Answer to mystery of Pluto's moons could depend on 2015 flyby

Pluto and its moon Charon act like a double-planet system with wreath of other, smaller moons. NASA's New Horizons mission could help explain how those moons got there.

Hubble Space Telescope/NASA/AP
This photo by the Hubble Space Telescope shows the five moons in their orbits around Pluto.

New research by two astronomers has the potential to make the current NASA mission to Pluto and beyond more than just a first close-up glimpse of the distant, demoted planet. It could help scientists understand how planets form around other stars.

The reason: While Pluto's companion, Charon, is widely considered a moon, its orbital relationship to Pluto is identical to that of stars in a binary-star system. Indeed, some astronomers hold that Charon is not a moon, but part of a binary dwarf-planet system, with Pluto as the senior partner.

With at least four other small moons orbiting beyond Charon, the Pluto system could be a unique laboratory for scientists.

"Not only could we try to understand the outer part of the solar system, we could actually have an idea of how planets form around binary stars and actually test it real life," says Scott Kenyon, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who performed the analysis along with University of Utah's Benjamin Bromley.

Charon is thought to have formed from a collision between Pluto and another object, Dr. Kenyon explains. To try to determine how the smaller outer moons might have formed thereafter, the researchers used computer simulations. Did the outer moons form from the debris of the collision? Or did they take shape long afterward from the primordial disk of dust, rock, and ice that Pluto-Charon captured from its general neighborhood?

The simulations suggest that both scenarios are possible, but that each would yield moons with different compositions. NASA's New Horizons mission could help prove if either scenario is right. New Horizons is now half way to Pluto and is expected to reach the dwarf planet in 2015.

The results of the calculations by Kenyon and Dr. Bromley have been submitted for publication and have been posted on an astrophysics website in hopes that the New Horizons science team can work in observations that would test these competing ideas into the mission's science plan during the Pluto flyby.

Over the years, the known size of the Pluto system has expanded. Pluto itself was discovered in 1930 by astronomer Clyde Tombaugh. It took another 48 years to pick out Charon. In 2005, astronomers discovered Nix and Hydra. And between July 2011 and July 2012, researchers detected another two moons – P4 and P5. (A recent nonbinding poll to name the two moons suggested Vulcan and Cerberus.)

Assuming a giant impact formed Charon, the raw material for the other moons could have come from debris that formed a disk outside Charon's orbit.

For moons to form in this way, there would have needed to be enough debris, and it would have needed to be orbiting Pluto and Charon at a distance relatively undisturbed by their gravity – so clumping could occur. "If you can't get material out past the orbit of P5 [the closest known moon to Pluto and Charon], then you're doomed," Kenyon says, because gravity from Pluto-Charon would sweep the material into those two objects.

But simulations of the impact scenario suggested that material did pass the orbit of P5 and that this scenario was the most efficient means of producing moons, Kenyon says. The collision yields more than enough debris to make moons with the masses astronomers think the system's moons have. Moreover, in the simulations, the innermost moon tends to settle into an orbit at a distance comparable to P5. 

But the approach that focuses on the primordial disk of dust and ice can also form moons, simulations found. At some point after the giant collision, the Pluto-Charon system could have drawn in a ring of dust and ice from material in the vicinity – material that was part of the solar system's original inventory of dust, gas, and ice.

"You just gradually accumulate stuff over millions and millions of years, and that coagulates into the satellites," Kenyon says.

But simulations found that the masses of the moons formed in this scenario are at the lowest end of the range of mass estimates astronomers have calculated for the moons in the Pluto-Charon system. And those less-massive moons would appear in orbits much farther from Pluto-Charon than the existing moons.

Either way, if both scenarios start out with the same amount of mass in the debris disks, the same number of satellites will form, but their composition will be different.

If the satellites are formed from the collision debris, their composition will look much like Charon's. Charon is less dense than Pluto, consisting of a roughly 50-50 mix of ice (mostly water ice) and rock with a very icy surface. This allows it to reflect a relatively larger amount of sunlight from its surface than would a more mixed surface composition.

If the satellites formed via gradual accretion of primordial ice and rock well after a giant impact, Kenyon adds, the satellites would be darker and with a higher proportion of rock to ice.

In that way, they would look more like typical objects in the Kuiper Belt – the broad expanse of rocky and icy objects left over from solar system's construction phase some 4.6 billion years ago. The belt's inner edge is about 2.8 billion miles from the sun, just beyond Neptune's orbit. The outer edge is thought to lie about 4.7 billion miles from the sun.

Pluto, which orbits the sun at an average distance of 3.7 billion miles, is the second largest known dwarf planet. The solar system's largest, most massive dwarf planet is Eris, which orbits the sun at an average distance of 6.3 billion miles.

Based on the simulations, New Horizons could find perhaps five to 10 more moons in the Pluto-Charon system, Kenyon says. They would be small, perhaps ranging from 1,000 feet to a mile or two across, and outside the orbit of Hydra. And there would be enough material for a tenuous disk of particles whose size are measured in inches.

New Horizons can begin its observations of the Pluto-Charon system about 70 days before its closest encounter and for some days after.

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