Wandering moons called ‘ploonets’ could be the culprits behind astronomical mysteries

All four gas giant planets in our solar system have moons orbiting them, but it’s unknown whether that’s true of any of the many gas giant exoplanets that have been discovered orbiting other stars. Researchers have a theory about it, and their concept could also be the mystery player behind other astronomical phenomena.

Astronomers have yet to find a confirmed “exomoon,” or a moon outside our solar system, even if they are predicted to form around massive planets. Exomoons are harder to pinpoint than exoplanets because of their smaller size.

In 2018, astronomers discovered what could be an exomoon, estimated to be the size of Neptune. It was found in orbit around a gigantic gas planet 8,000 light-years from Earth. But the scientists behind this discovery, hesitant to confirm that the new find is an exomoon due to some of its peculiarities, say more observation is needed. Their findings were published in the journal Science Advances.

Related article First suspected 'exomoon' discovered 8,000 light-years away

In a new study soon to be published in the Monthly Notices of the Royal Astronomical Society journal, other researchers modeled the formation of exomoons around gas giant exoplanets. They projected that the massive planets would kick moons out of orbit and send them on their way – or the researchers believe that angular momentum between the giant exoplanet and moon would allow the moon to essentially escape the gravity of the planet.

While half would probably be destroyed by this expulsion or a potential collision with the planet or star, the other half are projected to survive.

This is an artist's concept of the tiny moon Hippocamp that was discovered by the Hubble Space Telescope in 2013. Only 20 miles across, it may actually be a broken-off fragment from a much larger neighboring moon, Proteus, seen as a crescent in the background. This is the first evidence for a moon being an offshoot from a comet collision with a much larger parent body.

Related article Meet Hippocamp, Neptune's new moon

The remnants of the expelled moon would end up circling its star with an eccentric orbit similar to Pluto’s. Pluto has an angled, elliptical orbit on a different plane than the rest of the planets in our solar system. It takes 248 Earth years to complete one full orbit of the sun.

The researchers have dubbed these rogue exomoons “ploonets.”

Many of the early exoplanets discovered are so-called hot Jupiters, gas giant exoplanets that are closer to their stars than Jupiter is to its own, and warmer. These were common discoveries during the early days of exoplanet hunting because they were easy to find, but they represent only about 1% of known exoplanets now. And research suggests that some of them should have large moons.

But if they were ejected from orbit, that would explain why exomoons are missing from detection. Instead, the moons are basically on their own.

This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852, also known as Boyajian's Star or Tabby's Star.

Related article Why does the 'most mysterious star in the universe' act this way?

“These moons would become planetary embryos, or even fully-fledged planets, with highly eccentric orbits of their own,” said study author Jaime Alvarado-Montes of Macquarie University in Australia.

If they exist, the ploonets could explain the mystery of Tabby’s Star, why stars appear to cannibalize each other and even exocomets around other stars.

Related article 12 new moons discovered around Jupiter

“The strange changes in [Tabby’s Star’s] light intensity have been observed for years, but are still not understood. Ploonets could be the answer,” Alvarado-Montes said.

But actual evidence of ploonets remains elusive. It could be that they deteriorate quickly after escaping their planets’ orbit and can’t be observed.

“If the timescales are large enough, we could have real chances to detect them in the near and middle future,” the researchers wrote in the study.