Two small moons have been discovered orbiting Pluto, bringing
the planet's retinue of known satellites to three and leaving scientist 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.
"That suggests they probably formed at the same time as
Charon," Weaver told SPACE.com in a telephone interview Friday.
NASA planned 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 speculates. 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 wide and Charon's diameter is about 730 miles.
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
objectswith
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.
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, 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 denied 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 to 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 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.
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