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Pluto's Family

by David Tholen

Pluto and satellites

Image of the Pluto system taken on February 15, 2006, by the Hubble Space Telescope. NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRl), and the HST Pluto Companion Search Team.

Pluto has been in the news a lot lately, primarily for being demoted from full planet to "dwarf planet." But Pluto has also made news in scientific circles for the 2005 discovery of two small moons in orbit around it, bringing the number of its known satellites to three. Joining Charon, which was discovered in 1978, are Nix and Hydra.

It took the power and clarity of vision of the Hubble Space Telescope (HST) to spot the new members of Pluto's family. Both new satellites are approximately 5,000 times fainter than Pluto, which was discovered in 1930 by Clyde Tombaugh. They eluded detection until 2005 because most images of Pluto and Charon were made using exposure times optimized to show just those two bodies, so Nix and Hydra were grossly underexposed. A group of astronomers led by Hal Weaver (Applied Physics Laboratory) discovered them when they took purposely overexposed images of Pluto specifically to look for fainter satellites.

It was a discovery that almost didn't happen. Only about one in six proposals can be awarded time on HST because time requests exceed the time available. However, because one of HST's instruments, the Space Telescope Imaging Spectrograph, failed, the Space Telescope Science Institute, which operates HST, took another look at the proposals that didn't make the cut during the first review process and allocated time to the project to search for additional satellites of Pluto.

Since they are very faint and only recently discovered, little is known about Nix and Hydra at the moment. I am working with Marc Buie and Will Grundy of Lowell Observatory to establish their size, location, composition, and brightness more accurately. We have determined that the orbits of the new satellites are nearly coplanar with Charon's orbit, which strongly suggests a common formation mechanism for all three satellites. Whereas Charon has a nearly circular orbit almost 20,000 kilometers (12,000 miles) from Pluto, Nix orbits Pluto at a distance of about 49,000 kilometers, and Hydra orbits at a distance of about 65,000 kilometers. As dictated by Kepler's third law, the greater the distance from Pluto, the longer the orbital period. Charon completes one trip around Pluto in just 6.4 days (compared with 27.3 days for the Earth's Moon), whereas Nix requires 25.5 days and Hydra nearly 39 days.

The size of Charon is known with great accuracy because in 2005 it passed directly in front of a relatively bright star, thereby casting its shadow on Earth. Astronomers timed the duration of the star's disappearance. By combining that duration with the known velocity of Charon's motion, its size could be determined to stunning accuracy, a diameter of 1,212 kilometers (753 miles), slightly more than half the size of Pluto itself. (Both are smaller than our Moon, which has a diameter of 3,475 kilometers, or 2,159 miles.) In fact, Charon is large enough for its gravity to overcome the strength of its rock and ice composition, crushing itself into a round shape. Had the International Astronomical Union adopted the Planet Definition Committee's original proposed definition of a planet--round bodies orbiting the Sun--both Pluto and Charon would have qualified as planets, making it a binary planetary system.

The sizes of Pluto's new kin are not yet known with any accuracy. It is likely that both Nix and Hydra are smaller than 100 kilometers (about 60 miles) in diameter. That's less than one-tenth the size of Charon.

It's possible that there are more satellites of Pluto just waiting to be discovered, since Pluto's gravitational sphere of influence extends far beyond Hydra. But if additional satellites exist, they would almost certainly have to be even smaller and fainter than Nix and Hydra, otherwise they should have been seen in the same images used to discover the new moons. It's even possible that Pluto could have a ring. However, such discoveries will almost certainly have to wait for the flyby of NASA's New Horizons spacecraft, due at Pluto in July 2015. In the meantime, my collaborators and I will continue to refine our measurements of the size, location, composition, and brightness of these four fascinating objects, so that when the spacecraft's cameras zoom in for a close up, they are able to maximize the science return.