Researchers Might Have Found Two Hidden Moons Around Uranus

Reexamined data from Voyager 2 reveals a spooky surprise.


Uranus’s rings are dark and full of surprises. Researchers detected a strange, wavy pattern within the rings, which suggests there might be two tiny, never-before-seen moons orbiting close to the planet.

Compared to the other planets in the solar system, we know very little about the distant, icy world. To date, only one spacecraft has visited Uranus — Voyager 2 in 1986. Scientists have pieced together observations from many different ground-based and space-based observatories to try and understand this world.

Unlike the rest of the solar system, Uranus is tilted on its side, subjecting the planet to extreme weather and decades-long seasons. Like the other gas giants, Uranus has many moons — 27 we know of, and 10 of which were discovered during the Voyager flyby. However, the moons of Uranus are named for Shakespearean characters as opposed to the mythological figures we see in the Jovian and Saturnian systems.

The planet also has an unusual set of rings, which are much darker and narrower than the ones we see around Saturn. When the Voyager 2 spacecraft flew by the planet, it measured the amount of material in the rings by beaming a set of radio waves through the rings, and back towards Earth. By recording the amount of light that passes through the ring material, astronomers can measure how much material is there.

Uranus is seen in this false-color view from NASA's Hubble Space Telescope from August 2003. The brightness of the planet's faint rings and dark moons has been enhanced for visibility.

NASA/Erich Kakoschka

Two planetary scientists, Rob Chancia and Matt Hedman of the University of Idaho, recently took another look at the Voyager data, uncovering something surprising. Their findings, to be published in The Astronomical Journal, illustrates signs of “moonlet wakes — wave-like disruptions formed when the gravitational tug of the moons pulls dust particles into the rings. The perturbations could indicate the presence of a pair of tiny, previously undiscovered moons lurking close to the planet. (Previous studies showed similar ripples within the rings, caused by two of the planet’s other moons — Cordelia and Ophelia.)

“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,” Hedman said in a news release.

If the moons exist, they’re probably less than nine miles in diameter. Given the dark surface materials that cover previously discovered Uranian moons, it wouldn’t be shocking if the innermost pair had avoided detection.

“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.

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