Fourth Moon Adds to Pluto's Appeal
July 20, 2011

The tiny moon: With an estimated diameter of 8 to 21 miles (13 to 34 kilometers), P4 is the smallest moon discovered around Pluto. By comparison, Charon, Pluto's largest moon, is 648 miles (1,024 kilometers) across, and the other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter (32 to 113 kilometers).

On the anniversary of the first landing of men on our moon, New Horizons mission team scientists have announced the discovery of a fourth moon around Pluto – adding to the scientific treasure trove that awaits NASA’s Pluto-bound New Horizons when it arrives in 2015.

“Could this planet get any more interesting?” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder Colo. “We already know that when New Horizons provides the first close-up look at Pluto in July 2015, we’ll see planetary wonders we never could have expected. Yet this discovery gives us another hint of what awaits us in the Pluto system, and we’re already thinking about how we want to study this new moon with New Horizons. What a bonus for planetary science and for New Horizons!”

A Hubble Space Telescope observing team led by Mark Showalter of the SETI Institute, Mountain View, Calif., and Douglas Hamilton of the University of Maryland, College Park, detected the new moon in five sets of Hubble Space Telescope images taken over the past two months. Astronomers are still trying to better peg orbital details on the object, designated “S/2011 P1” or “P4” until it receives a permanent name. They’ve put its diameter at between 8 and 21 miles (13 to 34 kilometers) and estimate that it travels on a circular, equatorial orbit nearly 37,000 miles (about 59,000 kilometers) from Pluto – placing the new moon between the orbits of the moons Nix and Hydra.

Read the NASA/Hubble Space Telescope release

These two images, taken about a week apart by NASA's Hubble Space Telescope, show four moons orbiting the distant, icy dwarf planet Pluto. The green circle in both snapshots marks the newly discovered moon, temporarily dubbed P4, found by Hubble in June. The new moon lies between the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005. It completes an orbit around Pluto roughly every 31 days. The moon was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28, 2011. The sighting was confirmed in follow-up Hubble observations taken July 3 and July 18. P4, Nix, and Hydra are so small and so faint that scientists combined short and long exposures to create this image of Pluto and its entire moon system. The speckled background is camera "noise" produced during the long exposures. The linear features are imaging artifacts. The tiny satellite was uncovered in a Hubble survey to search for rings around the frigid dwarf planet.

Signs of the new satellite in the June-July 2011 images also sent the observing team – which includes Stern, Hal Weaver of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and Andrew Steffl and Leslie Young of Southwest Research Institute – combing though older Hubble snapshots. Knowing where to look, Weaver found strong evidence for P4 in images from June 2010. Steffl saw the same in a set taken in February 2006. Together these “archival” records add to the available suite of data needed for scientists to accurately determine the moon’s orbit.

The discovery of a fourth moon adds to Pluto’s complex and fascinating story. Discovered in 1930 by American astronomer Clyde Tombaugh, Pluto was first considered an icy oddball with a strange orbit – yet it harkened the existence of thousands of similar frozen mini-worlds in a planetary outpost known as the Kuiper Belt. Pluto’s moon Charon was discovered in 1978; it’s the largest moon in the solar system relative to the planet it orbits. (In fact, the pair orbits a common center of gravity that sits outside each body – earning it the rare status of a “double planet.”)

Charon got company in 2005, when a team led by Weaver and Stern found two smaller moons – subsequently named Nix and Hydra – again using Hubble.

“Pluto continues to amaze us,” says Weaver, the New Horizons project scientist. “Who would have thought a dwarf planet could support such a complex satellite system? The hunt will now be on for similarly complex systems around other Kuiper Belt objects.”

Aside from giving the New Horizons team something to plan for and look forward to, the discovery offers a peek into Pluto’s violent past. “The discovery of this moon reinforces the idea that the Pluto system was formed during a massive collision 4.6 billion years ago,” says Weaver. “The smaller satellites, including this one, probably came together in the resulting debris disk.”

This discovery also suggests other small bodies may lurk in the Pluto system – and, if so, New Horizons should root them out when it flies by. But more “targets” might not be the best thing for the mission.“The discovery of P4 is exciting, but it also raises the possibility that our New Horizons spacecraft may enter a more hostile environment than we previously imagined,” says Glen Fountain, New Horizons project manager from APL.

“We don’t want our spacecraft running into any debris that’s still hanging around from the massive collision that spawned the formation of Pluto’s smaller satellites,” adds Stern. “For this reason we’re going to look with New Horizons, and even before we get there, we’ll be looking with Hubble and other tools, we hope.”

New Horizons launched on Jan. 19, 2006, picking up a gravity boost from (and conducting scientific investigations of) Jupiter in February 2007. The spacecraft is now more than 1.8 billion miles (2.9 billion kilometers) from Earth, on course to fly past Pluto on July 14, 2015, then on to one or more other Kuiper Belt objects in a possible extended mission. The Johns Hopkins Applied Physics Laboratory built and operates the New Horizons spacecraft and manages the mission for NASA; Stern leads the mission as principal investigator.