Ask a Scientist: Why doesn’t Earth have rings like Saturn’s?

Stephen Reynolds is an astrophysicist and professor of physics at N.C. State. Here he explains the science behind planetary rings systems. Questions and answers have been edited.

Q. Where do planets get their rings?

A. The origins of rings are not clear. They could be left-over material from the formation of the planets that couldn't coalesce, material blasted off the surfaces of moons by collisions with other bits of rocks, or parts of a moon that was disrupted by “tidal forces” (the difference in gravity on the side closest to the planet compared to the side furthest). If a moon is held together only by its own (relatively weak) gravity, if it gets too close to its planet, those tidal forces can pull it apart.

Q. Why do some of the giant gaseous planets in our solar system have rings – while the smaller, closer-to-the-sun terrestrial planets do not?

A. Bits of matter could be orbiting in any direction, but the big gaseous planets also spin fast, so their equators bulge out. That “equatorial bulge” helps pull orbiting particles into a plane instead of their buzzing around in all kinds of orbits like a swarm of bees. None of the inner planets has nearly that much bulge.

Big planets are also more likely to have moons, so there are hundreds, down to boulder size, orbiting the four gas giants.These moons could also be shattered by asteroids, generating debris that would then fall into line to add to the rings around a planet.

But there are only three moons – of which two are the tiny moons of Mars – orbiting the four terrestrial planets

Q. What are those rings made of, and why are they different colors? Why are the rings horizontal on some planets and vertical on others?

A. Rings are made of independently orbiting small solid particles: everything from tiny dust grains to boulders. The solid particles may be rock, water ice or frozen versions of other gases. The bright colors you see in some images are actually false-color representations of slight differences in the composition of the ring particles detected by sensitive instruments. In reality, these rings would appear in varying shades of gray.

Because the rings (and moons) orbit around the equator of a planet, they can take on different orientations. Uranus happens to be tilted on its side, so the rings are also tilted.

Q. If Earth had rings, would that change its rotation, tides, weather, etc.?

A. Not much would change. Even the gigantic ring systems of Saturn wouldn’t generate enough gravity to affect Earth’s rotation or tides. The shadows cast from the rings might affect the amount of light reaching Earth and create cooler temperatures in some regions, but that would depend on the density of the rings.

However, rings would make it more hazardous to orbiting spacecraft! There's enough man-made junk in orbit now to cause serious concern for artificial satellites, which frequently get hit by tiny particles.

A ring system would look pretty cool. We would presume that a ring system around Earth would also orbit in Earth’s equatorial plane, so if you looked up from the Equator, you'd see a bright line in the sky (because the ring systems of other planets are very thin), or if you were looking from other latitudes, a series of bands.

This story was originally published April 24, 2016 at 3:00 AM with the headline "Ask a Scientist: Why doesn’t Earth have rings like Saturn’s?."