Wandering rogue moons to be as common as stars

An exomoon circles a gas giant planet in this artist’s impression. What happens to exomoons when their planets communicate with each other?


Rogue moons, torn from their planets and beat to the outside to drive by the space, as often as stars in the universe, a new study found.

Moons are common in the solar system, Jupiter, only owning at least 67. As such, Yu-Cian Hong, an astrophysicist at Cornell University, wanted to see how moons around the distant stars would behave, she said.

Previous research shows that many giant exoplanets — worlds orbiting distant stars — oval-shaped or “eccentric” orbits. Prior work suggested that the most likely explanation for this orbital eccentricity became the attraction-of-war between planets. This led Hong and her colleagues to investigate what the effect of this “planet-planet scattering” could have on the moons of those worlds.Hong’s team found that the vast majority — about 80 to 90 percent of moons around giant extrasolar planets have been removed from their original homes by planet-planet scattering. These exomoons may involve a number of “wild behavior,” said a Hong [The Strangest Alien Planets We Know]

For example, a destabilized moon can go to a collision with a planet or a star. The moon may eventually kidnapped by a different planet or go on an orbit around the star on its own as a “planet.”

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The researchers also found that the planet-planet scattering can drive planets and their moons to go “rogue” — that is, they can fly through space without a star, as a free-floating body. In addition, exomoons may rogue on their own without a host of planets. In fact, rogue moons “may be higher than the number of floating planets in the universe and maybe even as numerous as the stars,” Hong said.

The exomoons are most likely to survive the planet-planet scattering that orbit close to their host planets that orbit the larger planets with a stronger gravitational pull, the researchers said. For example, exomoons in orbits similar to Jupiter’s four largest “Galilean” moons have approximately 20 to 40 percent chance to survive.

A way to see how many moons are in a stable orbit around a planet or a free-floating, without a host of planets could be due to NASA’s upcoming Wide-Field Infrared Survey Telescope (WFIRST) infrared space observatory, although WFIRST danger of cancellation. In the event that the telescope is not closed, “I am looking forward to the stable exomoons, or floating moons discovered with the upcoming WFIRST mission,” Hong said.

The scientists detailed their findings online Jan. 10 in the Astrophysical Journal.

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