We Were Wrong About the Number of "Rogue" Exomoons

When you think of moons, a picture of our bright white luna orbiting the Earth probably comes to mind. But as it turns out, a vast majority of the moons in the universe aren’t like that at all.

These worlds, called exomoons, are simply natural satellites that orbit exoplanets. Exoplanets are, as their name suggests, found in star systems outside our solar system. Exoplanets are known to have oval-shaped orbits, which means their oblong trajectories can disturb the paths of other objects in their star system through a processes known as “planet-planet scattering.”

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Yu-Cian Hong, an astrophysicist at Cornell University, and her team decided to examine how this scattering could be affecting the exomoons that orbit these exoplanets. They found that about 80 or 90 percent of moons around giant exoplanets have been knocked out of their original orbit.

A simulation of planet-planet scattering.

In a paper published in the Astrophysical Journal, Hong and her colleagues explain that there might be as many “rogue” exomoons as there are stars. These free-floating bodies can end up crashing into a planet or a star, or being adopted by another planet.

Planet-planet scattering can even be responsible for turning an exomoon into planet or dwarf planet, depending on its size. If the abandoned moon begins to orbit a star on its own, it would technically not be considered a moon anymore.

Not all exomoons are destined for a life of exile, though. Hong’s study pointed out that objects that closely orbit large planets with powerful gravitational forces are pretty safe from planet-planet scattering.

Luckily the inner planets in the solar system have pretty circular orbits, meaning that we don’t need to worry about the effect of planet-planet scattering.

So remember, while our moon might seem like something completely ordinary, it’s actually pretty uncommon in a universe full of scattered exomoons.