A tiny, fourth moon has been discovered orbiting Pluto.

The new moon, temporarily named P4, was found by the Hubble telescope as it was searching for rings around Pluto in late June, NASA reported Wednesday.

Pluto used to be considered our solar system's ninth planet, but was demoted to the status of "dwarf planet." It has three other moons, Charon, which is about 1,043 kilometres across, and Nix and Hydra, which are each 32 to 133 kilometres wide.

P4 is just 13 to 32 kilometres across and located between the orbits of Nix and Hydra.

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km)," said Mark Showalter of the SETI Institute in Mountain View, Calif., in a statement. Showalter is leading the Hubble program that made the discovery.

Its goal is, in part, to plan the itinerary for NASA's New Horizons spacecraft, which is designed to observe Pluto and its moons. It was launched in 2006 and is expected to fly past Pluto in July 2015.

The new discovery means the New Horizons team can plan close-up observations of P4 during the flyby, said Alan Stern, principal investigator of the New Horizons project.

Hubble first photographed P4 on June 28. More photos were taken on July 3 and July 18.

NASA said P4 wasn't seen in earlier photos of Pluto because they used shorter exposure times.

Nix and Hydra were also discovered by Hubble, in 2005.