The giant planets of the Solar System have rings around them, but their origin and the differences between the spectacular rings of Saturn and the more modest ones around Neptune and Uranus have been unknown for a long time. Scientists think they now have answers to those questions.

An international group of researchers, led by Hyodo Ryuki from Kobe University in Japan, has performed sophisticated computer simulations to work out where the rings come from.

According to their model, the rings are formed from destroyed planetoids that once inhabited the Kuiper belt, the area of the Solar System beyond the orbit of Neptune. These objects are expected to have an icy exterior and a rocky inner core.

In the early days of the Solar System, in a period called the Late Heavy Bombardment, many of these objects moved inwards and the ones unfortunate enough to pass near a gas giant were pulverized and turned into rings.

The model also provides an answer as to why Saturn’s rings are rich in ice (95 percent) while the rings of Uranus and Neptune are darker and probably rich in rocky materials. The cause of this, the team believes, has to do with Saturn’s density.

Saturn has a significantly lower density than the other planets, so much so that it would float in water (don’t try this at home). Given its sizable mass and low density, Saturn has a larger radius with respect to its gravity compared to Uranus and Neptune.

These Kuiper belt objects can get close enough to Uranus to be completely disrupted, rocky core included, but for the rocky part to be disrupted by Saturn, they’d end up inside the planet. So, Saturn’s rings come from more distant interactions where only the icy top layers are affected.

According to the paper, published in Icarus, the giant planets are capable of stealing between 0.1 to 10 percent of the mass of the passing objects. This is enough to explain the present-day mass of the rings. The simulations also showed that the initial fragments, with a size of a few kilometers, experienced several high-speed collisions, shattering them, with the pieces eventually settling into the circular orbits we observe today.

This study provides an interesting solution to the origin of the rings of the outer planets, but it might soon have even wider applications. We are already observing exoplanets with rings, and they appear to be just as complex and mysterious.