Hidden Planet Pushes Star's Ring a Billion Miles Off-Center (Forwarded)

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June 11, 2007

Hidden Planet Pushes Star's Ring a Billion Miles Off-Center

A young star's strange elliptical ring of dust likely heralds the presence
of an undiscovered Neptune-sized planet, says a University of Rochester
astronomer in the latest Monthly Notices of the Royal Astronomical
Society.

Stars in the early stages of life are surrounded by dust clouds that thin
out and dissipate as the star reaches maturity, becoming rings in their
final stages. One star, however, has a dust ring that has long puzzled
astronomers because it is not centered around the star as usual. Instead,
the ring is elliptical, with the parent star off to one side.

"We wanted to know why this ring was off-center," says Alice C. Quillen,
Associate Professor of Astronomy and author of the study. "People guessed
there might be a planet in there, but nobody knew where it might be, or
how big it might be. Now we've got a very good idea."

Roughly 250 planets have been discovered so far around stars other than
our Sun. Most have been revealed by the way the planets influence their
parent stars, but Quillen has been working for years on understanding the
delicate interaction between stellar dust disks and the planets that shape
them. She is now one of the world's experts in predicting planet size and
position from the features of a star's dust ring.

Quillen used new images from the Hubble Space Telescope that caught the
star, Fomalhaut, and its surrounding ring almost edge-on and in more
detail than ever before. Fomalhaut, 25 light-years away, is the brightest
star in the autumn sky. Using a device called a coronagraph that blocks
out a star's light so dimmer objects near it can be seen, the Hubble
revealed that Fomalhaut was indeed off-center within its ring. The images
were also clear enough to show that the ring itself had a surprisingly
sharp edge.

That sharp edge was the clue Quillen was looking for. Since ascertaining
one of the first extra-solar planets using dust-ring analysis in 2002,
Quillen has greatly strengthened her planet-ring interaction models.
Treating the ring like a hydrodynamic structure, for instance, is
necessary for younger stars whose dust is relatively fine and acts more
like a fluid -- while the physics of dust collision become dominant in
older ring systems where the dust has begun clumping into larger bodies.

The sharp inside edge of Fomalhaut, Quillen calculated, demanded that a
relatively small, Neptune-size planet was tucked right up against the
inner side of the ring, using its gravity to toss dust in the area out of
orbit.

According to Quillen's calculations, the ring is elliptical because the
Neptunian planet's own orbit around Fomalhaut is elliptical -- a curiosity
in such a young system. When stars form from a giant cloud of gas and
dust, the angular momentum of the cloud carries over to all the objects
that form from the cloud, including new planets. Those new planets should,
initially at least, orbit in nice, circular paths -- not elliptical ones.
Fomalhaut's ring is offset by 1.4 billion miles, more than 15 times the
distance from the Earth to the Sun, suggesting the hidden planet's orbit
is also tremendously skewed.

"Something had to skew that planet, and that's what we're working on now,"
says Quillen. "There may have been fantastic planetary collisions early on
that changed their orbits. We're working on figuring out how many more
planets of what size you'd need to account for that elliptical orbit, and
to account for why there is no other dust inside that ring."

Quillen's model will remain just a theory until a new generation of
telescopes can actually see the Formalhaut planets in question. These
telescopes will be equipped with sophisticated coronagraphs that can block
out Formalhaut's light enough to let the planets themselves shine through.

This research was funded by the National Institutes of Science and NASA.

[NOTE: An image supporting this release is available at
http://hubblesite.org/newscenter/arc...05/10/image/a/ ]