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A new image of the disk of gas and dust around a sun-like star has
spiral-arm-like structures. These features may provide clues to the presence of
embedded but as-yet-unseen planets.
“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. Now, for the first time, we’re seeing these features,” says
Carol Grady, a National Science Foundation (NSF)-supported astronomer with
Eureka Scientific Inc.
The newly imaged disk surrounds SAO 206462, a star located about 456
light-years away in the constellation Lupus. Astronomers estimate that the
system is only about 9 million years old. The gas-rich disk spans some 14
billion miles, which is more than twice the size of Pluto’s orbit in our own
solar system.
“The surprise,” says Grady, “was that we caught a glimpse of
this stage of planet formation. This is a relatively short-lived phase.”
A near-infrared image from the National Astronomical Observatory of Japan
shows a pair of spiral features arcing along the outer disk. Theoretical models
show that a single embedded planet may produce a spiral arm on each side of a
disk. The structures around SAO 206462 do not form a matched pair, suggesting
the presence of two unseen worlds, one for each arm. However, the research team
cautions that processes unrelated to planets may also give rise to these
structures.
“What we’re finding is that once these systems reach ages of a few
million years, their disks begin to show a wealth of structure–rings, divots,
gaps and now spiral features,” says John Wisniewski, a collaborator at the
University of Washington in Seattle. “Many of these structures could be
caused by planets within the disks.”
Grady’s research is part of the Strategic Exploration of Exoplanets and
Disks with Subaru (SEEDS), a five-year-long near-infrared study of young stars
and their surrounding dust disks using the Subaru Telescope atop Mauna Kea in Hawaii. The
international consortium of researchers now includes more than 100 scientists
at 25 institutions.
“These arm-like structures have been predicted by models, but have
never before been seen,” says Maria Womack, program director for the
division of Astronomical Sciences at NSF. “It is the first observation of
spiral arms in a circumstellar disk, and an important test for models of
planetary formation.”
