Planetary rings might look sturdy and solid from afar, but they're made of tiny bits of ice and rock orbiting the planet.

"Rings are mostly made of ice and small rocks," said William Saunders, a planetary scientist at NASA's Langley Research Center. "Saturn's are mostly ice, Jupiter's are mostly dust, and Uranus' and Neptune's are made of both." Dust rings appear much fainter than ice rings, making them harder for us to see.

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What a planet's ring is made of also partly depends on where it is. In the outer solar system, it's cold enough for ice to form. But "if Earth had a ring, it wouldn't be made of ice because Earth is too close to the sun for ice to stay frozen in space," Saunders said.

A planet’s moons can also influence its ring system. For example, Jupiter has rings just like the other gas giants, but its rings are smaller and fainter than Saturn's. Jupiter's multiple large moons (namely, the Galilean satellites: Io, Europa, Ganymede, and Callisto) make it hard for a large ring to exist, as they'd disturb and destabilize it with their gravity.

There are a few ways a planet can get rings. If a large object hits a planet, it can break off bits of the planet and blast the impacting object to smithereens. Because of gravity, most of that debris doesn't just float away into space. Instead, it is gathered into an orbit around the planet, making a ring.

Alternatively, a planet can devour its moon to create a ring. If a moon gets too close to its planet, the planet's gravity can rip the moon apart, leaving a trail of debris that becomes a ring. (Don't worry, we're not at risk of this anytime soon!) Rings can also grow as a planet scoops up objects that pass nearby, like an asteroid that crosses into the planet's gravitational sphere of influence.

Astronomers still have questions about planetary rings. We don't know exactly where Saturn's rings came from, nor why each planet's rings are so incredibly different. Uranus' rings hold a particularly intriguing mystery.

"We think that Uranus' rings would be unstable without little, shepherding moons, but we haven't discovered those moons yet," Saunders said. "That's one of the many reasons we need to send an orbiter to Uranus!"