Image caption The Hubble Space Telescope observations of the moon were made in June and July

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 estimate the new moon's diameter to be 13-34km (eight 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.

"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."

The new moon carries the provisional designation S/2011 (134340) 1. At some point in the future, when the orbit is well known, the object will be given a Roman numeral designation and a more memorable name, says the International Astronomical Union's (IAU) Minor Planet Center.

The fourth moon 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 the famous observatory 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 diameter, and Nix and Hydra are in the range of 30-115km across.

Hubble first saw the new moon with its new Wide Field Camera 3 on 28 June. Follow-up observations this month confirmed its existence.

New Horizons will fly past Pluto in July 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 travel 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 (£432m) probe was launched in 2006, the same year that the IAU - astronomy's official nomenclature body - decided Pluto no longer merited full planet status, giving it the new classification of dwarf planet.

Jonathan.Amos-INTERNET@bbc.co.uk