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Planetary News: Space Telescopes (2005)

Hubble Spots Two New Rings at Uranus

And New Names Are Given to Two Tiny Moons

The Hubble Space Telescope's sharp eyes have revealed yet more rings around the tilted planet Uranus. The rings were observed as part of a long-term observing campaign by scientists Mark Showalter and Jack Lissauer. The newly discovered rings extend the diameter of Uranus' observed ring system by a factor of two. The campaign has also yielded the discoveries of new moons as well as the observation of chaotic behavior in the orbits of the previously known moons. Two moons that were discovered in 2003 are now named Cupid and Mab.

Two composite images captured by Hubble in 2003 and 2005 show Uranus and its newly extended family of rings. Shorter-exposure color observations show clouds in the Uranian atmosphere, while longer-exposure panchromatic observations show the faint outlines of the new rings, just barely visible above background noise. One of the newly named moons, Mab (formerly known as S/2003 U1), is co-orbital with the outermost of the newly discovered rings and makes a slightly brighter arc near the top of the 2003 image.

Click to enlarge > Newly discovered rings and moons of Uranus Source Credit: NASA, ESA, and M. Showalter (SETI Institute)

The ring and moon system is very tighly packed, as the diagram below shows. "The inner swarm of 13 satellites is unlike any other system of planetary moons," Lissauer remarked in 2003. "The larger moons must be gravitationally perturbing the smaller moons. The region is so crowded that these moons could be gravitationally unstable. So, we are trying to understand how the moons can coexist with each other." Showalter and Lissauer's observations, dating back to 1994, have revealed "random or chaotic" motions among the moons. According to their calculations, the system of moons and rings should be unstable on a time scale of just millions of years, a tiny fraction of the age of the solar system. They note that the other newly named moon, Cupid (formerly known as S/2003 U2), has an orbit that takes it within a mere 500 kilometers (300 miles) of much larger Belinda.

Click to enlarge > Orbits of Uranus' rings and moons Uranus' rings and moons form a bullseye of orbital paths around the tilted planets. Two new rings, U1 and U2, were announced in December 2005. Credit: NASA