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Posted by baalke on April 18, 2007, 6:12 pm
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http://www.jpl.nasa.gov/news/news.cfm?release=2007-043
Astronomers Map Out Planetary Danger Zone
April 18, 2007
Pasadena, Calif. - Astronomers have laid down the cosmic equivalent of
yellow "caution" tape around super hot stars, marking the zones where
cooler stars are in danger of having their developing planets blasted
away.
In a new study from NASA's Spitzer Space Telescope, scientists report
the first maps of so-called planetary "danger zones." These are areas
where winds and radiation from super hot stars can strip other young,
cooler stars like our sun of their planet-forming materials. The
results
show that cooler stars are safe as long as they lie beyond about 1.6
light-years, or nearly 10 trillion miles, of any hot stars. But cooler
stars inside the zone are likely to see their potential planets boiled
off into space.
"Stars move around all the time, so if one wanders into the danger
zone
and stays for too long, it will probably never be able to form
planets,"
said Zoltan Balog of the University of Arizona, Tucson, lead author of
the new report, appearing May 20 in the Astrophysical Journal.
The findings are helping astronomers pinpoint the types of
environments
where planets beyond our solar system, including some that might be
hospitable to life, are most likely to form.
Planets are born out of a flat disk of gas and dust, called a
protoplanetary disk, that swirls around a young star. They are
believed
to clump together out of the disk over millions of years, growing in
size like dust bunnies as they sweep through the dust.
Previous studies revealed that these protoplanetary disks can be
destroyed by the most massive, hottest type of star in the universe,
called an O-star, over a period of about a million years. Ultraviolet
radiation from an O-star heats and evaporates the dust and gas in the
disk, then winds from the star blow the material away. Last year,
Balog
and his team used Spitzer to capture a stunning picture of this
"photoevaporation" process at work
www.spitzer.caltech.edu/Media/happenings/20061003/ .
The team's new study is the first systematic survey for disks in and
around the danger zone, or "blast radius" of an O-star. They used
Spitzer's heat-seeking infrared eyes to look for disks around 1,000
stars in the Rosette Nebula, a turbulent star-forming region 5,200
light-years away in the constellation Monoceros. The stars range
between
one-tenth and five times the mass of the sun and are between 2 and 3
million years old. They are all near at least one of the region's
massive O-stars.
The observations revealed that, beyond 10 trillion miles of an O-star,
about 45 percent of the stars had disks - about the same amount as
there
were in safer neighborhoods free of O-stars. Within this distance,
only
27 percent of the stars had disks, with fewer and fewer disks spotted
around stars closest to the O-star. In other words, an O-star's danger
zone is a sphere whose damaging effects are worst at the core. For
reference, our sun's closest star, a small star called Proxima
Centauri,
is nearly 30 trillion miles away.
In addition, the new study indicates that a protoplanetary disk will
boil off faster in the zone's perilous core. For example, a disk two
times closer to an O-star than another disk will evaporate twice as
fast. "The edges of the danger zone are sharply defined," said Balog.
"It is relatively safe for protoplanetary disks outside it, whereas a
disk that gets dragged along by its star to be really close to an O-
star
could disappear in as fast as a hundred thousand years."
Despite this doomsday scenario, there is a chance some planets could
survive a close encounter with an O-star. According to one alternative
theory of planet formation, some gas giants like Jupiter might form in
less than one million years. If such a planet already existed around a
young star whose disk is blown away, the gas giant would stay put
while
any burgeoning rocky planets like Earth would be forever swept away.
Some astronomers think our sun was born in a similarly violent
neighborhood studded with O-stars before migrating to its present,
more
spacious home. If so, it was lucky enough to escape a harrowing ride
into any danger zones, or our planets, and life as we know it,
wouldn't
be here today.
Other paper authors include James Muzerolle, Kate Su, George Rieke and
Erick Young of the University of Arizona; and Tom Megeath of the
University of Toledo, Ohio.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the
Spitzer
mission for NASA's Science Mission Directorate, Washington. Science
operations are conducted at the Spitzer Science Center at the
California
Institute of Technology, also in Pasadena. Caltech manages JPL for
NASA.
Spitzer's multiband imaging photometer, which collected the new data,
was built by Ball Aerospace Corporation, Boulder, Colo.; the
University
of Arizona; and Boeing North American, Canoga Park, Calif. Co-author
Rieke is the instrument's principal investigator.
For more information and graphics, visit www.spitzer.caltech.edu/Media
http://www.spitzer.caltech.edu/Media and http://www.nasa.gov/spitzer .
------------------------------------------------------------------------
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
2007-043
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