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."
In tune
with Charon
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.
Multiple
systems the norm?
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.
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