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The Question

(Submitted October 27, 1998)

What keeps planetary rings in place?

The Answer

You can think of rings as a bunch of tiny moons, all in a similar orbit around the planet. Each particle is in free-fall, like the space shuttle in orbit --- so one could ask why would a ring not stay there forever instead.

They don't because they collide with each other, and are subject to forces other than the gravity of its parent planet (gravity of the bigger moons, magnetic field etc.). Such collisions and extra forces tend to spread rings out, and at the lowest altitude, friction with the tenuous outer atmosphere will cause particles to drop onto the planet.

Sometimes planetary rings are kept in place by the gravitational force of shepherd moons. Saturn has a very intricate ring system with lots of moons helping to keep its rings together. According to http://ringmaster.arc.nasa.gov/neptune/neptune.html, Neptune's rings are probably confined by one of its moons. In the case of Jupiter's rings, particles in them probably don't stay there for long (due to atmospheric and magnetic drag). Galileo (the spacecraft) found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field. (from http://www.seds.org/nineplanets/nineplanets/jupiter.html) Uranus, like the other gaseous giant planets in our solar system, also has a ring system.

You can read more about planetary rings here: http://www-space.arc.nasa.gov/displaypage.cfm?page=Cuzzi_rings&branch=sst

And more about the planets here: http://www.seds.org/nineplanets/nineplanets/

Hope this helps!

Maggie Masetti & Koji Mukai
for Ask an Astrophysicist

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