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Pluto has football-shaped moons that tumble chaotically through space

A rendering of the chaotic rotation of Pluto's moon Nix.

(STScI and Mark Showalter, SETI Institute)

It was only in the past decade that scientists discovered Pluto has at least five moons. Now they're discovering those moons are even stranger than we expected.

In a new study published Wednesday in Nature, Mark Showalter of the SETI Institute and Douglas Hamilton of the University of Maryland found that some of Pluto's moons are tumbling chaotically through space. Instead of spinning smoothly, like Earth, the tiny football-shaped moon Nix wobbles wildly back and forth, like a boat caught in a swale.

As a result, if you lived on Nix, "you would literally not know if the sun was coming up tomorrow," Showalter said during a press conference announcing the findings, which were made using data collected by the Hubble Space Telescope. The researchers found that the moon Hydra has a similarly chaotic rotation, and that Styx and Kerberos likely do as well.

Most large moons in the solar system (including our moon) are tidally locked, meaning that they rotate so the same side always faces their planets as they orbit them. But Pluto's four small moons appear to rotate chaotically due to the competing gravitational pull of Pluto and its largest moon, Charon.

These two large bodies orbit each other at the center of the system, which is why some have suggested that we call them a double-planet system. As a result, the smaller moons are caught in a wildly fluctuating gravitational field, leading to rotational tumult. The only other object we've seen with this sort of chaotic rotation is Saturn's irregularly shaped moon Hyperion.

Why Pluto's moons are fascinating for scientists

A scale illustration of Pluto's moons.

(STScI and Mark Showalter, SETI Institute)

What's more, Hamilton and Showalter found that three of the moons orbit Pluto nearly in sync together, a characteristic called orbital resonance. Over time, this likely prevented them from crashing in to one another — and, Hamilton said, "is one reason why tiny Pluto is able to have so many moons."

The researchers also found that most of the moons are shaped more like footballs than perfect spheres. Two of them (Nix and Hydra) are much smaller and brighter than previously thought, while Kerberos is extremely dark, reflecting back just 4 percent of the light hitting it.

All these findings are especially interesting because many scientists consider Pluto to be a sort of scaled-down model of a solar system. Showalter and Hamilton speculate that the chaos of Pluto's moons, for instance, might be a characteristic present in binary star systems, where two stars orbit each other much like Pluto and Charon. We can't collect this sort of data on distant star systems' planets yet, so studying Pluto is an effective substitute.

A composite photo of the Pluto system, as imaged by the Hubble Space Telescope.

(NASA/STScI/Showalter)

We're sure to learn a lot more about Pluto and its moons as NASA's New Horizons probe approaches it for a flyby in July. Already, the probe has sent back the best photos we've ever taken of Pluto, and it's going to provide unprecedented data on the dwarf planet's atmosphere and composition.

New Horizons might even spot a few more moons — as four of its moons were discovered in just the last decade, and simulations suggest Pluto might be orbited by as many as 10 tiny moons in total.