Relative to our sun, AU Mic is a modest star, with only half its mass and a tenth of its energy output. It's also a baby--just 12 million years old compared with the sun's 4.6 billion years. Paul Kalas of the University of California at Berkeley and his colleagues discovered AU Mic with the help of the James Clerk Maxwell Telescope on Mauna Kea in Hawaii. Of particular interest was the star's expansive disk of dust, which extends a whopping 210 astronomical units (AU), or 20 billion miles, from the star itself. Direct images of dust disks are quite rare, and AU Mic's is the closest one recorded since the discovery two decades ago of the disk around beta-Pictoris, which weighs in at about 2.5 times the mass of the sun and lies 65 light-years away. In fact, although AU Mic and beta-Pictoris currently reside in rather different parts of the sky, Kalas notes that they appear to be sister stars that formed at around the same time in the same region of space and are traveling together through the galaxy.
Analyses of the radiation from AU Mic suggest that dust particles are present only at large distances from the star: they do not appear to exist inside a radius of about 17 AU. "The dust missing from the inner regions of AU Mic is the telltale sign of an orbiting planet," says team member Michael C. Liu of the University of Hawaii. "The planet sweeps away any dust in the inner regions, keeping the dust in the outer region at bay."
Looking forward, the researchers plan to observe the disk in greater detail, hoping to obtain sharper views that perhaps reveal planets. "Unfortunately, we can't go back in time and observe our own solar system," Liu remarks. "But by studying these very young stars, we can examine how planets are forming around them, and thus indirectly learn about the origin of our own solar system." The findings are reported in the current issue of Science and in a forthcoming paper in the Astrophysical Journal. --Kate Wong