Subtle variations in the thin rings of Uranus may hint at the existence of an additional two moons in that planet’s orbit, according to a study set to be published in The Astronomical Journal.
Two University of Idaho physicists made an early draft of their findings available on arxiv.org on 7 October 2016. and on 21 October 2016 NASA issued a press release about their study:
A new study led by University of Idaho researchers suggests there could be two tiny, previously undiscovered moonlets orbiting near two of the planet's rings.
The researchers estimate that the two objects, if they exist, would be around two to seven km in radius.
While the findings are new, the two researchers—doctoral student Rob Chancia and his advisor Matthew Hedman—used decade’s old data from NASA’s Voyager 2 probe, which made a fly-by of the seventh planet from the Sun in January 1986.
The scientists analyzed two types of datasets from that mission: radio signals from the Voyager 2 probe sent back to Earth which traveled through the rings on their journey home, and light from distant stars temporarily obscured by the rings before being instruments on board Voyager 2 detected them.
These data, termed radio and stellar occultations, allowed Chancia and Hedman to generate a high resolution map of the structure of Uranus’ rings. Their analysis showed that—surprisingly—the structure of the rings varied with longitude. In the press release, Hedman said:
“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.”
The two researchers had honed the skills necessary to search for tiny objects in the rings of planets while studying the rings of Saturn. The researchers noted that the patterns they observed in Uranus’ rings were similar to patterns, termed moonlet wakes, identified in the rings of Saturn.
These would be the first moonlets discovered lurking within Uranus’ rings. Saturn’s first indirectly observed moonlets were discovered in 2006, and since then over a hundred more have been inferred in similar ways.
While the researchers made an effort to confirm visually the existence of the two moonlets, they fear it may be impossible given the 1970s era technology that Voyager was equipped with at the time. In the press release, the scientists said that they would leave the detection aspect to other researchers:
Confirming whether or not the moonlets actually exist using telescope or spacecraft images will be left to other researchers, Chancia and Hedman said. They will continue examining patterns and structures in Uranus' rings, helping uncover more of the planet's many secrets.
If the moonlets do indeed exist, they might help to explain another of Uranus’ mysteries. Scientists have long been baffled by why the planet’s rings, undiscovered until Voyager’s flyby, are so narrow. These moonlets may help contain (or “shepherd”) them in place, as NASA describes:
The moonlets, if they exist, may be acting as “shepherd” moons, helping to keep the rings from spreading out. Two of Uranus’ 27 known moons, Ophelia and Cordelia, act as shepherds to Uranus’ epsilon ring.