Earth’s nearest neighbor, the Moon, is a pretty reliable partner. As far as scientists know, the Moon originated during or shortly after the formation of Earth itself, and it’s been hanging around here every since. But not all moons behave this way, and due to the complicated nature of some exoplanet orbits it’s now believed that many moons end up being flung from their homes and sent drifting into space.
In a new research paper published in The Astrophysical Journal, researchers explain how a mechanic called “planet-planet scattering” could be responsible for a huge number of these free-floating “rogue” moons. These drifting worlds can become planets of their own if they find a home around a star, or be drawn in by the gravity of another planet, becoming a moon once more.
Before coming to this conclusion, scientists drew upon past findings that suggested many exoplanets have oval-shaped orbits. The most readily available explanation for why these orbits exist is that the planets are being pushed and pulled by the gravity of other nearby worlds. Using those observations as a starting point, the team sought to discover what might happen to a moon of one of those planets.
The combined effect of the moon’s orbit and the eccentric orbit of it host planet can lead to these smaller bodies being flung away from their homes. As Space.com reports, the data suggests that this is the case more often than not, and that up to 90 percent of exoplanet moons end up somewhere other than where they originated. The researchers believe that rogue moons may actually outnumber free-floating planets, and be as common as stars.
Once flung away from its host planet, the fate of a free-floating moon is unpredictable. If it falls into orbit around a star it can become a planet, but it’s also possible that another exoplanet will capture it in orbit or even collide with it. Likewise, the former moon could find itself drifting towards its star, where it would meet an untimely end.