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Hypothetical "ploonets" would blur the lines between planets and moons

Hypothetical "ploonets" would ...
An artist's illustration of an Earth-like exomoon orbiting a Saturn-like exoplanet
An artist's illustration of an Earth-like exomoon orbiting a Saturn-like exoplanet
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An artist's illustration of an Earth-like exomoon orbiting a Saturn-like exoplanet
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An artist's illustration of an Earth-like exomoon orbiting a Saturn-like exoplanet

You know what planets are. You're familiar with moons. But what do you get when you cross the two? "Ploonets," according to astronomers from Australia and Colombia. These weird, hypothetical objects start life as a moon circling a giant planet, but models show they can be exiled and essentially turn into planets themselves. If true, this theory could explain away some astronomical mysteries.

So far, over 4,000 exoplanets have been discovered, and if our solar system is anything to go by, it's natural to assume that many of these should also host their own moons. But so far, no exomoons have been directly detected – although evidence of them has been spotted.

To investigate where they might be hiding, astronomers modeled exomoons in orbit around "Hot Jupiters." These are the most commonly-detected type of exoplanet, and as the name suggests, they're gas giants the size of Jupiter or bigger, and are sizzling due to being quite close to their stars.

Interestingly, the study found that large exomoons orbiting Hot Jupiters could escape the gravity of their host planets. While some of those exiled exomoons might end up crashing into the planet or being flung into the star, roughly half are expected to survive and venture out on their own.

"These moons would become planetary embryos, or even fully-fledged planets, with highly eccentric orbits of their own," says Jaime Alvarado-Montes, co-lead researcher on the study.

The team has dubbed these objects "ploonets," and they start to blur the lines between moons and planets. Unfortunately, they may not be long-lived – the team suggests that they may end up dissolving within a few hundred million years. That might make them hard to detect, as could the fact that we might not be able to tell them apart from regular planets at this distance.

If they exist, ploonets could also plug a few cosmic plot holes. The weird dimming pattern of Tabby's Star, for instance, probably isn't an "alien megastructure" but might be a result of ploonets.

The next steps, of course, are to try to observe a ploonet in action.

The research was published online at arXiv.

Source: Macquarie University

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