Pluto may no longer be a planet, but this icy world at the edge of the Solar System keeps making the headlines. Latest news is that it has yet another moon.

Its fourth satellite, labelled simply P4 for now, was spotted as a faint smudge in photos taken with the Hubble space telescope.

Scientists had been checking to see if there were rings around Pluto in preparation for the arrival of NASA’s New Horizons probe which is currently racing across space for a rendezvous.

Instead, the tiny new moon was revealed, its motion in orbit around the planet clearly apparent in photos taken on 28 June and 3 and 18 July. It really is tiny too, with a diameter estimated at between 13 and 34 km (eight and 21 miles).

Pluto, which lost its status as a full planet when it was demoted by the International Astronomical Union in 2006, has three larger moons. The biggest, Charon, discovered in 1978, is 1,043 km (648 miles) wide, while Nix and Hydra, found by Hubble in 2005, are only 32 to 113 km (20 to 70 miles) across.

P4 appears to be in a circular orbit 59,000 km (37,000 miles) from Pluto, at a distance between Nix and Hydra.

Scientists believe that Pluto’s moons were created when it collided with another large icy body in the Kuiper Belt in the early days of the Solar System.

The discovery was made using Hubble’s Wide Field Camera 3. Mark Showalter of the SETI Institute, California, who led the observing program, 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 3 billion miles (5 billion km).”