Pluto Might Have Rings

The two moons discovered around Pluto last year were likely formed from the same giant impact that created the planet’s much larger satellite, Charon, scientists say.

The idea suggests that other Kuiper Belt Objects might also harbor multiple satellites and raises the possibility that Pluto is encircled by rings fashioned from debris ejected from the surface of the tiny moons.

The two moons, called P1 and P2 for now, were discovered in May 2005 using the Hubble Space Telescope. Scientists now think the two moons are roughly 37 and 31 miles (60 and 50 km) in diameter. Charon has an estimated diameter of about 750 miles (1,200 km).

The moons’ tiny sizes raise the possibility that even more satellites might be discovered around Pluto in the future.

“The very small masses of P1 and P2 relative to Charon beg the question of why ... there are not more small satellites of Pluto,” a team of researchers write in the Feb. 23 issue of the journal Nature. “Perhaps there are other, still fainter satellites that escaped detection.”

Because of how P1 and P2 move, scientists think they were formed from the same collision that, according to the leading theory, spawned Charon.

“The small moons are in circular orbits in the same orbital plane as Charon, and they are also in, or very near, orbital resonance with Charon,” said study leader Alan Stern of the Southwest Research Institute (SwRI).

For every twelve orbits Charon makes around Pluto, P1 makes almost two orbits and P2 completes nearly three. This ratio would likely not be constant if P1 and P2 were merely passing objects captured by Pluto’s gravity. The most likely explanation for this arrangement, scientists figure, is that all three moons were born of the same event.

Collisions between large objects helped shape many aspects of our solar system. The Moon, for example, is believed to have formed when a Mars-sized object slammed into Earth 4 billion years ago. The crash that’s thought to have created Pluto’s moons is believed to have occurred at around the same time.

Scientists will get a closer look at Pluto and its moons when NASA's recently launched New Horizon mission reaches the system in 2015.

Scientists have determined that up to a fifth or more of known Kuiper-belt objects (KBOs) harbor satellites or belong to binary systems; the new modeling suggests that there could also be numerous systems consisting of three, four or even more bodies grouped together.

But finding these systems is difficult because of the distances involved. The Kuiper belt is a region of space populated by asteroids and comets and other rocky, icy bodies; it is located beyond Neptune, between 30 and 50 AU from Earth. One AU is equal to the distance from the Earth to the Sun.

“Finding small satellites around KBOs is difficult because their large distances from the Sun makes them appear very faint,” said study team member Andrew Steffl of SwRI.

However, KBOs and their satellites are occasionally ejected from the Kuiper Belt and get flung closer to the Sun, where they become easier to spot. Steffl said a good way to determine whether KBOs with multiple satellites are unusual or the norm is to search for satellites around these outcasts, which are known as “Centaurs.”

“We hope to use Hubble to search for faint moons around some of them,” Steffl said.

The discovery of P1 and P2 also raises the intriguing possibility that impact debris from the small moons is captured by Pluto’s gravity and coalescing into rings or even arcs around the tiny planet. If confirmed, it would be the first example of a ring system around a solid body rather than a gas giant planet.

Related story. The links on Sciam appear to be dynamically generated, but perhaps they're just changed every day or something. You may wind up having to go into the main page and hunt down this article. That's what I had to do.

Two New Moons--and Maybe Some Rings--for Pluto
by David Biello
February 23, 2006
Scientific American
"We used Hubble's exceptional resolution to peer close to Pluto and pick out two small moons that had eluded detection for more than 75 years," says Hal Weaver, an astrophysicist at Johns Hopkins University and the discovery team leader. "That was somewhat surprising because ground-based observers had been trying for more than a decade to find new satellites around Pluto," adds astronomer Max Mutchler of the Space Telescope Science Institute, the first to see the moons in Hubble's images. Based on their brightness--and assuming that their surfaces are about as reflective as Charon's--the scientists believe the two moons are roughly 38 miles and 29 miles in diameter. Given that they share Pluto's distance from the sun--roughly three billion miles--but are 4,000 times fainter, it is not surprising that the satellites eluded detection until now, the researchers say.

The researchers say that it is somewhat surprising that they hadn't been seen before, but not surprising that they'd eluded detection? Either the writer or the editor should have noticed that discrepancy.

This SA story about the moons is also recent (FR has at least one topic) and has different moon size data than Nick's article from Space, and that's interesting. From 2003:

Pluto's Other Moons:
Why They Might Exist and Who's Looking
by Robert Roy Britt
6 May 2003
Earlier this year, around the time New Horizons was approved by NASA, Stern and colleagues announced they would conduct a search for moons of Pluto by year's end. Pluto has one known moon, Charon which, at 745 miles (1,200 kilometers) wide is more than half the size of Pluto and the largest satellite in our solar system relative to its planet... Pluto may have some distinct advantages over Earth in the satellite-garnering department... "The probability of a two-body gravitational capture is phenomenally low," Stern said, even for objects that pass reasonably close... It's more likely, however, that one or more other satellites could have formed directly around Pluto during its birth, Stern said. Or the planet could harbor a cloud of small objects that were broken up from a collision that shattered a satellite of Pluto long ago... Pluto, less massive but more distant from the Sun, could retain a satellite orbiting as far as 620,000 miles (1 million kilometers) away.

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