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Scientists coin the term ‘ploonets’ for the bizarre hybrids created when a moon becomes a planet

Some moons around distant, giant planets can break free of their orbits and end up circling the host star like a planet instead — as what experts are calling a ‘ploonet’.


Researchers reveal that ploonets can form when gas giants like Jupiter are forced to migrate closer to their star, causing a transfer of angular momentum to their moons.

Such exomoons — moons in other star systems — have around a 50 per cent chance of becoming a ploonet, else they get ejected into space or collide with their planet.

No longer shielded by the magnetic field of their original host, however, ploonets are fated for doom — as they are gradually eroded away in the glare of stellar radiation.

While the orbits in our solar system are relatively stable, it is possible for planets to gradually move in closer towards, or less commonly away from, their host stars.

This phenomenon is believe to explain the existence of so-called ‘Hot Jupiters’, gas giants similar to their namesake in the solar system but that lie far closer to their host star — suggesting they formed further out before settling into a tighter, stable orbit. 

It is believed that this drift might occur many different ways, including through interactions with gas or planetesimals and scattering by a larger planet’s gravity.

For astrophysicist Mario Sucerquia of the University of Antioquia, in Colombia, and colleagues, this begged the question — where might do these migrating planet’s moons end up?

‘If large exomoons form around migrating giant planets which are more stable, what happens to these moons after migration is still under intense research’, they wrote.

While such moons might just be flung off into deep space, the team explored whether a large, regular exomoons might be able to end up in direct orbit around the host star as well, becoming a sort of ‘planet’ in their own right.

The researchers dub this hypothetical space body a ‘ploonet’.

In simulations exploring how exomoons would respond to the migration of their host planet over millions of years, the researchers found that exomoons became ploonets around 50 per cent of the time.

The rest of the time, however, the moons were either ejected out of the star system or ended up colliding with their erstwhile host planet.

The findings mean the ploonets may exist in other star systems, and in principle could be detected by telescopes on or orbiting the Earth.

Actually detecting a real ploonet, however, will likely prove challenging.

‘The detectability of a ploonet not only depends on its size, but also on its orbital distance from the planet,’ the researchers wrote.

Furthermore, the researchers think that most ploonets might be essentially doomed from the outset — slowly evaporating in the radiation coming from the host star.

‘Volatile-rich ploonets are dramatically affected by stellar radiation,’ the researchers noted. 

‘Their radius and mass change significantly due to the sublimation of most of their material during time-scales of hundreds of millions of years.’

  ‘In contrast, moons still in circumplanetary orbits could be still protected by the magnetic field of their planets, reducing the rate of atmospheric erosion produced by the stellar wind,’ they added.

Even in the worse case scenario, however, an eroding ploonet will likely leave a little reminder of itself behind.

‘In a realistic situation, even under the harshest radiation conditions, a ploonet will not entirely evaporate, but probably leave a residue of a refractory material,’ the researchers wrote.

A pre-print of the article, which has not yet been peer-reviewed, can be read on the arXiv repository. 


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