'Ploonet' May Be the Most Adorable Name Ever for a Moon That Leaves the Nest

Computer models suggest some moons can grow up to be planets. (Photo: GiroScience/Shutterstock)

Imagine the Earth without its moon. It might seem like the ultimate "empty nest" syndrome.

After all, in a sense, the moon is the offspring of our planet. Research suggests it formed about 4.5 billion years ago, when a Mars-sized body smashed into Earth and sent a broken chunk into orbit.

They've been together ever since.

But the moons of other planets, called exomoons, may eventually leave home. They break free of their parent's orbit. Sometimes, it's the result of their own struggle to be free; sometimes it's their planet's decision to evict them.

These former moons are, rather adorably, referred to as "ploonets."

An international team of scientists floated the term in a research paper published June 27 in the preprint journal arXiv.org. The paper has yet to be peer-reviewed, but please, let's just make this happen.

The universe is chock-full of technical mouthfuls like period-luminosity relations, spectroscopic binaries and Widmanstätten patterns.

Just this once, can we please just mash the words planet and moon together?

Go ahead and say it out loud. And while we're murmuring about the moon, try "moonmoon." That's what certain fun-loving scientists suggest we call the offspring of those moons — although ultimately, they settled on the more sobering "sub-moon."

An illustration of a large body colliding with Earth.
Our own moon was likely born during a violent impact between Earth and another large body. SueC/Shutterstock

But we're getting ahead of ourselves. Before a moon can have a moon of its own, it has to become a ploonet. That's no easy feat, considering the immense gravitational thrall of an exomoon's host planet. And it's probably why there's scant evidence so far that ploonets even exist.

For their research, the team looked at hot Jupiters, a class of planet that's massive, gaseous and in scorchingly tight orbit with their host star. These planets may have been born much farther from their stars, but were gradually drawn closer to them.

Now, what happens to any moons that might be in the planet's entourage as it creeps closer? Through computer simulations, the team concluded that the combined gravitational forces from the hot Jupiter and the star would increasingly agitate the exomoon's orbit, eventually prying it from planetary lockdown. Once free, the exomoon could set up shop on its own, forming its own orbit around the sun.

And a ploonet is born.

Of course, that former moon would be unrecognizable from its former self. If it was encased in ice, for instance, the star would evaporate that ice in short order. But as the moon's ice melted, the researchers suggest, it could grow a comet-like tail — and that may even account for why some stars appear to flicker.

A slender moon in the sky.
If Earth should ever migrate closer to the sun, our own moon could break away. But, of course, no one would be around to miss it. yaalan/Shutterstock

In some cases, the ploonetary process could end in a tragic miscarriage, as gravitational strains push the exomoon not away from its host planet, but rather into it. Hence the debris fields astronomers occasionally spot encircling other planets, testifying perhaps to the sad fact that a ploonet died there.

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

Perhaps someday, if conditions are just right — as in, if that ploonet gets hit by a big enough object for a piece of it fall off — it may have babies of its own.

Adorable little moonmoo -- err... sub-moons.