BY DR EMILY BALDWIN
Posted: 06 January, 2009
Astronomers have turned to an unexpected place to study the evolution of planets: dead stars.
Using NASA's Spitzer Space Telescope, astronomers detected six ‘dead’ white dwarf stars littered with the remains of shredded asteroids. These remains, it turns out, could teach astronomers about the building materials of planets around other stars. Moreover, the finding suggests that the same materials that make up the Solar System's rocky bodies could be prevalent across the Universe, and, if these planetary building blocks are common, then rocky planets could be too.
White dwarf stars 'polluted' with shredded asteroid debris could shed light on planet formation across the Universe,. Image: NASA/JPL-Caltech.
"If you ground up our asteroids and rocky planets, you would get the same type of dust we are seeing in these star systems," says Michael Jura of the University of California, who spoke at the American Astronomical Society meeting held in Long Beach, California this week. "This tells us that the stars have asteroids like ours, and therefore could also have rocky planets."
Asteroids and planets form out of the dusty material that swirls around young stars. Over time, the dust builds up into larger and larger clumps and eventually into fully-fledged planets, with the left over debris forming the asteroids. When a star like our own Sun nears the end of its life, it swells up into a red giant that consumes its innermost planets, while jostling about the orbits of the remaining asteroids and outer planets. In a final death cry, the star expels its outer layers and collapses into a skeleton of its former self. That is, into a white dwarf. Occasionally, one of the remaining asteroids may stray too close to the white dwarf, upon which the gravity of the dead star shreds the asteroid to pieces. A similar thing happened to Comet Shoemaker Levy 9 when Jupiter's gravity tore it up into some 21 different fragments before the comet ultimately smashed into the planet in 1994.
In the new study, astronomers used Spitzer’s infrared spectrograph, an instrument that breaks light apart into a rainbow of wavelengths, to reveal the chemical imprints of shredded asteroid pieces tumbling around six different white dwarfs. Previously, Spitzer had analysed the asteroid dust around two so-called ‘polluted’ white dwarfs; the new observations bring the tally 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," says Jura.
This plot of data from NASA's Spitzer Space Telescopes shows that asteroid dust around white dwarf star GD 40 contains silicates, a common mineral on Earth. The yellow dots show averaged data from the spectrograph, while the orange triangles show older data from Spitzer's infrared array camera. Image: NASA/JPL-Caltech/UCLA.
In all eight systems Spitzer revealed that the dust contains a glassy silicate mineral similar to olivine, which is commonly found on Earth. "This is one clue that the rocky material around these stars has evolved very much like our own," says 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 also have relatively little carbon.
In each of the eight white dwarf systems astronomers suspect that a single asteroid – perhaps as large as 200 kilometres in diameter – broke apart within the last million years or so. When an asteroid is torn apart by a dead star it shatters into very tiny pieces, whereas asteroid dust around living stars comprises much larger particles. By continuing to use spectrographs to analyse the visible light from this fine dust, astronomers will be able to determine what elements are present and in what abundance, revealing much more about how other star systems sort and process their planetary materials.
Jura’s paper describing the results has been accepted for publication in the Astronomical Journal, with co-authors Ben Zuckerman from the University of California and Jay Farihi from Leicester University in England.