An image of Uranus and its rings captured by the Keck II telescope. Credit: W. M. Keck Observatory (Marcos van Dam)
Uranus may have a pair of previously undiscovered tiny moons orbiting near two of its rings, according to a study carried out using data from Voyager 2.
Researchers used information on Saturn’s rings captured by the Cassini spacecraft and applied that to old data from the Voyager 2 mission, which flew past Uranus 30 years ago.
The images showed a pattern in Uranus’s rings similar to structures seen in Saturn’s rings called ‘moonlet wakes’.
These are gaps caused by the presence of small moons orbiting close to a planet’s rings.
The researchers, Rob Chancia and Matt Hedman of the University of Idaho, noticed patterns in Uranus’s rings from Voyager data captured in 1986.
It showed the amount of material in the edge of one of the planet’s brightest rings - its alpha ring - varied periodically, and a similar pattern was seen in the same part of the beta ring.
"When you look at this pattern in different places around the ring, the wavelength is different - that points to something changing as you go around the ring.
There's something breaking the symmetry," says Hedman.
The pair looked at data from radio signals sent through Uranus's rings by Voyager 2 that were detected back on Earth, and also observations made by the spacecraft of how light from distant stars shines through the rings, helping reveal how much material they contain.
The study suggests the moonlets would be 4-14km in diameter; smaller than any other of Uranus’s 27 known moons.
The discovery could also provide an explanation as to why Uranus’s rings are so narrow. It could be that the moons’ orbits are keeping the rings from spreading out wider.
This is already known to happen with the planet’s moons Ophelia and Cordelia, which restrict the expansion of Uranus’s epsilon ring.
The two researchers have said they will leave confirmation of the moons’ existence to other researchers, who may be able to follow up with further telescope or spacecraft images.
"The problem of keeping rings narrow has been around since the discovery of the Uranian ring system in 1977 and has been worked on by many dynamicists over the years," says Chancia.
"I would be very pleased if these proposed moonlets turn out to be real and we can use them to approach a solution."