US astronomers using the Hubble Space Telescope have discovered a fifth and tiniest moon yet orbiting the icy dwarf planet Pluto.
The moon is estimated to be irregular in shape and between 10 km and 25 km across. It is visible as a speck of light in Hubble images, NASA said.
The newly discovered moon, named S/2012 (134340) 1 until it is formally named, could help reveal more on how the Pluto system came into existence and evolved.
It was detected in nine separate sets of images taken by Hubble's Wide Field Camera 3 taken during June and July.
The Pluto team is intrigued that such a small planet can have such a complex collection of satellites.
According to the favoured theory, all the moons revolving around Pluto are relics of a collision between the dwarf planet 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, the leader of the team that discovered the new moon.
Pluto's largest moon, Charon, which is about 1,000 kilometres across, was discovered in 1978. Hubble observations in 2006 uncovered another two smaller moons, Nix and Hydra.
"The inventory of the Pluto system we're taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft," added Alan Stern of the Southwest Research Institute in Boulder, the mission's principal investigator.
Last year Hubble had discovered another moon revolving around Pluto. A NASA spacecraft named New Horizons is currently en-route to Pluto and will arrive there in 2015.
New Horizons 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.
Pluto was discovered by American Scientist Clyde Tombaugh in 1930. It was regarded as the ninth full-fledged planet in the Solar System but astronomers have since demoted it to a dwarf planet in 2006.
Pluto was declassified as a planet due to the fact that it is one of several large, icy objects that reside in the Kuiper Belt, a region just beyond the orbit of Neptune.