The new names have been approved by the Paris-based International Astronomical Union (IAU), the international authority for assigning appellations to celestial bodies, a member of the IAU panel, Richard Binzel, told AFP.

Pluto, currently enshrined as the outermost planet of the Solar System, has a large moon, Charon, which was discovered in 1978.

Nix and Hydra were spotted in 2005 by US astronomers using the orbiting Hubble Telescope.

The new names are in line with the tradition of Solar System appellations inspired by Greek and Roman mythology.

In Greek mythology, Nyx is the goddess of darkness. As an asteroid, number 3908, already bears this name, so the IAU used the Egyptian equivalent, spelt Nix.

The hydra was a terrifying monster with the body of a serpent and nine heads.

Both names were chosen because they were fitting for Pluto, who was the god of the underworld.

Pluto is the ninth acknowledged planet of the Solar System, although this status is under challenge.

Some astronomers believe it is so small and its orbit so unusual that it should be downgraded to the status of a rock, rather than a planet.

And if it is a planet, its role as the farthest planet from the Sun could be supplanted by another object, provisionally named Xena, which was spotted in July 2005.

Hydra and Nix measure between 48 kilometers (30 miles) and 165 kilometers (103 miles) across -- there is not enough data yet to gauge their size exactly -- and take 38 and 25 days respectively to orbit Pluto, according to the discoverers, from the US Southwest Research Institute, the Johns Hopkins University, the Space Telescope Science Institute and Lowell Observatory.

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh and is one of the most enigmatic objects in the Solar System.

An unmanned US spacecraft, New Horizons, is due to fly by Pluto and the Kuiper Belt in 2015.

"Pluto doesn't reveal its moons easily," said Hal Weaver of Johns Hopkins' Applied Physics Laboratory.

"It took 48 years after the discovery of Pluto to find Charon and another 27 years to find Nix and Hydra.

"Perhaps we won't have to wait as long for the next discovery, because the New Horizons spacecraft will be making a rendezvous with Pluto in nine years and will be searching for other small satellites."

In January, astronomers made their most accurate measurement of the elusive Charon, estimating it to have a diameter of around 1,212 kilometers (757.5 miles), making it about half the size of Pluto's 2,300 kilometers (1,437 miles).

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