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The cosmos loves irony. Five years after Pluto was stripped of its planet status, astronomers have discovered yet another moon in orbit around it, bringing its entourage to four.
The tiny body may have been born in the same collision that gave birth to Pluto's other moons.
The Hubble Space Telescope spotted the new moon, which has been designated P4 for the time being. Astronomers estimate it is between 13 and 34 kilometres across. "I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles [5 billion kilometres]," says Mark Showalter of the SETI Institute in Mountain View, California, who led the observing team.
Three other moons of Pluto were already known. Charon, discovered in 1978, is by far the largest at about 1000 kilometres across. Nix and Hydra, discovered in Hubble images in 2005, are tiny by comparison: both are estimated to be between 32 and 113 kilometres in diameter.
All four moons are thought to have formed at the same time. "The discovery of this moon reinforces the idea that the Pluto system was formed during a massive collision 4.6 billion years ago," says discovery team member Hal Weaver of Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland.
"A big impact produced Charon, and the remaining three [moons] formed from the debris scattered further out," Showalter told New Scientist.
Team member Alan Stern of the Southwest Research Institute in Boulder, Colorado, also heads the New Horizons mission that will fly by Pluto in 2015.
He is excited by the discovery but says it reinforces the need to keep scanning the Pluto region for more objects that could pose a hazard to New Horizons.
"We don't want our spacecraft running into any debris that's still hanging around from the massive collision that spawned the formation of Pluto's smaller satellites," he says.
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