HERE鈥橲 to all the moons we鈥檒l never see. Gas giant planets risk wiping out their own satellites, and it鈥檚 all down to the steps of a gravitational dance.
Exomoons 鈥� moons around exoplanets 鈥� may be the most likely places to find alien life, but dedicated searches have found none so far. That may be because many don鈥檛 survive, suggest Christopher Spalding of the California Institute of Technology in Pasadena and his colleagues.
Models and observations indicate that giant planets usually form far from their stars and then drift inwards. As they do so, their orbital period shortens and the orbits of their moons tend to shift as well.
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This orbital dance can stretch and drag a moon鈥檚 path. If the planet鈥檚 inward drift continues, it鈥檚 only a matter of time before a moon veers too close to its planet. Eventually, the planet鈥檚 gravity either tears the moon apart or causes the two to collide. This shrinks the region around a planet where moons can survive.
To illustrate the theory, Spalding modelled what would happen in a simplified solar system containing just the sun and Jupiter, with Jupiter slowly migrating inwards from its present path at 5.2 times the radius of Earth鈥檚 orbit. He found that Jupiter鈥檚 moon Io would be destroyed when the planet had closed in to 0.6 times Earth鈥檚 distance from the sun ().
In a different solar system, this migration could eliminate moons that would have become suitable for life as their planet closed in on its star.
The thought scares David Kipping of Harvard University, who is racing to discover the first exomoon. 鈥淵our first thought is, oh no 鈥� does that mean we鈥檙e not going to have any moons left over?鈥� he says.
But he notes that the moons that are most at risk are also the hardest to see. 鈥淭hose moons are not really the moons we鈥檙e looking for,鈥� Kipping says.
This article appeared in print under the headline 鈥淕iant planets might eat their own moons鈥�