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Despite a lot of searching, we haven鈥檛 found a moon around an exoplanet yet. According to a new study, that could be because these exomoons have been ejected from orbit around their home planet and turned into miniature planets called 鈥減loonets鈥�. Our own moon may even become a ploonet one day.

Mario Sucerquia at the University of Antioquia in Colombia聽and his colleagues modelled the interplay between exomoons and the giant planets they could orbit 鈥� gas worlds between 0.5 and 1.8 times the mass of Neptune.

We have observed these so-called 鈥�hot Jupiters鈥� orbiting stars at the distance Mercury lies from the sun, but they could not have formed so close to their star because its heat would burn off large amounts of gas. That means they must migrate inward, which could disturb any moons they have and potentially fling them off into their own orbits around the star.

鈥淐loser-in giant planets are more prone to lose their moons because the tidal interaction between the star and the planet is stronger,鈥� says Sucerquia. 鈥淭his is, in fact, bad news for exomoon hunters because those around close-in exoplanets are more easily detectable.鈥�

Sucerquia and his team found that about 44 per cent of exomoons would collide with their planet in this process, about 6 per cent would be absorbed by the star, and about 2 per cent would be ejected from the planetary system entirely.

But the rest would become ploonets. Of those, 54 per cent would end up in orbits further from the star than their home planet. Another 14 per cent would end up on orbits closer to the star, and almost a third would take on eccentric orbits where the ploonet鈥檚 path would cross the planet鈥檚 orbit every once in a while.

鈥淓arth鈥檚 tidal strength is gradually pushing the moon away from us at a rate of about 3 centimetres a year,鈥� says Sucerquia. 鈥淭herefore, the moon is indeed a potential ploonet once it reaches an unstable orbit.鈥�

When they leave the protective magnetic field of their planet, ice-rich exomoons would be exposed to radiation from the star and their surface ice could evaporate and create a trail of dust and gas on their orbit.

That could explain the strange observations of Tabby鈥檚 star, in which astronomers have seen large, unexplained dips in its light. The erratic dips are so weird that even alien megastructures have been suggested as their source. Sucerquia and his team suggest that gas and debris from ploonets could be blocking the light.

鈥淚t鈥檚 a good theory but we still have to test it because there鈥檚 nothing that says this is a smoking gun quite yet, so it鈥檚 not like we鈥檝e got it solved,鈥� says Tabetha Boyajian at Louisiana State University, the discoverer of Tabby鈥檚 star. These ejected exomoons could plausibly pass聽in front of the star and obscure some of its light, she says, but we still don鈥檛 know how much gas and debris they would likely produce before extinguishing.

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