The cosmos may be awash in wandering, lonesome former moons, ejected from their respective planetary systems long ago.
Nobody has ever conclusively seen a moon orbiting a planet in another stellar system, partly because their small size and great distance makes them difficult to find with modern detection methods. But it’s also possible that there just aren’t very many of them.
at Cornell University in Ithaca, New York, at the French National Centre for Scientific Research in Paris and their colleagues used simulations of the formation of planetary systems to figure out what happens to moons as their planets are still forming.
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Planets are born in the chaos of early stellar systems, where they can be jostled out of position relatively easily by neighbouring worlds. They frequently brush by each other in the scramble for a stable orbit, often with catastrophic results for any early moons that might be hanging around.
Moons on the loose
The researchers’ model showed that in 80 to 90 per cent of cases planets shed their primordial moons. Most of them are flung out into interstellar space, and Raymond says there could be 1 to 100 wandering moons for every star in the Milky Way.
Moons like the large satellites of Jupiter are the only ones that survive simulations. Io, Europa, Ganymede, and Callisto all orbit quite close to Jupiter, making it harder to rip them out of the planet’s gravitational grip. Moons that orbit farther from their planets, though, end up on the loose.
“Basically, the dominant outcome is that moons are lost,” Raymond says. But he says that solar system shows no signs of such early instability and may be unusual in the grand scheme of things.
The biggest planets in our solar system are fairly far away from the sun, with extremely stable circular orbits. That could have spared their moons some of the chaos of the early solar system. “The Solar System giant planets might not have had very strong planet-planet scattering events, so the efficiency for making moons leave their host planet and become free-floaters might not be as high,” says Hong.
All is not lost
“The case is compelling that moon loss is probably a common occurrence,” says at Columbia University in New York. He says that his research into exomoons has also pointed toward the idea that moons closer to their planets are more common and stable than those that are farther out.
Raymond says one of the most intriguing outcomes from the model is the possibility that some of these cast off moons may not escape altogether but instead stay bound to their home stars. “That moon might find itself in a stable orbit around the star, and in that case, what is that moon? It’s a planet,” he says.
Sometimes, a planet disturbed by a passing neighbor can hold onto its moon even as it is flung away into interstellar space. If the moon orbits close enough to its planet, tidal heating from the planet’s gravity could make for an oddly habitable world around a planet with no sun in its sky. “It doesn’t seem like the worst place for life,” Raymond says.
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Read more: Possible exomoon may be an ocean-covered world as big as Saturn
