The Hubble Space Telescope can add another impressive achievement under its belt. Experts using the observatory to check for rings around the dwarf planet Pluto announce the discovery of a new moon orbiting the distant object, the fourth one so far.

Pluto is known to have one large moon called Charon, which is around 648 miles (1,043 kilometers) across. It is joined in orbit by two smaller objects, called Nix and Hydra, which are between 20 and 70 miles (32 to 113 kilometers) in diameter.

Hubble's new observations indicate that another moon, known at this point only as P4, is also orbiting the dwarf planet. The object is smaller that all the other natural satellites, with an estimated diameter of 8 to 21 miles (13 to 34 kilometers).

What is most impressive about this research is that the 20-year-old telescope was able to spot such a small space rock from so far away. Pluto lies between 4.4 and 7.4 billion kilometers from the Sun, the equivalent of 30 to 49 astronomical units (AU).

“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 kilometers),” says SETI Institute expert Mark Showalter, the leader of the Hubble observing program.

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

He is the principal investigator of the NASA New Horizons mission, a deep space probe that is currently heading towards Pluto. The spacecraft is scheduled to make a close fly-by of the Plutonian system in 2015, before heading farther out into the Kuiper Belt.

Hubble data place P4 between the orbits of Nix and Hydra. The two objects were also discovered using the telescope, about six years ago. Hubble also helped confirm the 1978 discovery of Charon, which was made by experts at the US Naval Observatory.

Astronomers believe that the three small moons orbiting Pluto were produced during a large collision that took place early on in the history of the solar system. It is conceivable that the impact was also roughly the same size as Pluto, maybe a little smaller.

The material that the impact ejected into space eventually went on to coalesce as moons. The same scenario is known to have occurred around other planets in the solar system, such as Earth and Mars.

“This surprising observation is a powerful reminder of Hubble's ability as a general purpose astronomical observatory to make astounding, unintended discoveries,” NASA Headquarters Astrophysics Division director Jon Morse comments.

The Wide Field Camera 3 (WFC-3) on the telescope is responsible for the discovery. The images that revealed P4 were snapped on June 28. Photos snapped on July 3 and July 18 confirmed the finding.