The NASA/ESA Hubble Space Telescope has discovered a new moon circling Pluto. P4, as it is currently called, is the smallest moon yet found orbiting Pluto, with an estimated size of 13–34 km. By comparison, Pluto’s largest moon, Charon, is 1043 km across, while Nix and Hydra are 32–113 km wide. The tiny, new satellite popped up in a Hubble survey searching for rings around the dwarf planet.


This composite of two Hubble images shows Pluto’s four satellites in motion.

The new moon lies between the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005.

P4 completes an orbit around Pluto roughly every 31 days. It was first seen with Hubble’s Wide Field Camera 3 on 28 June. The sighting was then confirmed in follow-up Hubble observations on 3 July and 18 July. Long exposures are needed to see the new moon and this creates the speckled background from ‘noise’ in the camera. The cross shape is also an artefact of the camera.

The finding is a result of ongoing work to support NASA’s New Horizons mission, scheduled to fly through the Pluto system in 2015. The mission is designed to provide new insights about worlds at the edge of our solar system. Hubble’s mapping of Pluto’s surface and discovery of its satellites have been invaluable to planning for New Horizons’ close encounter.

“This is a fantastic discovery,” said New Horizons’ principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo. “Now that we know there’s another moon in the Pluto system, we can plan close-up observations of it during our flyby.”

The new moon is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the U.S. Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.

The dwarf planet’s entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites observed around Pluto.

“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., who led this observing program with Hubble.


An artist’s concept of Pluto’s satellite system with newly discovered moon P4 highlighted. Credit: NASA, ESA, and A. Feild (STScI)

Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago. Scientists believe material blasted off Pluto’s moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.

“This surprising observation is a powerful reminder of Hubble’s ability as a general purpose astronomical observatory to make astounding, unintended discoveries,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

P4 was first seen in a photo taken with Hubble’s Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter. There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.

Images and more information about Hubble.

Filed under: Pluto