Planetary rings
Why doesn’t Earth have a ring?
A ring is ice or rock that orbits inside a limit where a moon would be pulled apart. Earth could wear one if a moon broke up that close. Our Moon is too far out and too well stuck together. Saturn’s rings on Saturn and its moons are the famous leftover — ice that never quite made a moon, or a moon that came too close.
The Roche limit
Too close to a planet, tides across a moon are stronger than the moon’s own gravity. The moon comes apart and the pieces spread into a ring. That distance is the Roche limit. Saturn’s bright rings live inside it. Titan lives outside it.
Earth has a Roche limit too, only a few Earth-radii up. Our Moon is about 60 Earth-radii away. It is in no danger of becoming a ring. If something ice-rich wandered in that close, Earth could wear a faint ring for a while — but nothing that size is on that path now.
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The giants all have some kind of ring
Jupiter, Uranus and Neptune have faint dusty rings as well as Saturn’s ice boulders. One idea is that a ring is a phase every giant goes through: moons grind down, dust is replaced, the show fades. Saturn’s would then be a bright snapshot of something the others have already finished. Earth never had the leftover ice at the right distance, and its one large moon formed much farther out.