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Planetary News: Pluto (2006)

Nix and Hydra Join Pluto's Family

By Amir Alexander
22 June 2006

The dark family at the far edge of our solar system has now expanded to include four members. Until now the planet Pluto, named for the Roman god of the underworld, was accompanied only by its large moon Charon, the untrustworthy boatman who ferries the souls of the dead to their final destination. This week the pair was joined by Nix, the goddess of darkness who is also mother of Charon, and Hydra, the giant monster keeping watch at the gate of the underworld, with the body of a serpent and nine heads. These were the names assigned by the International Astronomical Union (IAU) to the two small moons discovered in the summer of 2005 circling Pluto outside the orbit of Charon. The IAU is the recognized body charged with officially naming celestial objects.

Nix and Hydra, previously known as S/2005 P2 and S/2005 P1 respectively, were discovered last year during a Pluto moon search conducted in preparation for the launch of spacecraft New Horizons, currently on its way to Pluto. The two moons are small – only a few dozen kilometers in diameter compared to Charon's 1200 kilometers (700 miles) and Pluto's 2400 kilometers (1400 miles), with Hydra, the outermost moon, about 10%-15% smaller than Nix. They move in near circular orbits around Pluto on the same orbital plane as Charon, and their orbital periods are in resonance with that of their large neighbor, meaning that they bear a simple ratio (e.g. 1:2 or 2:3) to the period of Charon. Scientists believe that this configuration is strong evidence that the three moons were formed by a single cataclysmic event, when a large impact on the surface of Pluto sent vast amounts of debris into space. Over time, these rocks and dust particles in orbit around the planet accreted to form the three moons we see today.

Pluto and its Moons
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Pluto and its Moons
Pluto, Charon, Nix, and Hydra, as observed by the Hubble Space Telescope on February 15, 2006 Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the Hubble Space Telescope Pluto Companion Search Team

"We're very pleased with the decision of the IAU" said Alan Stern of the Southwest Research Institute, who is Principal Investigator on New Horizons and co-leader of the team that discovered Nix and Hydra. "You're going to be hearing a lot more about Nix and Hydra in coming years" he promised, adding that New Horizons will map the two in detail when it flies through the Plutonian system in 2015. Hal Weaver of the Johns Hopkins University Applied Physics Laboratory, the other co-leader of the discovery team, noted that although it had taken 48 years to find Pluto's first moon, and another 27 to find the next two, future discoveries may come easier. New Horizons will be on the lookout for any additional moons during its visit to the system nine years from now.

Apart from the dark connotations of the designations Nix and Hydra, appropriate for Pluto's companions, other considerations also contributed to the selection of the two names. The first letters of the two new moons, "N" and "H" are a tribute to the New Horizons mission just as the two first letters of the name "Pluto" were a tribute to Percival Lowell, the astronomer and observatory founder who foresaw the discovery of the planet. Hydra's nine heads are a nod to the ninth planet, Pluto, and its first letter "H" also honors the Hubble Space Telescope that was used in the moons' discovery. Nix is the second solar system body to be named for the goddess of darkness. Since asteroid 3908 is already designated by the Greek appellation "Nyx," the Plutonian moon is named "Nix," its Egyptian equivalent.

Meanwhile, as Pluto's family grows, its status as the 9th planet of our solar system seems shakier than ever. Pluto's planetary designation has been questioned ever since it became clear that it was not a solitary object, but rather the brightest memebr of a whole class of "Kuiper belt objects" (KBO's). The discovery of giant KBO's such as Sedna and Quaoar in recent years, only slightly smaller than Pluto, increased the pressure to clearly define what counts as a planet, but it was the detection last year of the KBO designated 2003 UB313 which brought the matter to a head. For 2003 UB313 is actually slightly larger than Pluto itself, which raises the inevitable question: if Pluto is a planet, shouldn't 2003 UB313 be one as well? Or conversely, if 2003 UB313 is not a planet, shouldn't Pluto be demoted to the status of a simple KBO as well?

In August, the IAU will be meeting to resolve the question, by finally providing a firm definition for the term planet. By September the solar system may have only 8 planets, if Pluto is excluded, or it may have as many as 20 planets, if all giant KBO's are deemed planets. Either way, the traditional 9-planet solar system seems to have run its course.