Evidence of an asteroid belt around Vega
Astronomers have discovered what appears to be a large asteroid belt around the star Vega, the second-brightest star in the northern night sky. The scientists used data from NASA’s Spitzer Space Telescope and the European Space Agency’s Herschel Space Observatory, in which NASA plays an important role.
The discovery of a band of asteroid-belt-like debris around Vega makes the star similar to another observed star called Fomalhaut. The data is consistent with the two stars having warm inner belts and cold outer belts separated by a gap. This architecture is similar to the asteroid and Kuiper belts of our own solar system.
What maintains the gap between the hot and cold belts around Vega and Fomalhaut? The results strongly suggest that the answer is multiple planets. Our solar system’s asteroid belt, located between Mars and Jupiter, is held together by the gravity of terrestrial planets and giant planets, and the outer Kuiper belt is sculpted by the giant planets.
“Our findings echo recent findings showing that multi-planet systems are common beyond our sun,” said Kate Su, an astronomer at the University of Arizona’s Steward Observatory in Tucson. Su presented the findings 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 warmer, bluer color in visible light. The two stars are relatively close, about 25 light-years away. The stars are thought to be around 400 million years old, but Vega might be closer to its 600 millionth birthday. Fomalhaut has only one 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 hot 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 the two stars. Comets and collisions of rocky chunks replenish the dust in these bands. The inner belts of these systems cannot be seen in visible light because the brightness of their stars eclipses them.
Both the inner and outer belts contain much more material than our own asteroid and Kuiper belts. The reason is twofold: the star systems are much younger than our own, which has had hundreds of millions of years more to clean up, and the systems probably formed from a cloud of gas and dust initially more massive than our solar system.
The gap between the inner and outer debris belts for Vega and Fomalhaut also corresponds proportionally to the distance between the asteroid belts of our sun and Kuiper. This distance corresponds to a ratio of about 1:10, with the outer belt being 10 times farther from its host star than the inner belt. As for the large gap between the two belts, it is likely that there are several undetected planets, Jupiter-sized or smaller, creating a dust-free zone between the two belts. A good star system for comparison is HR 8799, which has four known planets sweeping the space between two similar debris disks.
“Overall, the large gap between the hot and cold belts is a panel that points to several planets likely orbiting Vega and Fomalhaut,” Su said.
If unseen planets do indeed orbit Vega and Fomalhaut, these bodies are unlikely to remain hidden.
“New upcoming facilities such as NASA’s James Webb Space Telescope should be able to find the planets,” said paper co-author Karl Stapelfeldt, head of the Exoplanet and Stellar Astrophysics Laboratory. at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.