Pluto’s four littlest moons probably born in a crash

Of Pluto’s five satellites, four — Styx, Nix, Kerberos and Hydra — are packed together about as tightly as possible, researchers report in the June 4 Nature. The four orbits, plus that of the largest moon, Charon, are nearly synced, which suggests that the family formed from debris left behind after something big slammed into Pluto long ago.  

Like Jupiter’s four largest moons, the moons of Pluto keep coming back to nearly the same configuration time and time again, the researchers found. Charon completes three orbits, for instance, in roughly the same time Styx loops around once. Since it’s unlikely that all five moons became synced by chance, they probably have a common origin. The tight spacing also indicates that there are probably no other moons or rings lurking among the four smallest moons, since most debris would eventually get snatched by one of those satellites.

“Think of it as archaeology,” says Mark Showalter, a planetary scientist at the SETI Institute in Mountain View, Calif. The properties of the moons today hint at how they formed, he adds. Showalter, along with planetary scientist Douglas Hamilton of the University of Maryland at College Park, investigated the Pluto family using Hubble Space Telescope images that span seven years.

Kerberos appears much darker than its siblings. “The small satellites should have very similar surfaces,” says Marc Buie, a planetary scientist at the Southwest Research Institute in Boulder, Colo. “But Kerberos appears to be off-the-charts different.” Kerberos might be part of the body that struck Pluto, Showalter says, whereas Pluto’s other moons could be remnants of the dwarf planet itself.

The four small moons tumble unpredictably along their orbits as they are nudged by Charon and Pluto. “On Nix, you wouldn’t know if the sun was coming up tomorrow,” says Showalter. The sun might rise in the east one day and come up in the north the next, if it rose at all. “You’d be living on a random world with no day-night cycle,” he says.

The New Horizons spacecraft will visit Pluto and its posse on July 14. As the probe tears past at nearly 50,000 kilometers per hour, it should get the best view yet of the moons, Buie says.

“Pluto has not let us down yet,” Showalter says. “It’s always been a bit more interesting than expected.”