Giant moons escape fatal plunge into planets thanks to ‘safety zone’

9 Mar 2020

An artist’s impression of a satellite forming around a giant gas planet which is itself still forming around a star. Image: Nagoya University

Astronomers have discovered a ‘safety zone’ around planets similar to Saturn, which saves giant moons from plunging into their parent planet.

Numerical simulations have uncovered evidence of the existence of ‘safety zones’ surrounding giant gas planets.

Writing in Astronomy and Astrophysics Letters, researchers from Nagoya University and the National Astronomical Observatory of Japan have found that the temperature gradient in the gas disc of a young gas giant could play a critical role in keeping a single giant moon in orbit.

In the case of Saturn, for example, this would be Titan. It’s believed that many of the moons we see in the solar system – especially large moons – formed along with their parent planet. This would mean moons are formed from the gas and dust spinning around the infant planet.

However, previous simulations found that large moons should be swallowed up by their parent planet, or result in a number of large moons. The situation we observe around Saturn, with many small moons but only one large moon, does not fit in either of these models.

In this new simulation, the researchers created a new model of circumplanetary discs with a more realistic temperature distribution. They considered multiple sources of opacities including dust and ice.

‘A red-hot issue’

Researchers then simulated the orbital migration of moons considering pressure from disc gas and the gravity of other satellites. This revealed the existence of safety zones where a moon is pushed away from its parent planet. In this zone, warmer gas inside the orbit pushes the moon outwards to prevent it from falling into the planet.

“We demonstrated for the first time that a system with only one large moon around a giant planet can form,” said researcher Yuri Fujii. “This is an important milestone to understand the origin of Titan.”

However, fellow researcher Masahiro Ogihara warned that it would be difficult to know whether the same process is happening with Titan.

“Our scenario could be verified through research of satellites around extrasolar planets,” he said.

“If many single-exomoon systems are found, the formation mechanisms of such systems will become a red-hot issue.”

Colm Gorey was a senior journalist with Silicon Republic