Spitzer Space Telescope finds shredded asteroids around other stars

January 9, 2009--Observations made with NASA's Spitzer Space Telescope reveal six dead "white dwarf" stars littered with the remains of shredded asteroids, say researchers at the Jet Propulsion Laboratory (Pasadena, CA).

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January 9, 2009--Observations made with NASA's Spitzer Space Telescope reveal six dead "white dwarf" stars littered with the remains of shredded asteroids, say researchers at the Jet Propulsion Laboratory (Pasadena, CA). While this might sound bleak, it turns out the torn-up asteroids are teaching astronomers about the building materials of planets around other stars.

The Spitzer Space Telescope has an f/12 0.85 m beryllium mirror cooled to below 5.5 K and covers the 3 to 180 micron wavelength range, with a diffraction limit of 6.5 microns. It has imaging and photometry capabilities in the 3 to 180 micron range, spectroscopy between 5 to 40 microns, and spectrophotometry between 50 to 100 microns. The telescope is positioned in an Earth-trailing heliocentric orbit.

So far, the new results suggest that the materials that make up Earth and our solar system's other rocky bodies could be similar to those elsewhere in the universe. If the materials are common, then rocky planets could be, too.

"If you ground up our asteroids and rocky planets, you would get the same type of dust we are seeing in these star systems," said Michael Jura of the University of California, Los Angeles, who presented the results on January 5 at the American Astronomical Society meeting (Long Beach, CA). Jura is the lead author of a paper on the findings accepted for publication in the Astronomical Journal.

Asteroids and planets form out of dusty material that orbits young stars. The dust sticks together, forming clumps and eventually full-grown planets. Asteroids are the leftover debris. When a star like our sun nears the end of its life, it becomes a red giant that consumes its innermost planets, while jostling the orbits of remaining asteroids and outer planets. As the star continues to die, it blows off its outer layers and shrinks down into a skeleton of its former self--a white dwarf.

Sometimes, a jostled asteroid wanders too close to a white dwarf and meets its demise, in which the gravity of the white dwarf shreds the asteroid to pieces. (A similar thing happened to Comet Shoemaker Levy 9 when Jupiter's gravity tore it up, before the comet ultimately smashed into the planet in 1994.)

Spitzer observed shredded asteroid pieces around white dwarfs with its infrared spectrograph (see figure). Previously, Spitzer analyzed the asteroid dust around two so-called polluted white dwarfs; the new observations bring the total to eight.

"Now, we've got a bigger sample of these polluted white dwarfs, so we know these types of events are not extremely rare," said Jura.

In all eight systems observed, Spitzer found that the dust contains a glassy silicate mineral similar to olivine and commonly found on Earth. "This is one clue that the rocky material around these stars has evolved very much like our own," said Jura. The Spitzer data also suggest there is no carbon in the rocky debris--again like the asteroids and rocky planets in our solar system, which have relatively little carbon.

A single asteroid is thought to have broken apart within the last million years or so in each of the eight white-dwarf systems. The biggest of the bunch was once about 200 km in diameter.

Jura says the real power of observing these white dwarf systems is still to come. When an asteroid comes apart around a dead star, it breaks into very small pieces. Asteroid dust around living stars, by contrast, is made of larger particles. By continuing to use spectrographs to analyze the visible light from this fine dust, astronomers will be able to see exquisite details, including information about what elements are present and in what abundance. This will reveal much more about how other star systems sort and process their planetary materials.

Other authors are Ben Zuckerman at the University of California, Los Angeles, and Jay Farihi at Leicester University, England.

--posted by John Wallace
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