Close encounters with interstellar gas could have given the
dust-filled disks of solar systems — where planets are thought to form — the
odd shapes that some of them have taken on, a new study suggests.
Stars across the galaxy have disks
of dusty debris generated by the collisions of small comet- and
asteroid-like bodies orbiting each star.
Astronomers have noticed that many of these debris disks are
a bit wonky-looking, with lobes of dust sticking out in odd directions. One
team noticed just such an oddly-shaped disk while using the Hubble
Space Telescope to investigate the composition of the dust around the star
HD 32297, which lies 340 light-years away from Earth in the constellation
Orion.
John Debes of NASA's Goddard Space Flight Center in Greenbelt, Md., noticed that the interior portion of this star's dusty disk — a
region comparable to the size of our own solar system — was warped in a way
that was similar to other distant star systems.
Astronomers have previously attributed these warped shapes
to the presence of undiscovered planets or past encounters with another star.
But Debes and his colleagues used a model to show that the odd shapes aren't
likely due to one of these exotic factors, but instead are likely caused by the
interstellar environment that the star and its attendant disk are moving
through.
"It's important to consider the ecology of these debris
disks before running to such conclusions, and this model explains a lot of the
weirdly shaped disks we see," Debes said.
Interstellar breeze
The model, described in the Sept. 1 issue of the
Astrophysical Journal, simulates the movement of a star and its dust disk as
they orbit the center of the galaxy. As it moves through the galaxy, it
encounters patches of thin gas clouds that litter the space between stars and
create an effect like an interstellar breeze.
When the dust particles meet a relatively dense clump of
gas, the breeze, or flow, creates a drag force on the dust particles.
"The small particles slam into the flow, slow down and
gradually bend from their original trajectories to follow it," Debes said.
The resulting shapes of the dust disk depend on the
orientation of the star system to the gas cloud. If the system hits the cloud
face-on, which seems to be the case for the star HD 61005 in the constellation
Puppis, the disk bends gently away from the direction of motion. Fine dust
trails behind, forming a cylindrical wake.
If, on the other hand, the dust disk slices edgewise through
the gas cloud, the headwind blows the fine dust away from the portion of the
disk inside the cloud, resulting in a lopsided disk.
"The drag from interstellar gas only affects the
outskirts of the disk, where the star's gravity can't really hold onto the
material," said study team member Alycia Weinberger at the Carnegie Institution
of Washington, headquartered in Washington, D.C.
There was also evidence from previous studies that
interstellar gas lurked in the region around HD 32297.
"The pieces came together to make me think that gas
drag was a good explanation for what was going on," Debes said.
The systems studied are about 100 million years old and
resemble our own solar system shortly after the major planets formed. Although
astronomers don't know whether planets lurk within the disks of these systems,
a better understanding of processes affecting a disk's outer regions will shed
light on how "ice giant" planets like Uranus and Neptune — and the more distant
swarm of small, icy bodies known as the Kuiper Belt — formed within the solar
system.
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