Why Saturn Has Rings

Gravity pulls more strongly on the near side of a body than on the far side. Usually, this difference is small and has no effect. But near a massive planet, it becomes stronger than the debris pile's own gravity, so the pieces can no longer stick together.

This boundary is called the Roche limit, and almost all the rings of known planets lie inside it, while moons lie outside. It creates a division of labor: beyond the boundary, material gathers into moons; inside it, the material remains scattered.

Where the material itself came from is a separate question. There are two main theories: leftover material from the planet's formation, or a body torn apart by tides, such as a moon or a large comet that came too close.

The proportions are hard to wrap your head around. If you shrank the rings to the size of a soccer field, their thickness would be less than that of a sheet of paper. That's why every few years, when we see them edge-on, the rings simply disappear from view.

They consist mostly of water ice, from dust particles to house-sized chunks. Ice reflects light well, which explains their brightness: the rings are visible through even a modest telescope, although by cosmic standards they contain very little material.

Measurements from the latest mission to Saturn pointed to something unexpected: the rings have less mass than expected, and the ice is too pure. If they had existed for billions of years, dust would have contaminated them by now. That puts their estimated age at hundreds of millions of years rather than the age of the planet itself.

On top of that, material from the rings is gradually falling onto the planet. At the current rate, estimates suggest this will continue for another hundred million years. The figures are disputed and still being refined, but the overall conclusion is intriguing: we happened to catch Saturn while it had rings.