Two small moons have been discovered orbiting Pluto, bringing the planet's retinue of known satellites to three and leaving scientists to wonder how it could be.
The newfound moons orbit about 27,000 miles (44,000 kilometers) from Pluto, more than twice as far as Charon, Pluto's other satellite. They are 5,000 times dimmer than Charon.
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 Baltimore.
"That suggests they probably formed at the same time as Charon," Weaver told Space.com in a telephone interview Friday. NASA conducted a teleconference with reporters Monday at 1 p.m. ET to announce the discovery.
While scientists had predicted there might be more moons, the newfound setup is surprising nonetheless, in part because Pluto is smaller than our own 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 speculated. It can't 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 30 and 100 miles (45 to 160 kilometers) in diameter, Weaver said. There is not enough data to pin their size down exactly, however. Pluto is 1,430 miles (2,290 kilometers) wide and Charon's diameter is about 730 miles (1,170 kilometers).
The moons were found using the Hubble Space Telescope.
Piece of the puzzle
The discovery represents one more piece of an increasingly complex puzzle in the outer solar system, a place that astronomers look to 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 can't 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 hadn't been discovered when Pluto was found 75 years ago. The region includes other round objects with moons, and one recently discovered world 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.
There could be more moons to find, too, 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 across in the Pluto system."
Easy to find
The moon-hunting project was rejected by Hubble planners several times and took years to get approved, and only then after a failed instrument on Hubble last year caused project leaders to add several previously unaccepted observing programs to fill the schedule.
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 too dim to be seen with a typical backyard telescope, but "relatively easy to see with Hubble," Weaver said.
Then the astronomers dug up old Hubble observations done by colleague Marc Buie of the Lowell Observatory, to see if the same objects had been imaged before.
Weaver said they are pretty sure they've located the moons in the archived photos, and the combination of data is what suggests the moons' circular orbits in the plane of Charon's path.
More Hubble observations are planned for February to confirm the discoveries and pin down the orbits.
The moons are cataloged 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.