NEW HORIZONS IS practically knocking on Pluto’s door. The spacecraft is less than six weeks away from its razor-close flyby of the dwarf planet. In anticipation, Pluto research has gone into overdrive, as astronomers and planetary scientists rack up questions to ask of the coming data. Fortunately, the little-known underdog of the solar system continues to surprise—especially when it comes to the weird configuration of its five motley moons.
Pluto is so far away that astronomers only discovered four of those orbiting bodies in the last decade (the largest, Charon, was discovered in 1978). New analysis of Pluto’s system, gleaned from Hubble Space Telescope images, suggests that three of the small moons—Styx, Nix and Hydra—are locked in close rotation. That keeps them from colliding as they circle the “binary planet” formed by Pluto and Charon. But that alignment can be thrown into chaos thanks to interactions with those larger bodies and the recently discovered moon Kerberos. Astronomers Mark Showalter and Douglas Hamilton hope their findings, published in Nature today, will help explain how planets and their satellites form.
Chaotic interactions between moons and their orbitees are uncommon, but not unknown, says Showalter, of the SETI Institute in Mountain View, Calif. Saturn’s cratered, potato-shaped moon Hyperion also has a wobbly rotation, one that is impossible to forecast in advance, unlike the majority of well-behaved, synchronously rotating moons in the solar system.
More curious than that, however, may be Pluto’s moons’ orbital configuration. The ratios of their orbital periods (the amount of time they take to complete their orbit around the Pluto-Charon pair) are very close to 1:3:4:5:6. That simple integer ratio is called the Laplace resonance—a gravitational relationship seen in Jupiter’s moons Io, Europa, and Ganymede, which have a period ratio of 1:2:4.
Aside from being mathematically cool, the Laplace-like resonances seen around Pluto narrows down some of the possibilities for how the system formed. In the past, the relationships between the moons’ movements may have been more perfectly aligned, until they were knocked slightly out of place by some unknown force, speculates Scott Kenyon, an astrophysicist at the Smithsonian and author of an accompanying Nature article commenting on Showalter and Hamilton’s study. The probability that this relationship developed by chance, he says, is very slight (about 1 percent).
Kerberos, the most recently discovered member of the Pluto-moon family, continues to add to the system’s mystery. Observations suggest that its mass is about one third of the mass of Nix and Hydra. Yet it reflects so little sunlight (about 5 percent as much as Nix and Hydra) that it is probably made of a different material. “We expect it is about as black as coal,” says Showalter.
We won’t have to wait long to find out, says Alan Stern, principal investigator of NASA’s New Horizons mission. When the spacecraft reaches Pluto, in less than a month and a half, it will be able to send images of the planet and its moons back, confirming if Kerberos really is composed of a different material. If true, that would also be extremely rare.
The majority of planetary systems have moons that seem to be made of the same material as the planet and each other. If Kerberos is made of something dramatically different it may be the relic of a body that crashed into Pluto, sprayed the region with debris, and contributed to the formation of its fellow moons. New Horizons will clarify how likely that is to be true.
Keep tuned in, dear reader. Pluto’s story is just beginning to be told.
Source: http://www.wired.com/2015/06/odd-orbits-deepen-plutos-mystery/ by Danielle Venton