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Pluto, the most distant planet in our Solar System, has been known for nearly 30 years to have a moon - Charon - about half as wide as the planet itself. Although some scientists suspected that the planet may have other, smaller moons, at such a great distance from the Earth they would be very hard to spot.

Now two such moons have been seen for the first time. Hal Weaver and co-workers report in Nature that they have found them in images taken by the Hubble Space Telescope. Compared with Charon, the two new moons, called P1 and P2, are tiny. Their exact size is hard to gauge, but reasonable assumptions about their reflectivity indicate that they are both between 48 and 165 kilometres across, compared with Charon's diameter of about 1,200 kilometres. The researchers estimate that P1 orbits Pluto once every 38 days, and P2 every 25 days.

Where did these moons come from? Charon is believed to have formed, like our own Moon, from the debris created when another object slammed into its parent planet. In a second, related paper, Alan Stern and colleagues suggest that a small amount of the material from this impact on Pluto gathered together under its own gravity to form P1 and P2.

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