Though Pluto was demoted to dwarf planet status back in 2006, the distant, icy rock at the fringe of our solar system has three more moons than Earth, according to scientists with the Space Telescope Science Institute (STSI).

On Wednesday, they announced that they have discovered a fourth satellite orbiting the distant dwarf planet.

Temporarily designated P4, it is the smallest of Pluto's known satellites, with an estimated diameter of eight to 21 miles, according to its discoverers.  At 746 miles across, Pluto's largest moon, Charon, was discovered in 1978.  The other two, Nix and Hydra, are between 20 and 70 miles in diameter and were spotted in Hubble images back in 2005.

P4 is located between the orbits of Nix and Hydra.

STSI scientists first spotted P4 in a photo taken with Hubble's Wide Field Camera 3 on June 28, they said in a statement.  Its existence was then confirmed in two more pictures taken July 3 and July 18.

The dwarf planet's entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup tossed material that coalesced into the family of satellites observed around Pluto.

Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago.

Pluto's fourth moon was spotted while Hubble was being used to search for rings around the dwarf planet, scientists said.

I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than three billion miles, Mark Showalter of the SETI Institute in Mountain View, California, who led a team mapping Pluto with the Hubble Space Telescope, said in a statement.

P4's discovery was the result of work done to support NASA's $700-million New Horizons mission.  The mission hopes to send a probe through the Pluto system in 2015.

Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby, said Alan Stern of the Southwest Research Institute.  This is a fantastic discovery, he added.

Hubble is a project of international cooperation between NASA and the European Space Agency.