The Hubble Space Telescope has revealed that Pluto appears to have not just one, but three moons.

by Dr Tony Phillips

Pluto was discovered in 1930. The planet resides 3 billion miles from the sun in the heart of the Kuiper Belt, a vast region of icy, rocky bodies beyond Neptune's orbit. In 1978, astronomers discovered Charon, Pluto's only confirmed moon.

"If, as our new Hubble images indicate, Pluto has not one, but two or three moons, it will become the first body in the Kuiper Belt known to have more than one satellite," said Hal Weaver of the Johns Hopkins Applied Physics Laboratory, Laurel, Md. He is co-leader of the team that made the discovery.

The candidate moons, provisionally designated S/2005 P1 and S/2005 P2, are approximately 27,000 miles (44,000 kilometers) away from Pluto--in other words, two to three times as far from Pluto as Charon.

These are tiny moons. Their estimated diameters lie between 40 and 125 miles (64 and 200 kilometers). Charon, for comparison, is about 730 miles (1170 km) wide, while Pluto itself has a diameter of about 1410 miles (2270 km).

Pluto, as seen from its new moon P1

The team plans to make follow-up Hubble observations in February to confirm that the newly discovered objects are truly Pluto's moons. Only after confirmation will the International Astronomical Union consider permanent (and catchier) names for S/2005 P1 and S/2005 P2.

The Hubble telescope's Advanced Camera for Surveys observed the two new candidate moons on May 15, 2005. "The new satellite candidates are roughly 5,000 times fainter than Pluto, but they really stood out in these Hubble images," said Max Mutchler of the Space Telescope Science Institute and the first team member to identify the satellites.

Three days later, Hubble looked at Pluto again. The two objects were still there and appeared to be moving in orbit around Pluto.

"A re-examination of [older] Hubble images taken on June 14, 2002 has essentially confirmed the presence of both P1 and P2 near the predicted locations based on the 2005 Hubble observations," said Marc Buie of Lowell Observatory, Flagstaff, Ariz., another member of the research team.

Hubble pictures show the new moons on the move

The team looked long and hard for other potential moons around Pluto, but they didn't find any.

"These Hubble images represent the most sensitive search yet for objects around Pluto," said team member Andrew Steffl of the Southwest Research Institute, "and it is unlikely that there are any other moons larger than about 10 miles across in the Pluto system," he concludes.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. The Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington, under contract with Goddard.