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 solar system.
What is maintaining the gap between the warm and cool belts around Vega and Fomalhaut? The results strongly suggest that 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 from the Steward Observatory at the University of Arizona in Tucson.
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 asteroid and Kuiper belts. The reason is twofold: The star systems are far younger than ours, 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 is 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 likely will not stay hidden.
“Upcoming new facilities such as NASA’s James Webb Space Telescope should be able to find the planets,” said Karl Stapelfeldt of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.