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Our Moon Could Escape Earth’s Orbit and Become a “Ploonet”

A hypothetical new type of world could explain a lot about our universe.

7. 10. 19 by Kristin Houser
NASA/Victor Tangermann
Image by NASA/Victor Tangermann

Meet the Ploonets

Our Moon might not always be the dedicated companion to Earth that it is now.

An international team of researchers has proposed a hypothetical new type of world it calls a “ploonet”: a former moon that escaped its host planet’s orbit and began circling its host star instead.

The team thinks ploonets could explain several unusual astronomical phenomena — and that our own Moon could one day join their ranks.

Simulation Game

Hot Jupiters are a class of exoplanets that orbit incredibly close to their host stars. However, some astronomers believe they may have actually formed on the outskirts of their solar systems and migrated inward.


In a yet-to-be-peer-reviewed paper published on the pre-print server arXiv, the researchers detail their simulations of what might happen if a hot Jupiter started that migration with an exomoon in tow.

Based on their simulations, about 48 percent of the exomoons would detach from their hot Jupiters and begin orbiting their stars instead — as ploonets.

Theory of Some Things

The team believes ploonets could explain several unusual astronomical phenomenon.

An icy moon’s water could evaporate as it escapes its planet’s orbit and moves toward its star, for example, giving the ploonet a comet-like tail. The passage of such a ploonet across its star might explain why some stars appear to flicker.


Meanwhile, a ploonet that eventually crashed into its former host could create debris that might explain the strange rings found around some exoplanets.

“Those structures [rings and flickers] have been discovered, have been observed,” researcher Mario Sucerquia told Science News. “We just propose a natural mechanism to explain [them].”

Moon Light

Ploonethood could also explain why astronomers have yet to definitively find any exomoons despite predictions that the universe should be rife with them — the moons may get kicked out of their planets’ orbits before we can detect them. If we see the former moons after that point, we might just mistake them for new exoplanets.

Based on the researchers’ simulations, ploonets also have incredibly short lifespans, astronomically speaking — roughly 50 percent crash into their star or former host planet within half a million years, while others meet the same fate after less than a million years of ploonethood — which could further explain why we haven’t found any.


As for Earth’s own Moon, Sucerquia told Science News that it “is a potential ploonet” given that it moves about 4 centimeters farther away from Earth every year. But we don’t have to worry about it going its own way any time soon — at this rate, it won’t break free from Earth’s orbit for about 5 billion years.

READ MORE: Moons that escape their planets could become ‘ploonets’ [Science News]

More on the exomoons: Researchers Think They’ve Discovered the First Moon Outside Our Solar System

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