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Alien dust found around distant proto-planets
Posted: 07 January 2010

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Dusty debris found around planetary embryos in a 500 light year distant system by astronomers using the Gemini South telescope bears no resemblance to the planetary building blocks of our own Solar System.

The dusty debris around the star HD 131488 is thought to have formed from colliding planetary embryos. “Until now, warm dust found around other stars has been very similar in composition to asteroidal or cometary material in our Solar System,” says lead researcher Carl Melis, who presented the results at the American Astronomical Society meeting in Washington yesterday. “This newly discovered dusty star is a compelling exception.”

Artist’s impression of what HD 131488’s inner planetary system might look like as two large rocky bodies collide. Image: Lynette Cook for Gemini Observatory/AURA.

Warm dust is present close to the star out to a distance comparable to the Earth-Sun separation (a region known as the terrestrial planet zone), while cold dust resides out to about 45 times that distance, analogous to the Kuiper Belt in our Solar System. The type of dust, however, is currently unknown. “Typically, dust debris around other stars, or our own Sun, is of the olivine, pyroxene, or silica variety, minerals commonly found on Earth,” says Melis. “The material orbiting HD 131488 is not one of these dust types. We have yet to identify what species it is – it really appears to be a completely alien type of dust.”

The quantity of warm dust is unusually large, say the astronomers, who propose that the most likely explanation is a recent collision of two rocky bodies. This would also provide a source for the dust particles. The distal cold dust, on the other hand, is most probably left over from planet formation that took place further away from the sun.

The location of HD 131488Ős dust belts (top) and comparable regions to our own Solar System (bottom). TPZ = terrestrial planet zone. Image: Lynette Cook for Gemini Observatory/AURA.

“Although dusty telltales of planetary formation processes in the outer regions surrounding young stars have often been seen with infrared-sensitive space telescopes, for some reason stars that have large amounts of orbiting warm dust do not also show evidence for the presence of cold dust,” says Benjamin Zuckerman. “HD 131488 dramatically breaks this pattern.”

The star joins five other systems bearing suns a few times more massive than our Sun that also show evidence of dust in their terrestrial planet zone. But this is five out of thousands of intermediate mass stars – why are these warm dust rings apparently rare? The astronomers note that all five of these stars have ages in the range of 10-30 million years, suggesting that catastrophic collisions – like the one that resulted in the formation of the Moon in our own Solar System – occurred in this narrow age range for stars of this mass.

Further study of HD 131488 and of other similar systems is planned by the team. The infrared imaging and spectroscopic observations in this study were conducted using the T-ReCS instrument on the Gemini South telescope located in Chile. Other observations were made using the Infrared Astronomical Satellite (IRAS) space-based infrared observatory, the NASA Infrared Telescope Facility (IRTF), and the Keck II telescope and optical observations from the Keck I telescope, the Siding Spring Observatory (SSO) 2.3-metre telescope, and the Tycho space-based imager/astrometer that flew aboard the ESA Hipparcos mission.