Astronomers using the Hubble Space Telescope have identified another moon around the dwarf planet Pluto.
It becomes the fourth object known to be circling the distant world after the long-recognised Charon and recently observed Nix and Hydra satellites.Scientists are temporarily calling the new moon P4 and estimate its diameter to be 13 to 34 km (of 8 to 21 miles).
Pluto, controversially demoted from full planet status in 2006, will be the target of a big space mission in 2015.
Nasa's New Horizons probe is due to fly past the icy world and should get a good look at the moons, also.
"This is a fantastic discovery," said New Horizons' principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado. "Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby."
P4 sits between the orbits of Nix and Hydra, which Hubble identified in 2005. The space telescope did not discover Charon - that was done by the US Naval Observatory in 1978 - but it was the first astronomical instrument to resolve it as a separate body from Pluto.
For comparison, Pluto itself is a little over 2,300km across, Charon about 1,200km in diamter, and Nix and Hydra in the range of 30 to 115km across.
Hubble first saw P4 with its new Wide Field Camera 3 on 28 June. Follow-up observations this month confirmed its existence.
New Horizons will fly by Pluto in the July of 2015. The spacecraft's seven instruments will carry out detailed mapping of the object's surface features, composition and atmosphere.
The probe will go to about 10,000km from Pluto and about 27,000km from Charon, before pressing onwards.
With extra Nasa approval and funding, the probe will then be maintained to travel on to other objects in the Kuiper Belt, a region of space that contains many frozen leftovers from the construction of our Solar System.
The $700m probe was launched in 2006, the same year the International Astronomical Union - astronomy's offical nomenclature body - decided Pluto no longer merited full planet status, giving it the new classifcation of dwarf planet.
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