Imagine an exoplanet, a gas giant orbiting its star while a moon orbits the planet itself. Now suppose the moon turns rogue as it moves nearer to its star, breaking away — or being forced out of its orbit by the planet — and going off on its own trip, in effect behaving like a planet in its own right. What is it now: a moon or a planet? Call it “ploonet”, suggests an international team of astronomers.

They modelled the likely behaviour of giant exomoons predicted to form around massive planets — and found that they would be expelled and sent packing, they say in a statement released by Australia’s Macquarie University.

Roughly 50% of these ejected moons would survive the immediate expulsion and avoid any subsequent collision with the planet or the star, ending up in orbit around the star, but in eccentric orbits. If such a scenario does take place, these “ploonets”, could potentially explain several puzzling phenomena.

Why have astronomers not yet confirmed the existence of a single exomoon, when they found over 4,000 exoplanets? Have the exomoons turned into ploonets?

In a paper to be published in the journal Monthly Notices of the Royal Astronomical Society, the researchers explain that the angular momentum between the planet and its moon results in the moon escaping the gravitational pull of its parent. They concede, however, that ploonets remain hypothetical.