The Hubble Space Telescope discovered t wo small “moons” orbiting Pluto, bringing the planet’s retinue of known satellites to three and leaving scientists wondering how the new bodies were formed .
The newfound presumed moons orbit about 44,000 kilometers from Pluto, more than twice as far as Charon, Pluto’s other satellite.
Preliminary observations suggest they are in circular orbits around Pluto and in the same plane as Charon, said Hal Weaver of the Johns Hopkins Applied Physics Laboratory in Laurel, Md. They are 5,000 times dimmer than Charon, Weaver said.
“That suggests they probably formed at the same time as Charon,” Weaver said in a telephone interview Oct. 28. NASA held a teleconference with reporters Oct. 31 to announce the discovery.
While scientists had predicted the possible existence of more moons, the newfound setup is surprising in part because Pluto is smaller than Earth’s Moon.
“It’s almost like a mini solar system,” Weaver said. “How can something about 70 percent the size of Earth’s Moon have all these satellites? How can that happen? We’re going to have to explain that.”
The leading theory for the formation of Charon involves a large object striking Pluto. The debris from that collision could have formed the two smaller moons, Weaver said . It cannot be ruled out that they might have been captured into the system, but that seems very unlikely, he said.
The two new moons are between 45 to 160 kilometers in diameter, Weaver said. However, t here is not enough data to pin their size down exactly . Pluto is 2,300 kilometers wide and Charon’s diameter is about 1,200 kilometers .
Easy to find
The moon-hunting project was denied by Hubble planners several times and took years to get approved. Following a failed instrument on Hubble last year, which caused project leaders to add several previously unaccepted observing programs to fill the schedule, the project was given the go-ahead.
For Hubble, this one was easy.
Unlike many observing projects that require several Hubble orbits — often 15 or more and sometimes many dozens — Weaver’s team needed just two orbits. On the first set of observations they spotted the two points of light, then on the second orbit they found them again and made sure they moved against the background of relatively fixed stars.
The presumed moons are 23rd magnitude, far to dim to be seen with a typical backyard telescope but “relatively easy to see with Hubble,” Weaver said.
Then the astronomers compared the new images to old Hubble observations done by colleague Marc Buie of the Lowell Observatory in Flagstaff, Ariz., to see if the same objects had been captured before.
Weaver said they are pretty sure they ha ve located the moons in the archived photos, and the combination of data suggests the moons’ circular orbits are in the plane of Charon’s path.
Piece of the puzzle
The discovery represents one more piece of an increasingly complex puzzle in the outer solar system, a place where astronomers look for clues in understanding how it all formed 4.5 billion years ago in the wake of the Sun’s birth.
Lately, so many objects have been found in so many configurations out there, that astronomers cannot even agree on what to call them.
Though popularly considered a planet, Pluto is now viewed by most astronomers to be a member of the Kuiper Belt, a vast sea of frozen worlds beyond Neptune that had not been discovered when Pluto was found 75 years ago. The region includes other round objects with moons, and one recently discovered body is larger than Pluto.
For now, Pluto is the only Kuiper Belt object known to have more than one companion.
“Our result suggests that other bodies in the Kuiper Belt may have more than one moon as well,” said team co-leader Alan Stern of the Southwest Research Institute in Boulder, Colo.
Stern heads up the New Horizons mission to Pluto and the Kuiper Belt, slated for launch early next year. He had long predicted other moons around Pluto.
There could be more moons to find, but they would be small.
“These Hubble images represent the most sensitive search yet for objects around Pluto,” said team member Andrew Steffl of the Southwest Research Institute, “and it is unlikely that there are any other moons larger than about 10 miles [16 kilometers] across in the Pluto system.”
More Hubble observations are planned for February to confirm the discoveries and pin down the orbits.
The moons are catalogued as S/2005 P1 and S/2005 P2 for now. Once they are confirmed, the discoverers will suggest names to be approved by the International Astronomical Union.