Ploonets

Researchers recently added a new term to the astronomy vocabulary: ploonet. It describes an object that started out as a moon, but became a planet.

The researchers were studying a class of exoplanets known as hot Jupiters. They’re giants like Jupiter, the biggest planet in our own solar system. But they’re extremely close to their parent stars. As a result, their upper atmospheres are heated to hundreds or thousands of degrees.

Astronomers have expected to find giant moons orbiting some of these planets. But so far, not a single “exomoon” has been seen.

A team of astronomers from Australia and South America came up with a possible explanation: the moons might escape.

Most models say that hot Jupiters are born farther from their host stars and migrate closer in. As they move closer, the combined gravity of the planet and star could give any orbiting moons a boost. The moons would enter higher and higher orbits, and eventually drift off on their own.

Escaping might not be such a good thing for the moons, though. The astronomers found that about half of the runaway moons would slam into their planet or star within a million years. Such an impact might leave a giant ring of debris around the parent star — perhaps explaining the rings seen in several star systems.

If it survives that initial trial period, though, a former moon could enter a stable orbit around the star. So it would take up a new life as a ploonet — a planet that started as a moon.

Script by Damond Benningfield