Pluto, the most distant recognized planet of the Solar System, has two tiny satellites in addition to Charon, which was discovered in 1978.

US astronomers used the orbiting Hubble telescope to spot the moonlets, which have been labelled S/2005 P1 and and S/2005 P2 until formal names are approved after the International Astronomical Union (IAU) vets the find.

P1 and P2 appear to measure between 48 kilometres and 165 kilometres across and take 38 and 25 days respectively to orbit Pluto, according to their paper, published today in Nature, the weekly British science journal.

Pluto, discovered in 1930 by the American astronomer Clyde Tombaugh, is the outermost of the acknowledged planets, although a new contender for that title emerged last year in the form of an object called 2003 UB313.

Last month, astronomers made their most accurate measurement of the elusive Charon, estimating it to have a diameter of around 1,212 kilometres, making it about half the size of Pluto's 2,300 kilometres.

Charon's density is also very similar to that of Pluto's, which appears to back theories that the planet whacked into a large space object, causing a large chunk to break off and eventually be enslaved as a satellite.

P1 and P2 may have resulted from the same collision, says Alan Stern of the US Southwest Research Institute, in a review of the latest discovery, whose lead author is Hal Weaver of the Johns Hopkins University.

Weaver's team tentatively announced the discovery of the two moons last November after a nearly six-month analysis of images sent back by the Hubble.