why do planets have rings around them
Planetary ring systems are complicated, notes
Director, and they are more common than once believed. For ages, Saturn was thought to be the only planet in our solar system with a ring system. But in recent years ring systems have been discovered around Jupiter, Uranus and Neptune as well. There are various theories about planetary rings, like the fantastic rings around Saturn, but we cannot say for sure how they are formed, explains Lattis. One theory is that the rings formed at the same time as the planet and its major moons.
In this case, if material is close to the planet, the planet s gravitational pull is too strong to coalesce into a moon and the particles that would otherwise form a moon spread out in orbit around the planet as a ring. Another idea is that a close call by a moon or comet results in the planet s gravitational tidal force breaking up those bodies, the debris of which then becomes a ring system. Although astronomers can t say for certain what causes planetary rings, Lattis says that the Cassini spacecraft now in orbit around Saturn is beginning to provide tantalizing new clues to the forces that govern the physics of planetary rings.
Building on what hartacus said, there's a couple other reasons involved as well: Saturn is about 746 million miles (1. 2 billion km) Uranus is about 1. 7 billion miles (2. 5 billion km) Neptune is about 2. 7 billion miles (4. 3 billion km) So Uranus and Neptune are very far away. and they're also smaller, AND their rings are not as dense. In an amateur telescope, it's very hard to see Uranus as anything other than a small, dim, fuzzy, blueish disk.
Neptune is just a blue-green dot. The rings are so insubstantial, in comparison, that they just don't stand out. In the case of Jupiter, the problem is simply that they're so insubstantial. The Jovian rings are very thin when compared to Saturn's. Two flyby missions, Pioneer 10 and Pioneer 11, flew by Jupiter without noticing them at all. Combined with the difference in composition and albedo, they're just not visible to smaller Earth-based telescopes.
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