Planet rings could be behind the formation of solar system satellites

The results of this research are published in the Nov. 30, 2012 edition of Science.

There is a fundamental difference between giant planet systems, such as Jupiter and Saturn, and the terrestrial plants, such as Earth or Pluto. Whereas the giants are surrounded by rings and a myriad of small natural satellites, the terrestrial planets have few moons, or just one, and no rings. Until now, two models have been commonly used to explain the presence of regular satellites in our solar system. These indicate that the satellites of the terrestrial planets like Earth or Pluto were formed following a giant collision. They also indicate that the satellites of the giant planets were formed in a nebula surrounding the planet. They do not, however, account for the specific distribution and chemical composition of the satellites orbiting the giant planets. Another theory therefore seemed necessary.

In 2010 and 2011, a French research team developed a new model to describe how Saturn's moons came into being based on numerical simulations and Cassini probe data. The researchers discovered that Saturn's rings, which are very thin disks made up of small blocks of ice surrounding the planet, in turn gave birth to ice satellites. This is due to the fact that the rings spread over time and, when they reach a certain distance from the planet (known as the Roche limit or Roche radius), their ends agglomerate and form small bodies that break off and move away. This is how rings give birth to satellites orbiting the planet.

In this new study, two research lecturers, Aurélien Crida from the Université Nice Sophia Antipolis and the Observatoire de la Côte d'Azur and Sébastien Charnoz from the Université Paris Diderot and the CEA, set out to test the new model and discover whether it could be extended to other planets. Their calculations have brought several important facts to light. This model for satellite formation from planet rings explains why the largest satellites are located farther away from the planet than the smaller satellites. It also points to the accumulation of satellites close to the Roche limit, their "place of birth," on the outer edge of the rings. This distribution is in perfect agreement with Saturn's planetary system. The same model can also apply to the satellites of the giant planets, Uranus and Neptune, which are organized according to a similar layout. This suggests that these planets once had massive rings similar to Saturn's, which they then lost in giving birth to their satellites. Lastly, the model could also be applied to the formation of terrestrial planet satellites. And, according to the researchers' calculations, special cases exist where a single satellite may be formed from the ring around the planet. This is the case for Earth and the Moon, and for Pluto and Charon.

Thus, this planetary ring spreading mechanism alone could explain how the great majority of regular satellites were formed in our solar system.