Moons of Uranus, Neptune may have come from large rings, model finds

The moons revolving around Uranus and Neptune may have been formed from large rings that used to surround the planets. According to a report published Thursday in the journal Science, such rings may in fact be the source of most moons in the solar system.

The analysis is based on a mathematical model of moon formation created by the study’s authors, Aurélien Crida and Sébastien Charnoz, who work at the Université de Nice Sophiaantipolis in Nice, France, and the Sorbonne in Paris.

That model relies on what’s called the “Roche radius,” which is the distance away from a planet beyond which orbiting material will not aggregate into a larger mass because of planetary tides. The farther you get outside the Roche radius, the larger the objects that orbit the planet will become because larger masses are able to collect there without interference from the planet below.

That prediction lines up extremely well with the moons that orbit Saturn, Uranus, and Neptune, and suggests that Uranus and Neptune used to have large rings that migrated outside of the Roche radius, allowing those planets’ moons to form.

An important prediction of the model is that the speed with which the rings drift away from their planet predicts just how many moons will form. If the drift is slow, as Crida and Charnoz believe it was with Uranus and Neptune, many moons will form. But if the drift is fast, as it was with Earth and Pluto, only one moon will form.

According to the authors, it remains unclear just how such giant rings formed around planets such as Uranus and Neptune in the first place. Nevertheless, their model, they argue, provides a common mechanism by which giant planets like Uranus, Neptune and Saturn and terrestrial planets like Earth might both end up with moons.

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