For a long time, Pluto was considered an oddball planet — and although that perception has changed, astronomers now report that its tiniest moons rank among the oddest in the solar system.
"The Pluto system is more interesting than we thought," said Mark Showalter, a planetary scientist at the SETI Institute who's one of the authors of a paper about Pluto's moons in this week's issue of the journal Nature.
A detailed analysis of observations by the Hubble Space Telescope suggests that the orbital dance between Pluto and its largest moon, Charon, throws off the orbits of four smaller moons ever so slightly — and sends them tumbling like cosmic footballs.
One moon, Nix, apparently flips around. "If you were living on the north pole, you would suddenly find yourself on the south pole," Showalter told NBC News. Later, he told reporters that "you would literally not know if the sun is coming up tomorrow."
Another moon, Kerberos, is markedly darker than Pluto's other moons — and that suggests its origins may be different from those of the other moons. Showalter said Kerberos' composition might well provide evidence for a cosmic collision that astronomers say took place billions of years ago, and gave rise to Pluto and Charon as we know them today.
"We could be seeing a piece of the 'bullet' that broke apart Pluto," he said. "But that's just speculation at this point."
Ready for their close-up
Pluto was once considered the oddest of the solar system's nine planets, but over the past decade, more objects like Pluto have been discovered on the solar system's icy edge, more than 3 billion miles (5 billion kilometers) from the sun. Now Pluto is considered one of the largest objects in a category known as dwarf planets.
"'Dwarf' is an honorific. It's like saying 'Sir Pluto,'" joked John Spencer, a space scientist at the Southwest Research Institute who was not involved in the Nature study but is part of the science team for NASA's New Horizons mission to Pluto.
After a cruise of more than nine years, the piano-sized New Horizons spacecraft will capture unprecedented views of Pluto during a flyby on July 14. The findings outlined in Nature are based on Hubble observations made between 2005 and 2012, primarily to make sure New Horizons won't slam into something as it zooms past.
The observations led to the discovery of the moons Nix and Hydra in 2005, Kerberos in 2011, and Styx in 2012. In contrast to Charon, which is about half as wide as Pluto itself, these four smaller moons are thought to be no wider than 36 miles (60 kilometers) — and Styx's diameter could be less than 5 miles (8 kilometers).
Showalter said that he and the study's co-author — Douglas Hamilton, an astronomer at the University of Maryland — started out intending merely to tie up the loose ends in the Hubble observational campaign. But as they looked more closely at the readings, the moons "started getting weirder and weirder and weirder," he said.
The moons are almost, but not quite, locked together in a complex 1:3:4:5:6 resonance, which helps keep them from getting in each other's way. Styx, Nix and Hydra have a particularly tight relationship. "This is one reason why tiny Pluto is able to have so many moons," Hamilton said in a news release.
Showalter and Hamilton say the smaller moons wobble chaotically in their orbits because of the shifting gravitational effects of Pluto and Charon. The brightness readings for Nix and Hydra suggests that they're oblong, like footballs, rather than spherical — and that their orientation with respect to Pluto varies as well.
"Like good children, our moon and most others keep one face focused attentively on their parent planet," Hamilton said. "What we've learned is that Pluto's moons are more like ornery teenagers who refuse to follow the rules."
Showalter said he and Hamilton have been sharing their findings with the rest of the New Horizons team as they prepare for next month's big Pluto flyby. For example, the Hubble analysis supports indications from New Horizons' high-resolution camera that there are no thick rings, overlooked moons or other potential hazards that would stand in the spacecraft's way.
The moons appear to be so closely packed in their orbits that "it's just very difficult to add another moon in between," Showalter said. Any undiscovered moons would have to lie within Charon's orbit, or outside Hydra's, he said.
New Horizons could shed more light on the questions surrounding the tiny moons' quirky orbits, or on the nature of dark Kerberos, the astronomers said. Mission team member John Spencer said New Horizons' findings "will prove Mark and Doug right, or possibly wrong."
In addition to showing whether or not Pluto has additional moons or rings, New Horizons is expected to reveal never-before-seen details of the icy world. Pluto may or may not have ice volcanoes that well up from beneath the surface, or clouds in the atmosphere above. But in any case, the Pluto system is already showing that dwarf planets and their even more dwarfish moons can offer giant mysteries.
"Independent of the new discoveries in store, we have already learned that Pluto hosts a rich and complex dynamical environment, seemingly out of proportion to its diminutive size," Showalter and Hamilton wrote in Nature.
SETI Institute astronomer Mark Showalter will discuss the weird properties of Pluto's moons with NBC News' Alan Boyle on Wednesday's installment of "Virtually Speaking Science," an hourlong talk show airing at 8 p.m. ET via BlogTalkRadio. You can also watch the show as part of a live virtual audience in the Exploratorium's Second Life auditorium. If you miss the live show, never fear: You can always catch up with the podcast in BlogTalkRadio's archive or on iTunes. May's show featured a discussion of the Large Hadron Collider and its potential future discoveries with theoretical physicist Matt Strassler.