Fourth Moon Discovered Orbiting Pluto

Artist impression of the surface of Pluto based on scientific information.
Artist rendering of Pluto based on scientific information.

Astronomers using the Hubble Space Telescope have discovered a fourth moon orbiting the icy dwarf planet Pluto.  The tiny new satellite, temporarily designated P4, was uncovered in a Hubble survey searching for rings around the dwarf planet.

The new moon is the smallest discovered around Pluto.  It has an estimated diameter of 8 to 21 miles.  By comparison, Charon, Pluto’s largest moon, is 648 miles across, and the other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter.

“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,” said Mark Showalter of the SETI Institute in Mountain View, Calif., who led this observing program with the Hubble Space Telescope.

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 in planning for the 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.

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 rings 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.

Pluto was discovered in 1930 by Clyde Tombaugh, an amateur astronomer working at Lowell Observatory, Flagstaff, Ariz., and it was hailed as the ninth planet in our Solar System.  It was subsequently “demoted” to the status of “dwarf planet” by the International Astronomical Union (IAU) in 2006.  The IAU decision was quite controversial at the time, and is not yet universally accepted.  For example, in Tombaugh’s home state of New Mexico, the Legislature passed a resolution stating that Pluto would always be considered a planet in New Mexico.


You can contact Bob Eklund at [email protected], or visit his websites at and

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