Pluto's fifth moon discovered

This photo from the Hubble Space Telescope shows Pluto and its five known moons, including a newly discovered satellite indicated as P5. Its provisional name is S/2012 (134340) 1.
This photo from the Hubble Space Telescope shows Pluto and its five known moons, including a newly discovered satellite indicated as P5. Its provisional name is S/2012 (134340) 1.M. Showalter / SETI Institute / NASA / ESA

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By Alan Boyle, Science Editor, NBC News

Astronomers using the Hubble Space Telescope have discovered Pluto's fifth moon, a little more than three years before a NASA space probe is due to sail past the dwarf planet and its tribe of satellites.

The irregular moon, estimated to be 6 to 15 miles (10 to 25 kilometers) across, was found in the course of checking out the potential collision hazards facing NASA's New Horizons spacecraft for the Bastille Day flyby on July 14, 2015. "The inventory of the Pluto system we're taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft," the mission's principal investigator, Alan Stern of the Southwest Research Institute, said in a Hubble news release.

Stern and his colleagues suspect this fifth moon won't be the stuff they find in Pluto's neighborhood. "The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system," said Harold Weaver of the Johns Hopkins University Applied Physics Laboratory.

Call it P5 ... for now

The fifth moon is currently known only by its provisional names: S/2012 (134340) 1, or P5 for short. It'll be up to the discoverers to propose a more lyrical name to the International Astronomical Union, which classified Pluto as a dwarf planet in 2006.

P5 was detected in 14 separate sets of images taken by Hubble's Wide Field Camera 3 on June 26, 27 and 29 plus July 7 and 9. The Hubble team says it's in a 58,000-mile-diameter circular orbit around Pluto that steers clear of the dwarf planet's four other satellites — including the biggest moon, Charon. Two other moons, Nix and Hydra, were discovered in 2006, and the fourth moon (P4) was found in Hubble data last year.

"The moons form a series of neatly nested orbits, a bit like Russian dolls," team leader Mark Showalter of the SETI Institute said in today's news release. He told me that small moons have now been found in the Pluto-Charon system at close to a 1-to-3 orbital resonance with Charon (P5), a 1-to-4 resonance (Nix), 1-to-5 (P4) and 1-to-6 (Hydra). This suggests that the moons were formed from debris blasted away by the collision that led to the coalescence of Pluto and Charon as we know them today.

"This is a very tidy system, and what that means is, it's an orbitally evolved system," Showalter said. "Literally there are shells where the orbits are stable."

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Pluto's moons are traditionally named after Greek mythological characters associated with the underworld. Nix, for example, is an alternate spelling for Nyx, the name of the Greek goddess of the night and the mother of the Fates. (The more typical spelling, Nyx, was used previously in the name of an asteroid.) Hydra is the serpentine monster that guarded the gates of the underworld. "It's a very colorful cast of characters," Showalter told me.

For P4 and P5, the team members are holding off on proposing names for now, just in case a P6 comes along. "It's still a moving target, because we don't know what might come along," Showalter said. "I expect that in a month or two, we'll have finished everything we're going to find until New Horizons gets close." Only then will the team seriously consider what the two (or more) moons will be named. If things stay as they are, P4 and P5 will probably be named after a pair of characters with Greek underworld connections, such as Orpheus and Eurydice. (The name Orpheus is already taken, but they could go with a variant, such as Orfeo.)

As of today, Showalter says there are no other Plutonian moon candidates in sight. "Of all the things that we have looked at, that we thought might be moons, none of them has ever been convincing until this came along," he said of P5. "There is no P6 in our back pocket at this time."

The detection ... and the debate

Finding P5 was hard enough. Showalter told me that he first spotted the moon in the data on Saturday, the 7th. He and his colleagues then went back and found signs of the moon in the data gathered earlier, as well as the follow-up imagery captured on Monday. The object is just 0.001 percent as bright as Pluto, and 4 percent as bright as Nix, Showalter said. "We're really at the edge of what we can accomplish with Hubble," he said. "I don't know of any instrument that's going to be better than that."

In an IAU circular issued today, the team reports that P5's brightness is about magnitude 27, which makes it half as bright as P4. The brightness was used to estimate P5's size.

Today's announcement revived the debate over whether Pluto should be counted as a planet, period, rather than a dwarf planet. The difference, as outlined by the IAU almost six years ago, has to do with how much a celestial body has "cleared out the neighborhood of its orbit." In my book, "The Case for Pluto," I set out the argument for counting Pluto and other worlds that have a basically roundish shape as types of planets, even if they're put in the dwarf category.

"The name 'dwarf planet' really doesn't bother me," Showalter said. "When you think of a bonsai tree, it's still a tree, and what's interesting about it is that it's really, really small. I think of Pluto the same way. ... It only gets more interesting with each one of these discoveries that comes along. If you don't like the term 'dwarf planet,' call it a 'bonsai planet.'"

I like that approach. But what about you? Feel free to weigh in below with your comments on Pluto, P5, or your suggestions for the names of the bonsai planet's newest moons.

More about Pluto:

Last updated 4:30 p.m. ET.

In addition to Showalter, Weaver and Stern, members of the discovery team include A.J. Steffl and M.W. Buie.

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.