The Hubble Space Telescope has discovered a fifth moon circling the dwarf planet Pluto.
The new moon, visible as a speck of light in Hubble images, is estimated to be irregular in shape and between 10km and 25km across.
Scientists are intrigued that such a small world can have such a complex collection of satellites.
The moon - known only as P5 - could help shed light on how the Pluto system formed and evolved.
According to one idea, all the moons are relics of a collision between Pluto and another large icy object billions of years ago.
"The moons form a series of neatly nested orbits, a bit like Russian dolls," said Mark Showalter from the Seti Institute in Mountain View, US, the leader of the team that discovered the new moon.
Pluto's largest moon, Charon, was discovered in 1978. Hubble observations in 2006 uncovered two additional small moons, Nix and Hydra.
In 2011, another moon, known as P4, was found by Hubble.
Provisionally named S/2012 (134340) 1, or P5, the latest moon was detected in nine separate sets of images taken by Hubble's Wide Field Camera 3 taken during June and July.
New Horizons, an unmanned Nasa spacecraft, is currently en route to Pluto, with a flyby of the object scheduled for 2015.
It will return the first ever detailed images of the Pluto system, which is so small and distant that even Hubble can barely see the largest features on its surface.
Discovered in 1930 by the American Clyde Tombaugh, Pluto was regarded as the ninth planet in the Solar System until its demotion in 2006.
The decision to recategorize Pluto as a "dwarf planet" was driven by a recognition that it is one of several large, icy objects that reside in the Kuiper Belt, a region just beyond the orbit of Neptune.
It'll be interesting to see if any of the other dwarf planets have similar small moons that haven't been noticed yet. Haumea's known moons, Hi'aka and Namaka, and Eris's, Dysnomia, all have diameters in the hundreds of metres.
Any surprise about Pluto having so many moons? Hm.
I'd bet that it has to do with the very large distances between objects- it's easy for a system of loosely gravitationally bound objects to hold themselves together out there, because they never run into anything big enough to seriously disturb them. And the moons are so tiny that their own gravitational influences on each other are practically nil.
Since an object seems to still be considered a moon even if it's only a kilometre or so across, and a moonlet, if it's hundreds or even tens of metres across, I expect that we'll keep on finding them (at least around the gas giants and dwarf planets: I'd be surprised if any more are found around the inner planets, given how much we've studied them). Also, I've just noticed that the larger Haumean moon is in fact named Hi'iaka, not Hi'aka. Sorry about that.
