Astronomers discover extrasolar asteroid belt

Posted: June 5, 2001

Astronomers announced Monday that they had found evidence for what could be a belt of asteroids forming around another star, a discovery that may help them better understand how solar systems like our own form.

A pair of UCLA astronomers said the characteristics of a previously discovered dust disk around the star Zeta Leporis suggest that asteroids, or even planets, are forming around the young star. The announcement was made at a meeting of the American Astronomical Society in Pasadena, California.

Illustration of Zeta Lep. Photo: UCLA, Dept. of Physics and Astronomy, C.H. Chen & M. Jura
"Because of the conditions we identified near Zeta Lep, we believe that the dust around this star may contain asteroids that appear to be colliding violently with each other," said Michael Jura, a professor of astronomy at UCLA.

The presence of a dust disk of some kind around the star had been known since 1983, when it was detected by the Infrared Astronomy Satellite (IRAS). Followup observations ten years ago found that the disk was unusually warm, suggesting that either the disk was closer to its parent star than expected or that some other energy source was generating infrared radiation.

Jung and graduate student Christine Chen observed the disk around Zeta Leporis earlier this year using the Long Wavelength Spectrometer, an infrared instrument on one of the twin 10-meter Keck Observatory telescopes. Those observations let them measure the size of the disk, which turned out to be only 12.2 astronomical units (1.8 billion kilometers) in diameter. Placed in our solar system, it would extend about 1 AU (150 million kilometers) beyond the orbit of Jupiter.

Jung and Chen were also able to estimate the temperature of the disk by observing it at two different wavelengths. They found the average temperature of the disk to be about 70 degrees Celsius, which they consider to be a relatively high temperature. This would mean particles in the disk could be as close as 2.5 AU (375 million kilometers) to the star.

The size and temperature observations can be best explained, the astronomers said, if the disk was composed not just of dust grains but of larger objects, like asteroids or even protoplanets. "There must be objects larger than dust around Zeta Lep, which may resemble asteroids in our own solar system, that are creating the infrared-emitting dust by violently colliding with each other," said Jura.

If confirmed, this discovery would mark the first time an asteroid belt was discovered around another star. Astronomers have detected several dozen extrasolar planets and many protoplanetary disks of dust, but had yet to find evidence of intermediate-sized bodies like asteroids.

Illustration of our solar system. Photo: UCLA, Dept. of Physics and Astronomy, C.H. Chen & M. Jura
This discovery could also help astronomers understand how solar systems are put together. "Zeta Lep is a relatively young star: approximately the age of our Sun when the Earth was forming," said Jura. "The system we observed around Zeta Lep is similar to what we think occurred in the early years when planets and asteroids were created."

"In simplest terms, our planets formed when smaller objects smashed together," explained Chen. "Dust that surrounds a star will eventually either fall into the star, or collide with itself and create bigger particles. The particles we can identify around Zeta Lep may be forming chunks of rock or larger objects; asteroids or even planets may be forming or have already formed around Zeta Lep."

Zeta Leporis may not be the most hospitable locale in the galaxy, though. The young star -- about 100 million years old -- is about twice the mass of the Sun and 20 times as bright. The star is about 70 light-years from the Earth in the direction of the constellation Lepus, the Hare.

Jung and Chen plan to follow up their work with additional observations in an effort to obtain spectra of the disk, which would provide clues to its composition. "We want to know if the asteroids around this star are similar in composition to objects in our Solar System, and we want to learn if the processes we now see unfolding on Zeta Lep can help us understand how the planets in our own Solar System formed," said Chen.