Observations of warm dust swaddling a young, nearby star suggest that astronomers have found the first asteroid belt outside the solar system. And where there are asteroids, there could be planets.
Several features of the dust surrounding the star Zeta Leporis indicate that the particles were generated by a massive collection of asteroids smashing into each other, says Christine H. Chen of the University of California, Los Angeles.
The material’s temperature, mass, and proximity to the youthful star are all indicators that “the system we observed around Zeta Leporis is similar to what we think occurred in the early years [of the solar system] when planets and asteroids were created,” says study collaborator Michael Jura of UCLA. Chen and Jura reported the finding last week in Pasadena, Calif., at a meeting of the American Astronomical Society.
Zeta Leporis lies some 70 light-years from Earth, and astronomers estimate its age to be 50 million to 500 million years. That’s young enough that the star could still be forming planets and asteroids but old enough that any dust that surrounded the star at its birth would already have coalesced into larger objects or spiraled into the star, Chen notes.
Indeed, she and Jura calculate that any dust grains orbiting Zeta Leporis would last only about 20,000 years before being absorbed by the star.
The observed dust must therefore have come from some reservoir that continually replenishes the grains, Chen asserts. The most likely source, she notes, is collisions between large bodies, such as asteroids. In our solar system, collisions between members of the asteroid belt replenish dust in the region where the inner, terrestrial planets reside.
Astronomers have known since 1983 that dust orbits Zeta Leporis. That year, NASA’s Infrared Astronomy Satellite detected a larger-than-expected infrared signal from the star’s vicinity. That’s a sign of dust, which absorbs visible light from the star that it surrounds and reemits the radiation at infrared wavelengths.
Last February, Chen and Jura examined Zeta Leporis with an infrared camera on one of the Keck telescopes atop Mauna Kea in Hawaii. The camera revealed that the dust is confined to a region around the star no larger than 6.1 astronomical units (AU) in radius and could lie as close as 2.5 AU. One AU is the distance between the sun and Earth. The belt of asteroids in our solar system lies between 2.1 and 3.3 AU from the sun.
Chen and Jura calculate that to produce the amount of dust they observed, the asteroids orbiting Zeta Leporis must have a total mass about 200 times that of all the rocky denizens of the solar system’s asteroid belt.
An asteroid belt circling Zeta Leporis “is a solid inference” of the new findings, comments Mark V. Sykes of the University of Arizona in Tucson. He suggests that the gravity of a planet as massive as Jupiter may be required to stir up the belt, generating the collisions necessary to produce the amount of dust Chen and Jura observed.
Trying to find evidence of such a massive planet indirectly, by studying the wobble it would induce in the motion of Zeta Leporis, could prove difficult because the star’s rapid rotation would interfere with those observations, Sykes says. As telescope optics continues to improve, however, astronomers might be able to discern a planet by direct imaging, he adds.