Go on, make all the jokes you like, but there are probably two “dark moons” hiding near Uranus. Finished sniggering now? Because once you're done we can get on to talking about astronomy.
Like all the outer planets, Uranus has a lot of moons, mostly quite small. Prior to the latest announcement, the number was thought to be 27. So two more probably rather small ones wouldn't change much, were it not for the way they were found, by looking for distortions in nearby rings.
William Herschel shook up 18th-century science with the discovery of Uranus, the first planet unknown to the ancients. Between this discovery and when Voyager 2 passed Uranus in 1986, five moons were found, along with the Solar System's second ring system. Voyager added 11 to the moon count, and since then Hubble and ground-based telescopes have added 11 more. Yet 40 years after Voyager's approach, two astronomers have picked up something that was missed at the time.
In the Astrophysical Journal (preprint available on ArXiv.org), University of Idaho postgraduate student Rob Chancia and his supervisor Dr Matthew Hedman point out patterns in Uranus' alpha and beta rings are indicative of them being shepherded by small moons. The patterns could be seen when the rings passed between Voyager and a star, blocking out the light.
The authors estimate that the moons orbit about 100 kilometers (62 miles) further from the planet than the rings they shape.
This schematic of Uranus' rings and moons might need an update. Ruslik0/Wikimedia Commons
Uranus' ring system is not nearly as large as Saturn's, but it is considerably more impressive and complicated than that of either Jupiter or Neptune. However, it is formed from tiny particles that are surprisingly dark, and were only discovered when they blocked the light of a star in front of which Uranus was passing, rather than being directly observed.
This darkness has contributed to our ignorance about the nine-fold ring system. Two moons have been shown to be responsible for confining the epsilon ring, and similar explanations have been offered for other rings. Without these moons the particles within the rings would be expected to spiral outwards, causing the ring to slowly dissipate. However, the survival of the alpha and beta rings has remained a mystery.
Using data from the Voyager mission, Chancia and Hedman found that the rings have wavy edges that indicate the particles are bunching up in the manner expected if there were small moons slightly further out whose gravity was tugging them out of shape. A similar technique was used to detect the location of Saturn's moon Pan, whose existence was subsequently confirmed by searching through images taken by Voyager. The moon Ophelia has a similar effect on Uranus' epsilon ring.
Finding these two moons will be harder, however. Besides being closer to the planet than any known moon, they're probably between 2 and 7 kilometers (1 to 4 miles) in radius and Voyager would not have been able to detect anything at the smaller end of that scale. If made of similar material to the rings, they are also dark, further reducing the chances of detection. Nevertheless, New Scientist reports plans are afoot to seek these objects with the Hubble Space Telescope.
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