WHY ARE DENSE PLANETARY RINGS ONLY FOUND BETWEEN 8 AND 20 AU?

The recent discovery of dense rings around the Centaur Chariklo ( and possibly Chiron) reveals that complete dense planetary rings are not only found around Saturn and Uranus, but also around small bodies orbiting in the vicinity of those giant planets. This report examines whether or not there could be a physical process that would make rings more likely to form or persist in this particular part of the outer solar system. Specifically, the ring material orbiting Saturn and Uranus appears to be much weaker than the material forming the innermost moons of Jupiter and Neptune. Also, the mean surface temperatures of Saturn's, Uranus's, and Chariklo's rings are all close to 70 K. Thus the restricted distribution of dense rings in our solar system may arise because icy materials are particularly weak around that temperature.