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| This artist's concept illustrates an asteroid belt around the bright star Vega. [Image credit: NASA/JPL-Caltech] |
Washington: Astronomers have discovered what appears to be a large
asteroid belt around the star Vega, the second brightest star in northern night
skies. The scientists used data from NASA's Spitzer Space Telescope and the
European Space Agency's (ESA) Herschel Space Observatory, in which NASA plays
an important role.
The discovery of an
asteroid belt-like band of debris around Vega makes the star similar to another
observed star called Fomalhaut. The data are consistent with both stars having
inner, warm belts and outer, cool belts separated by a gap. This architecture
is similar to the asteroid and Kuiper belts in our own solar system.
What is maintaining
the gap between the warm and cool belts around Vega and Fomalhaut? The results
strongly suggest the answer is multiple planets. Our solar system's asteroid
belt, which lies between Mars and Jupiter, is maintained by the gravity of the
terrestrial planets and the giant planets, and the outer Kuiper belt is
sculpted by the giant planets.
"Our findings
echo recent results showing multiple-planet systems are common beyond our
sun," said Kate Su, an astronomer at the Steward Observatory at the
University of Arizona. Su presented the results Tuesday at the American
Astronomical Society meeting in Long Beach, Calif., and is lead author of a
paper on the findings accepted for publication in the Astrophysical Journal.
Vega and Fomalhaut
are similar in other ways. Both are about twice the mass of our sun and burn a
hotter, bluer color in visible light. Both stars are relatively nearby at about
25 light-years away. The stars are thought to be around 400 million years old,
but Vega could be closer to its 600 millionth birthday. Fomalhaut has a single
candidate planet orbiting it, Fomalhaut b, which orbits at the inner edge of
its cometary belt.
The Herschel and
Spitzer telescopes detected infrared light emitted by warm and cold dust in
discrete bands around Vega and Fomalhaut, discovering the new asteroid belt
around Vega and confirming the existence of the other belts around both stars.
Comets and the collisions of rocky chunks replenish the dust in these bands.
The inner belts in these systems cannot be seen in visible light because the
glare of their stars outshines them.
Both the inner and
outer belts contain far more material than our own asteroid and Kuiper belts.
The reason is twofold: the star systems are far younger than our own, which has
had hundreds of millions more years to clean house, and the systems likely
formed from an initially more massive cloud of gas and dust than our solar
system.
The gap between the
inner and outer debris belts for Vega and Fomalhaut also proportionally
corresponds to the distance between our sun's asteroid and Kuiper belts. This
distance works out to a ratio of about 1:10, with the outer belt 10 times
farther away from its host star than the inner belt. As for the large gap
between the two belts, it is likely there are several undetected planets,
Jupiter-sized or smaller, creating a dust-free zone between the two belts. A
good comparison star system is HR 8799, which has four known planets that sweep
up the space between two similar disks of debris.
"Overall, the
large gap between the warm and the cold belts is a signpost that points to
multiple planets likely orbiting around Vega and Fomalhaut," said Su.
If unseen planets do
in fact orbit Vega and Fomalhaut, these bodies will not likely stay hidden.
"Upcoming new
facilities such as NASA's James Webb Space Telescope should be able to find the
planets," said paper co-author Karl Stapelfeldt, chief of the Exoplanets
and Stellar Astrophysics Laboratory at NASA's Goddard Space Flight Center in
Greenbelt, Md.