SEATTLE - A
flurry of lint-like particles discovered swirling around a small, distant star could help explain how miniscule
interstellar dust grains clump together to form planets, astronomers say.
"We
have seen many seeds of planets and we have seen many planets, but how they go
from one to the other is a mystery," said study team member James Graham
of the University of California, Berkeley. "These observations help us to
fill in that gap."
The
newfound fluffy particles are about ten times larger than interstellar dust
grains and about as porous as newly fallen snow, which is composed of about 97
percent air and only 3 percent ice.
Space
lint
Another
good comparison is lint from a drying machine, Graham said. The two types of materials
have roughly the same porosity and are held together by the same molecular
forces.
The space
lint was detected in a disk encircling AU Microscopii, a relatively young red
dwarf star, just 12 million years old, located 32 light-years from Earth [image].
The
researchers used polarizing filters on the Hubble Space Telescope's
Advanced Camera for Surveys to blot out the main portion of Au
Microscopii's light so that only light reflected off the star's debris disk was
visible.
By
examining the reflected starlight, the researchers deduced information about
certain properties of the particles, including size and shape.
"The
way that light scatters depends on the refractive index of the material...[which]
tells you how light rays get bent," Graham explained. The refractive index
of a material depends on how porous the material is, and a low value is
indicative of porous materials.
Researchers
think the fluff was shed when larger, soft-ball sized objects-also fluffy-collided.
Dubbed "parent bodies," these objects are thought to be the precursors
to even larger entities which eventually coalesce to form planets.
"Once
they exceed softball sizes, they start to become compactified. The vacuum in between
the grains get filled with more material," Graham said in a news
conference Sunday at the 209th meeting of the American Astronomical
Society. "The fact that we see this low porosity means that the parent
bodies haven't grown too big."
The
"birth ring"
The parent
bodies are thought to reside within a tumultuous region located near the center
of the star called the "birth ring" [image].
First
predicted by astronomers in 2005, the birth ring is thought to extend out to a
distance of up to 4.6 billion miles from the star. Within this ring, collisions
are more likely to occur and it is in this region that planets are most likely
to form.
Crashing parent
bodies are thought to shed the fluff, which is then blown outwards by the
star's intense solar winds.
The
discovery of the fluffy particles could affect astronomers' theories about how
planets form, said study team member Paul Kalas, also of UC Berkeley.
For
example, he said, to calculate the mass of a rock with no porosity, scientists
cube the value of its radius. But for a porous object, this relationship no
longer holds, and mass is a function of the radius squared.
"So
let's say you have a theoretician wants to make a model of how everything in
the solar system forms," Kalas said. "If he starts off with that
basic physics result that mass is proportional to radius cubed, he would get
the wrong answer because we're starting from very porous fragments."
Kalas said
that future research might focus on whether a similar fluff surrounds other
stars.
"It
would be interesting to see how this porosity changes with the mass and
luminosity of the star," he said.
Editor's
Note: All
week, SPACE.com is providing complete
coverage of the 209th meeting of the American Astronomical Society.
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