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.