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Migrating gas planets might destroy their life-friendly moons
Januar 14, 2016

In our own solar system, there are several moons which astrobiologists regard as life-friendly and which have attracted particular interest in recent years. These moons include Europa, Io and Ganymed in orbits around Jupiter as well as Titan and Enceladus as the moons of Saturn. They contain liquid water and, due to the gravitational force of the gas planets, have an internal activity whereby material is exchanged between the surface and the underlying zones - both important preconditions for the emergence of life.

From such moons scientists infer the existence of moons around extrasolar planets. The detection of such "exomoons" is technically very complex, but not fundamentally impossible. Some groups, e.g. by David Kipping from Harvard University, are currently trying to find such moons. One of the most exciting discoveries would be a life-friendly moon within the habitable zone of its star.

Christopher Spalding and his colleagues from the California Institute of Technology have also thought about the existence of such moons [1]. In their opinion there should be only a few life-friendly moons. They argue that gas planets form in the outer regions of a star system and only get into the habitable zone by migration to inner orbits. But during this migration they would destroy most of their moons.

The most important reason for this is that the moons of the gas planets do not describe a perfect circle with the planet as a center, but rather an elliptic orbit. This also changes their position in relation to the planet. Seen from a perpendicular direction, the orbit of the moon, like the petals of a flower, will once orbit the planet, and the nearest and farthest points of the orbit will change their position.

Spalding and his colleagues simulated the situation for the moons Io and Europa in an alternative scenario. They started with Jupiter at its present position, about 5.2 astronomical units away from the Sun. Then they let the planet migrate up to 0.6 astronomical units to the Sun and observed how this affected the moons. As a result, they saw that the moons temporarily came dangerously close to the planet and eventually collided with it. Io was destroyed first, then Europa.

The migration of a gas planet from the outer, cold regions of a star system into warmer fields would thus have a sad irony. At the end of the migration, especially those moons would have been destroyed which would have provided suitable conditions for the formation and maintenance of life in the new position.


Artistic representation of a life-friendly moon orbiting an extrasolar gas planet. Such moons might exist, but a great number of them might have been destroyed during migration of the planet into the habitable zone.



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