[extropy-chat] binaries have planets too

Damien Broderick thespike at satx.rr.com
Fri Mar 30 19:00:57 UTC 2007

Jet Propulsion Laboratory, Pasadena, Calif.

News Release:  2007-036
March 29, 2007

NASA Telescope Finds Planets Thrive Around Stellar Twins

The double sunset that Luke Skywalker gazed upon in the film
"Star Wars" [[not to mention a zillion sf covers since the dawn of 
time]] might not be a fantasy.

Astronomers using NASA's Spitzer Space Telescope have observed
that planetary systems - dusty disks of asteroids, comets and
possibly planets - are at least as abundant in twin-star systems
as they are in those, like our own, with only one star.  Since
more than half of all stars are twins, or binaries, the finding
suggests the universe is packed with planets that have two suns.
Sunsets on some of those worlds would resemble the ones on Luke
Skywalker's planet, Tatooine, where two fiery balls dip below
the horizon one by one.

"There appears to be no bias against having planetary system
formation in binary systems," said David Trilling of the
University of Arizona, Tucson, lead author of a new paper about
the research appearing in the April 1 issue of the Astrophysical
Journal. "There could be countless planets out there with two or
more suns."

Previously, astronomers knew that planets could form in
exceptionally wide binary systems, in which stars are 1,000
times farther apart than the distance between Earth and the sun,
or 1,000 astronomical units. Of the approximately 200 planets
discovered so far outside our solar system, about 50 orbit one
member of a wide stellar duo.

The new Spitzer study focuses on binary stars that are a bit
more snug, with separation distances between zero and 500
astronomical units. Until now, not much was known about whether
the close proximity of stars like these might affect the growth
of planets. Standard planet-hunting techniques generally don't
work well with these stars, but, in 2005, a NASA-funded
astronomer found evidence for a planet candidate in one such
multiple-star system


Trilling and his colleagues used Spitzer's infrared, heat-
seeking eyes to look not for planets, but for dusty disks in
double-star systems. These so-called debris disks are made up of
asteroid-like bits of leftover rock that never made it into
rocky planets. Their presence indicates that the process of
building planets has occurred around a star, or stars, possibly
resulting in intact, mature planets.

In the most comprehensive survey of its kind, the team looked
for disks in 69 binary systems between about 50 and 200 light-
years away from Earth. All of the stars are somewhat younger and
more massive than our middle-aged sun. The data show that about
40 percent of the systems had disks, which is a bit higher than
the frequency for a comparable sample of single stars. This
means that planetary systems are at least as common around
binary stars as they are around single stars.

In addition, the astronomers were shocked to find that disks
were even more frequent (about 60 percent) around the tightest
binaries in the study. These coziest of stellar companions are
between zero and three astronomical units apart. Spitzer
detected disks orbiting both members of the star pairs, rather
than just one. Extra-tight star systems like these are where
planets, if they are present, would experience Tatooine-like

"We were very surprised to find that the tight group had more
disks," said Trilling. "This could mean that planet formation
favors tight binaries over single stars, but it could also mean
tight binaries are just dustier. Future observations should
provide a better answer."

The Spitzer data also reveal that not all binary systems are
friendly places for planets to form. The telescope detected far
fewer disks altogether in intermediately spaced binary systems,
between three to 50 astronomical units apart. This implies that
stars may have to be either very close to each other, or fairly
far apart, for planets to arise.

"For a planet in a binary system, location is everything," said
co-author Karl Stapelfeldt of NASA's Jet Propulsion Laboratory
in Pasadena, Calif.

"Binary systems were largely ignored before," added Trilling.
"They are more difficult to study, but they might be the most
common sites for planet formation in our galaxy."

Other authors on the paper include: John Stansberry, George
Rieke and Kate Su of the University of Arizona; Richard Gray of
the Appalachian State University, Boone, N.C.; Chris Corbally of
the Vatican Observatory, Tucson; Geoff Bryden, Andy Boden and
Charles Beichman of JPL; and Christine Chen of the National
Optical Astronomical Observatory, Tucson.

JPL manages Spitzer 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. The multiband imaging photometer for Spitzer was
built by Ball Aerospace Corporation, Boulder, Colo.; the
University of Arizona; and Boeing North American, Canoga Park,
Calif. Co-author Rieke is the principal investigator.

For more information and graphics, visit:

www.spitzer.caltech.edu/Media and 

More information about extrasolar planets and NASA's planet-
finding program is at:


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