A Dusty Young Star’s “Spiral Arms”

Astronomers are racking up discoveries of extrasolar planets by the hundreds and (thanks to Kepler) by the thousands. But no one really understands how all these worlds formed in the first place. Sure, there's consensus that big interstellar clouds collapse and organize themselves into central stars surrounded by a big, turgid disks of gas and dust.

But then what? Researchers certainly have some general ideas, backed by ever-more-sophisticated computer simulations. Yet the exact hows, whys, whens, and wheres of planet formation have yet to be worked out.

"Spiral arms" around SAO 206462
Two spiral arms emerge from the gas-rich disk around SAO 206462, a young star in the constellation Lupus. This image, acquired by the Subaru Telescope and its HiCIAO instrument, is the first to clearly show such features.
NAOJ / Subaru
Now they'll have a real-life situation to test those models, thanks to remarkable images from the Subaru Telescope that reveal never-before-seen details of a solar system in the making. The central star is SAO 206462, a 9th-magnitude solar-type star that's some 450 light-years away in the constellation Lupus. It's surrounded by a substantial dust-rich disk about 1 arcsecond across; at the star's distance, that's about 14 billion miles (22 billion km), roughly twice the size of Pluto's orbit.

Remarkably, the nearly face-on disk is not a nice neat circle but instead sports two distinct "wings" that arc outward from the main mass, creating an overall appearance much like a spiral galaxy. Such extensions have been seen around individual stars before, but never with such clarity. It's a credit to Subaru's 8.2-m aperture, its adaptive-optics imager (called HiCIAO), and the stable, rarefied air atop Mauna Kea, Hawaii.

Astronomer Carol Grady (Eureka Scientific) presented the new view two weeks ago at a NASA-hosted conference dubbed Signposts of the Planets. The distorted disk hints that one or more massive young planets are lurking inside it. "Detailed computer simulations have shown us that the gravitational pull of a planet inside a circumstellar disk can perturb gas and dust, creating spiral arms," Grady explains in a NASA press release. "Now, for the first time, we're seeing these dynamical features."

The computer models suggest that a single planet could create two arclike extensions, but in that case they'd be symmetrical. The arms extending outward from SAO 206462 are not, suggesting that two planets are in play.

Astronomers have been paying close attention to this star for a couple of decades, ever since they detected more far-infrared energy coming from it than could be explained by a star alone. This infrared excess, they reasoned, must result from a ring of warm dust encircling the star, which is estimated to be less than 9 million years old. By 1995 observers had deduced that the dust is carbon-rich, a hint that it contained organic matter. A recent spectroscopic study by Michael Sitko (University of Cincinnati) and others suggest that the putative planets have orbital periods of at most a few years and are sweeping up matter at rates that change with time.

But Grady and her team note that processes unrelated to planets could be creating the spiraling arcs. So now the search is on to detect the planets directly.

One thought on “A Dusty Young Star’s “Spiral Arms”

  1. Rod

    "It’s surrounded by a substantial dust-rich disk about 1 arcsecond across; at the star’s distance, that’s about 14 billion miles (22 million km), roughly twice the size of Pluto’s orbit."

    Just to clear things up a bit, 1 arcsecond size at 450 LY distance or 138 pc is about 138 AU in diameter. Diameter of Pluto orbit is about 80 AU.

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