Ploonets: When Moons Become Planets

By: Patrick J. Kiger  | 

Exoplanets — those that exist outside the solar system — can throw off their moons, which then begin to orbit on their own, in effect becoming small planets, hence the name "ploonets." NASA/JPL-CALTECH

What do you call a planet's moon that's gone rogue, slipped its usual orbit and stopped circling that planet, taking up a new solitary orbit around one of its own stars instead?

Well, give yourself a break if you don't know the answer to that question, because it's a strictly hypothetical proposition at this point, and it's only very recently that scientists have come up with a cutesy little name for these rogue moons.


In a paper published June 28, 2019, in Monthly Notices of the Royal Astronomical Society, an international team of astronomers proposes for the first time that some of the moons of giant planets outside the solar system might be pushed out of their orbits and begin to circle their stars separately, in effect becoming small planets.

So, of course, the researchers decided to smash together moons and planets and call these new beings — yep, you've got it now — ploonets.

In their paper, the researchers describe how they created simulations to figure out what might happen to the moons of so-called hot Jupiters, which, astronomers previously have theorized, are gas giants that migrate from distant orbits to become closer to stars. As one of these gas giants approaches a star, the gravitational forces of the two would make a moon's orbit more energetic, so that it is forced farther and farther away from its planet. Eventually, it would be so far away that it would be out on its own path.

While some expelled moons eventually would collide with the star — or with their former planet — and be destroyed, a subset of them, fewer than 50 percent, would survive and begin to orbit the star on their own, and even absorb other material floating in space, and begin behaving like planets.

"This process should happen in every planetary system composed of a giant planet in a very close-in orbit," Sucerquia told Science News. "So ploonets should be very frequent."

So far, the ploonets remain hypothetical, since astronomers haven't yet observed one. But as this article notes, once astronomers figure out the light signature of a ploonet, they can search through data accumulated by space telescopes and possibly locate some of them.


ScienceAstronomy TermsFloating PlanetScienceAstronomyHow Nomad Planets WorkScienceSpace ExplorationHow Planet Hunting WorksScienceThe Solar SystemWhy is Pluto no longer considered a planet?ScienceFuture SpaceHow Will We Colonize Other Planets?ScienceGeophysicsHow much does planet Earth weigh?ScienceThe Solar SystemWhy Did It Take So Long to 'Discover' Planet Nine?ScienceThe Solar SystemWhat's the Order of the Planets in the Solar System?ScienceThe Solar SystemDoes it rain on other planets?ScienceThe Solar SystemJupiter: Yokozuna of Gas Giants, Banisher of PlanetsScienceThe Solar SystemHow do planets form?ScienceStarsWhite Dwarfs Can Shred Planets to PiecesScienceThe Solar SystemWho Named Planet Earth?ScienceSpace ExplorationDoes a planet need continents to support life?ScienceThe Solar SystemIs Planet Nine Actually a Primordial Black Hole?ScienceSpace ExplorationHow many planets in our universe could support life?ScienceStarsCould a planet exist without a host star?ScienceThe Solar SystemWhy Are Planets Almost Spherical?ScienceThe Solar SystemNASA Announces New Solar System Packed With Seven PlanetsScienceThe Solar SystemPluto: Is It a Planet After All?ScienceThe Solar SystemHaumea, a Dwarf Planet in the Kuiper Belt, Has Its Own RingScienceSpace ExplorationNew NASA Satellite Is Hunting for Distant PlanetsScienceThe Solar SystemAncient Obliteration of Dwarf Planets May Have Created Saturn's RingsScienceThe Solar SystemIs Earth the Only Planet With Tectonic Plates?ScienceStarsHow do astronomers detect that a star has a planet orbiting it?ScienceSpace ExplorationCan we detect water on exoplanets?ScienceThe Solar SystemThe Truth Behind the Rogue Planet NibiruScienceThe Solar SystemUranus: The Planet on a Very Tilted AxisScienceThe Solar SystemPloonets: When Moons Become PlanetsScienceAstronomy TermsPlanetariumScienceSpace Exploration10 Remarkable ExoplanetsScienceSpace ExplorationClosest Exoplanet Yet Confirmed By European Southern ObservatoryScienceStarsSpotted: Early Planetary Formation Around a Binary Star SystemScienceStarsThis Is How We'll Detect Life on Distant ExoplanetsScienceSpace ExplorationNASA's Kepler Mission Adds 100 Alien Worlds to Exoplanet TallyScienceSpace ExplorationCan amateur astronomers spot exoplanets?ScienceFuture Space10 Best Ideas for Interplanetary CommunicationScienceSpace ExplorationLISA: Detecting Exoplanets Using Gravitational WavesScienceThe Solar SystemHow NASA Planetary Protection WorksScienceAstronomy TermsPlanetesimal Hypothesis