidates for astrobiological study, especially if they contain land masses with lakes and rivers. Even better, they should be detectable with remote sensing in the near future, given their large size and nearness to the central star.