Pluto and Its Moons Are Weirder Than We Thought

Increasingly it looks like Pluto is its own, fascinating thing.

In a teleconference today, astronomy professor Douglas Hamilton cracked that the "dwarf" in Pluto's title of "dwarf planet" should be seen as an honorific. After all, we have eight planets already; increasingly it looks like Pluto is its own, fascinating thing.

Hamilton co-wrote a study published today in the journal Nature that drew from Hubble telescope data to look at the distant system and explore its idiosyncrasies. Like good science should, the study answers a few questions and leads to many more, enough to get us pumped all over again for NASA's Kuiper Belt-exploring New Horizons mission to fly past the once-smallest planet next month (!).

"This system is a relic left over from, we believe, the beginning of the solar system," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate, "so this is helping us answer the question of our own origins."

An oblique view of the Pluto–Charon system showing that Pluto orbits a point outside itself. Pluto's orbit is shown in red and Charon's orbit is shown in green. Image: Lanthanum-138 at en.wikipedia

Pluto and its largest and closest moon Charon form a "binary system"—Charon causes Pluto to rotate around a spot outside of itself, instead of on its axis like Earth. Pluto and Charon, Like Earth and the Moon, are synchronously locked, keeping the same face to each other as they turn.

Pluto's other four, smaller moons, named Hydra, Nix, Styx, and Kerberos, rotate around the Pluto-Charon pair. Study co-author Mark Showalter described the pair of Pluto and Charon as a "very-uneven dumbbell" that the other moons go around, and that dynamic causes some interesting gravitational field problems. As Charon comes by, this largest moon tweaks the orbits of its smaller neighbors and the results are chaos, according to study co-author Mark Showalter.

While we aren't sure about Styk or Kerberos, Nix and Hydra aren't in synchronous orbit with Charon and Pluto, Showalter explained. Instead the moons are tumbling unpredictably through space.

"You can never really know enough about the orientation of Nix at any given time to really predict what it's going to do very far into the future," Showalter said. "If you can imagine what it would be like to live on this body, it's a very strange world. You would literally not know if the Sun is coming up tomorrow, and for that matter the Sun might rise in the west and set in the east—it might rise in the west and set in the north for that matter."

The orbits of Hydra, Styx and Nix are all synced up to each other with a clockwork regularity though, in a "three-body resonance," much like the large moons of Jupiter. Kerberos is left as a sort of black sheep of the system, in more ways than one.

This illustration shows the scale and comparative brightness of Pluto's small satellites. The surface craters are for illustration only and do not represent real imaging data. Image: NASA/ESA/A. Feild (STScI)

Discovered in 2011, Kerberos is only 6 to 20 miles in diameter. It only reflects about 4 percent of the light that strikes it, meaning it's like a charcoal briquette. The researchers believe the same impact event caused a cloud from which all of the moons formed, so Kerberos should be made of the same stuff as the other moons, which are bright, like dirty snow.

Why is it different? While the researchers speculated that perhaps Kerberos is left overs from the object that did the impacting, they really don't know. More questions for New Horizons. One month more...