A DUST cloud around a young red dwarf star 33 light years away promises astronomers a close-up view of planet formation. The cloud, recently spotted around AU Microscopium, appears hollow, suggesting that most of the interior dust has been swept up as planets have formed. The Hubble Space Telescope or large ground-based telescopes with adaptive optics should be able to reveal what is inside.
鈥淭his is going to be one of those really special systems,鈥� says Michael Liu of the University of Hawaii in Honolulu, whose findings will appear in Science. AU Microscopium is very young for our region of the galaxy, at just 12 million years old, but even at this age planet formation is expected to be nearly finished. The dust cloud stretches out to about seven times Neptune鈥檚 distance from the sun (see Graphic), and because it is cold it is unlikely to come closer to the star than Uranus is to the sun. The area closer in is nearly empty, suggesting that this is where planets have formed.
The dust disc is only the fourth to be seen directly by telescopes, and is also the closest spotted. Astronomers are particularly excited because AU Microscopium appears to be a sibling of Beta Pictoris, the first star to be discovered (in 1983) with a dust disc. The two are about the same age and are following parallel paths in space, although they are widely separated in the sky.
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They are also quite different kinds of star. Beta Pictoris is 2.5 times the mass of the sun and nine times as bright. AU Microscopium is much smaller, with only half the sun鈥檚 mass and only a tenth of its brightness. Comparing the discs should help astronomers understand planet formation around different types of star, says Paul Kalas of the University of California at Berkeley.
Kalas and Liu have applied for time on bigger telescopes when AU Microscopium returns to the night sky in the summer. 鈥淲e know so little right now that anything we get is going to be exciting,鈥� Kalas says.
Liu wants to look for molecular hydrogen with the Spitzer Space Telescope, to see how fast gas dissipates from the disc. Kalas wants to look for patterns in the disc that might reveal the planets forming.