Pluto Moons Have Complex Orbits: New Study

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New research shows that three out of four of Pluto’s moons move together in a mutual orbital dance. The new data offers important context for NASA’s New Horizons spacecraft, which is on a mission to find new moons orbiting Pluto when it makes a fly-by of the distant planet in mid-July.

More on unusual orbits of Pluto’s moons

According to lead study author Mark Showalter, a planetary scientist at the SETI Institute in Northern California, Pluto’s largest moon (arguably co-planet), Charon, weighs close to 11% of the mass of Pluto. Charon probably formed during a collision billions of years ago. Astronomers think the debris from the collision may have created the much smaller moons Styx, Nix, Kerberos and Hydra.

This new study shows that the orbits of Styx, Nix and Hydra became locked together by gravity at some point in the past. “If you lived in the Pluto system and were sitting on Nix, you would see Hydra go around three times every time Styx goes around twice,” Showalter commented. This orbital system is called three-body resonance, and will remain stable. The phenomenon is related to the Laplace resonance that holds Jupiter’s moons Europa and Ganymede in a mutual orbit.

“If you know where Hydra and Nix are, you can determine where Styx is,” notes Showalter. “It is a little piece of order in the system.” He went on to say that Styx is the smallest of the three moons, and it was probably captured by the others as the last phase of their orbital tango a trois.

Together with colleague Douglas Hamilton, a planetary astronomer at the University of Maryland, Showalter searched for the resonance by combing through Hubble Space Telescope observations of how the moons move over time. The astronomers, who published their research in the academic journal Nature, also discovered that Kerberos is a good bit more dark than either Nix or Hydra, meaning it is likely made of a different material.

New data could be useful for NASA’s New Horizons mission

The new work will help researchers to understand how the Pluto’s moon were created, says Marina Brozovic, a planetary astronomer at the Jet Propulsion Laboratory Lab, who has studied Pluto’s moons. “But off the top of my head, I do not see how a Laplace resonance between Styx, Nix and Hydra will shape the science results that come out of New Horizons,” she commented.

The New Horizons spacecraft is currently less than 50 million kilometers from Pluto. It has begun searching for smaller moons that the Hubble space telescope could not locate. No new moons have been spotted to date, but tiny moons could still be found in several places.

Very small moons (under 10% the mass of Styx) could also be hiding out between Styx and Hydra, notes Simon Porter, a researcher at the Southwest Research Institute in Colorado.

Lead study author Showalter believes that any new moons, if they exist, are probably quite small and are orbiting farther out than Hydra, the outermost known  moon. “But anything I say about Pluto right now could easily be obsolete by next week,” he noted with a chuckle. “Or tomorrow.”

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