Sometimes a good mystery is hiding just under your nose. That’s exactly what two astronomers at the University of Idaho found out when they discovered a pair of dark moons hiding near Uranus. The discovery was made then the two planetary scientists, Rob Chancia and Matthew Hedman, were combing through data gathered by the Voyager 2 spacecraft in 1986. Something in the Voyager data didn’t sit right with the scientists, who re-examined the craft’s 20-year old findings to determine what unexplained phenomenon was afoot around Uranus.
According to the astronomers’ pre-print publication on arXiv.org, the presence of two moonlets was detected through analyzing gravitational fluctuations in Uranus’ rings. These fluctuations implied that two small bodies were orbiting the seventh planet from the Sun and in fact might be helping keep Uranus’ rings in orbit around the planet:
Based on the observed structures in the rings, we estimate that the moonlets would need to be located about 100 km exterior to the rings' semi-major axes and be 2-7 km in radius. Such moonlets could help keep the rings confined. Due to their small radii and presumed low albedo, the expected brightness of these moonlets is on the order of the noise in Voyager 2 images.
The tiny moons are so small (between 4 and 14 km) that their presence on images and data gathered by Voyager’s cameras, low resolution by today’s standards, could have been written off as merely background noise. Furthermore, the composition of the two moons means that they reflect very little light, making them “dark” on images taken by Voyager.
Uranus already has twenty-seven known moons, so the presence of two more moons isn’t exactly a groundbreaking surprise in the astronomy world. However, the fact that new moons and planets are still being discovered in our own cosmic neighborhood shows that there might be much more mystery than we think lying just out of sight in places thought to be well-known to us like Uranus.