In a famous battle scene from Star Wars Episode V: The Empire Strikes Back, spacecraft dash through a swarm of tumbling asteroids. A NASA satellite now has found a dense band of asteroids around a nearby star where the action does get fast and furious. A bright ribbon of dust close to the star points to frequent crashes within an alien asteroid belt 25 times as massive as the one in our solar system, but astronomers think most planetary systems are not nearly so crowded.
Thousands of small rocky asteroids populate the gap between the orbits of Mars and Jupiter. Occasional collisions create a haze of particles that spreads among the planets. If the sky is dark, observers on Earth can see this dust as a faint band of reflected sunlight above the horizon before sunrise or after sunset. Astronomers have tried to detect asteroid belts around stars like our sun, but until now they had only found evidence of asteroids circling two massive and much younger stars.
In the new study, NASA's Spitzer Space Telescope examined infrared light from 85 stars with ages and sizes similar to our sun's. Just one of the stars, an object called HD 69830 about 40 light-years away, showed a warm glow of excess heat from material orbiting around it. Spitzer's detectors measured dust particles ranging from room temperature to an oven-roasting 200°C. To get so hot, the dust must orbit very near the star--as close as the orbit of Venus around our sun, says astronomer Charles Beichman of NASA's Jet Propulsion Laboratory in Pasadena, California.
The bright band of dust cannot last long unless rocky bodies collide once every 1000 years or so, Beichman says. Otherwise, the particles would quickly spiral into the star. A single large comet could supply the dust, but Beichman says it would be a "freak event" for astronomers to see the short-lived comet at just the right time. His team will report its research in an upcoming issue of the Astrophysical Journal.
If a planet like Earth arose near HD 69830, it would be bombarded with stray asteroids and could not support life, says planetary scientist Jonathan Lunine of the University of Arizona, Tucson. However, Spitzer's survey suggests that dense asteroid belts usually do not exist close to mature stars elsewhere--probably, Lunine notes, because large planets like Jupiter "sweep up the debris."