A closer look at the far-away Pluto system has revealed that it’s a lot more peculiar than scientists expected.
Observations made with the Hubble Space Telescope suggest the dwarf planet's four smallest moons — Nix, Hydra, Kerberos and Styx — tumble unpredictably as they follow their wobbly orbits around Pluto and its largest moon, Charon.
“If you stood on Nix, the sun might rise in the west and set in the east one day, and rise in the east and set in the north on another,” said Mark Showalter, a senior research scientist at the SETI Institute in Mountain View, Calif. “And if you bought real estate in the north pole, you might discover one day that you are now in the south pole.”
Saturn's moon Hyperion is the only other moon in the solar system known to exhibit a similarly chaotic rotation.
In a study published this week in the journal Nature, Showalter and Douglas Hamilton of the University of Maryland, College Park, wrote that this unpredictable lunar behavior is the result of the gravitational pull exerted by Charon, the innermost of Pluto's moons.
“Charon formed in the middle of this system some 4 billion years ago,” Showalter said. “Everything else has been chaotic ever since.”
Pluto and Charon are close enough in size that instead of one orbiting the other, they jointly orbit a common center of gravity.
Picture two unequal weights at the ends of a dumbbell, and the dumbbell is rotating. This is the type of system that astronomers call a “binary planet.”
Adding to the absurdity, Pluto’s four small moons appear to be shaped more like footballs than spheres. That’s probably because they don't have enough mass for their internal gravity to pull them into a round shape.
Each time these oblong moons pass by Charon, it keeps them from settling down, leading to the chaos the scientists observed.
Computer models show that if Charon weren’t there, the other moons would follow smooth, circular orbits. But Charon is there.
“Each time Charon passes by one of these moons, it pulls on it a little bit more strongly,” Hamilton said. “The orbits are nearly circular, but with Charon going by every so often, they move in less predictable ways.”
Pluto's strange, small moons were discovered quite recently. Nix and Hydra were first found in 2005, followed by Kerberos in 2011 and Styx in 2012.
The entire Pluto system lies 3 billion miles from Earth, and so even a telescope as powerful as Hubble can only see them as tiny points of light.
Showalter and Hamilton first realized that Nix and Hydra were moving in unpredictable ways by watching those points move and grow ever-so-slightly brighter and dimmer in nearly a decade's worth of Hubble data. They expected to find patterns in the way the light changed, but they couldn't.
“It is a fundamentally unpredictable situation,” said Showalter.
Styx and Kerberos are too dim to measure the same way, but Showalter said there is no reason to think they’d behave differently than their lunar siblings.
The Hubble observations revealed another intriguing finding as well. The astronomers reported that Nix, Hydra and Charon are the color of dirty snow or desert sand, while Kerberos is as dark as charcoal briquette.
This discovery is particularly head-scratching because the researchers believe that all of Pluto's moons formed as the result of an impact between the dwarf planet and another body early in the solar system's history. The impact would have caused a cloud of dust to orbit the planet, eventually coalescing into the moons we see today.
But if all the moons were formed of the same stuff, why would one be black and the others light?
One possible explanation is that the object that collided with Pluto billions of years ago was itself dark as coal, and perhaps Kerberos is made up of material just from that object.
Astronomers may soon gain new insight into these mysteries, with the arrival of the first spacecraft to visit the remote dwarf planet.
After a 9.5-year journey through the solar system, NASA's New Horizons mission is less than six weeks away from its closest approach to Pluto. The spacecraft will take extremely high resolution images of Pluto and Charon as well as collect information about the smaller moons that will help astronomers better understand how big they are, what they’re made of, and how much light they actually reflect.
John Spencer, a scientist at the Southwest Research Institute in Boulder, Colo., who works on the New Horizons mission, said it will add valuable information to the study of the Pluto system.
“It will revolutionize our knowledge of these moons,” Spencer said. “It will prove Mark and Doug right — or possibly wrong.”