Planetary rings are at the origin of Solar System satellites
Two French researchers published the first model that is cabable of explaining the origin of the vast majority of regular planetary satellites  in our Solar System from planetary disks. First tested in 2010 with the moons of Saturn, this model can explain the distribution of the numerous satellites surroundng the so-called “giant” planets, and can also explain the presence of satellites surrounding the so-called “terrestrial” planets, such as those around the Earth and Pluto . These results are decisive for understanding and explaining in a universal manner the formation of planetary systems.
These results were published in the November 30, 2012 issue Science.
The planetary systems encircling the giant planets (Jupiter, Saturn, Uranus and Neptune) are fundamentally different from those surrounding the terrestrial planets (such as the Earth and Pluto). Whereas the giant planets are surrounded by both rings and a myriad of natural satellites, the terrestrial planets possess only a few moons, or even a single unique moon, and no rings. Up until the present, two models were commonly used to explain the presence of regular satellites in our Solar System. For the case of the terrestrial planets, it was suggested that a collision with a large body was at the origin of the formation of their satellites. For the case of the giant planets, it was suggested that the satellites formed from a gas disk surrounding the planet. However, these models could not explain the specific distribtion and chemical composition of the satellites in orbit about the giant planets. Another theory was necessary.
In 2010 and 2011, as a result of numerical simulations and data from the Cassini mission, a French research group developed a new model that described the formation of the moons of Saturn. These researchers discovered that the rings of Saturn, which are very thin disks of small blocks of ice, gave rise to the formation of icy satellites. Over time, the rings spread outward, and when they reached a specific distance, called the Roche limit, the material near the edges coallesced, forming small bodies that detached from the rings and moved outward. The rings gave birth to the satellites in orbit about the planet.
In this new study, two researches, Aurélien Crida of the University Nice Sophia Antipolis and of the Observatoire de la Côte d’Azur, and Sébastien Charnoz of the Universiy Paris Diderot and CEA, wanted to test this new model to see if it could be generalized to other planets. Their calculations illuminated several points. This model of forming satellites from planetary disks explains why the largest satellites are found further from the planet than smaller satellites. In addition, the model predicts the presence of an accumulation of satellites near the “Roche limit”, the place where they are born just exterieur to the disk. This distribution is in perfect agreement with the satellites of Saturn. The same model can equally be applied to the giant planets Uranus and Neptune, which are organized with the same architecture. This suggests that these planets once possessed in the past rings that were as massive as those of Saturn, but that they were lost by giving birth to their presently observed satellites. Finally, this model can also explain the formation of the satellites surrounding the terrestrial planets. According to calculations carried out by these researchers, in certain cases, a single satellite could form from a disk surround a planet: this is the case of the Moon that orbits the Earth, and Charon that orbits Pluto. Thus, using a sole mechamism, it is possible to explain the formation of the vast majority of regular satellites in our Solar System.
Artists view of the planet Uranus, and a portion of its system of natural satellites, at the time when it possessed massive rings. The outer boundary of the disk, which is rich in dust, is the birth place of new satellites. The inner boundary of the disk falls into the atmosphere of the planet.
© Graphic design : Frédéric Durillon/www.animea.com
This work is the fruit of a collaboration between the laboratory Lagrange (Université Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur) and the laboratory AIM (Université Paris Diderot, CEA, CNRS).
The numerical calculations were made possible thanks to the support of the Sorbonne Paris Cité’s program UnivEarthS.
A. Crida and S. Charnoz
Formation of Regular Satellites from Ancient Massive Rings in the Solar System, Science, Nov/30/2012.
CEA : Coline Verneau – 01 64 50 14 88 – firstname.lastname@example.org
Observatoire de la Côte d’Azur : Marc Fulconis – 04 92 00 19 70 – Marc.Fulconis@oca.eu
 The regular satellites orbit close to the equator of the central planet, such as is the case of the Earth’s Moon.
 Pluto is classified as a dwarf planet.
 The Roche limit is located at a radius of approximately 2.5 times the radius of the planet. When the rings of a planet extend beyond this distance, they become unstable and form agregats that are capable of sticking to each other. This eventually gives rise to the birth of a moon that detaches itself from the ring.
Translation by Mark A. Wieczorek