In 1986, NASA’s Voyager 2 spacecraft flew by Uranus coming within 50,600 miles from the planet. During the flyby, the spacecraft gathered an abundance of data on the planet, its atmosphere, its rings and its moons. Now, decades later, a study from University of Idaho researchers suggests the planet could have two previously undiscovered moonlets.
The discovery was made by University of Idaho doctoral student Rob Chancia, who was inspecting images taken of Uranus three decades ago. During his examination, he noticed a discrepancy in the amount of ring material on the edge of the planet’s alpha ring—one of the five narrow rings that encircles Uranus. The volume of ring material varied intermittently and the same variation was seen on the beta ring as well.
"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,” said Matt Hedman, an assistant professor of physics at the University of Idaho, in a statement. “There's something breaking the symmetry."
Chancia and Hedman estimate the supposed moonlets in the rings of Uranus, the seventh planet from the sun, are two to nine miles in diameter. This would make them smaller than the planet’s 27 known moons, which are as small as eight - 10 miles across. Thus far, the moons are merely a hypothesis, as Uranian moons are difficult to identify due to their surfaces being covered in dark material.
"We haven't seen the moons yet, but the idea is the size of the moons needed to make these features is quite small, and they could have easily been missed," Hedman said. "The Voyager images weren't sensitive enough to easily see these moons."
Should the moons exist, then they are likely “shepherd” moons that prevent the rings around the planet from spreading out. This wouldn’t be unheard of since two other moons—Ophelia and Cordelia—serve as shepherd moons for Uranus’ epsilon ring.
The discovery of more shepherd moons offer scientists an explanation as to why Uranus has narrow rings, as opposed to Saturn’s extensive ring system.
"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," said Chancia, who published the study in The Astronomical Journal. "I would be very pleased if these proposed moonlets turn out to be real and we can use them to approach a solution."
Both Chancia and Hedman are experts on planetary rings, as they study Saturn’s rings using data from NASA’s Cassini spacecraft. The new data from the Cassini mission allows them to better understand how planetary rings behave and to apply that knowledge to previously gathered data from Voyager 2. As such, the pattern of ring material spotted in Uranus is similar to that in Saturn’s rings, suggesting the presence of moonlets.
Confirmation on whether the moonlets exist will require a telescope or spacecraft and Chancia and Hedman are going to leave that for other researchers in the field.