Astronomers using the Hubble Space Telescope to look for potential rings around dwarf planet Pluto have instead uncovered a fourth moon orbiting the distant icy world.

The moon, currently assigned the temporary name P4, is the smallest of Pluto’s satellites – astronomers estimate that its diameter is between 13 and 34 kilometres. The largest moon Charon is 1,043km wide and Nix and Hydra are in the range of 32-113 kilometres. P4 is 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 5 billion kilometres,” says Mark Showalter of the SETI Institute, who led this Hubble observing program. The moon was first identified in a Wide Field Camera 3 image taken on 28 June, and confirmed in follow-up images snapped on 3 and 18 July. The new images had much longer exposure times than those previously taken of Pluto’s neighbourhood, which is why P4 has been overlooked until now.

The hunt for rings around Pluto stems from the idea that material blasted off Pluto’s moons by micrometeorite impacts may go into orbit around the planet. So far no evidence for the proposed ring has been identified, but the Hubble observations will continue to prime the New Horizons spacecraft for when it encounters the outer Solar System dwarf planets in 2015 for close-up investigation.