Planetary systems can form around binary stars (Forwarded)

Carnegie Institution of Washington

Contact:
Alan Boss, 1-202-478-8858

January 10, 2006

PLANETARY SYSTEMS CAN FORM AROUND BINARY STARS

Washington DC -- New theoretical work shows that gas-giant planet
formation can occur around binary stars in much the same way that it
occurs around single stars like the Sun. The work is presented today by
Dr. Alan Boss of the Carnegie Institution's Department of Terrestrial
Magnetism (DTM) at the American Astronomical Society meeting in
Washington, DC. The results suggest that gas-giant planets, like
Jupiter, and habitable Earth-like planets could be more prevalent than
previously thought. A paper describing these results has been accepted
for publication in the Astrophysical Journal.
"We tend to focus on looking for other solar systems around stars just
like our Sun," Boss says. "But we are learning that planetary systems
can be found around all sorts of stars, from pulsars to M dwarfs with
only one third the mass of our Sun."

Two out of every three stars in the Milky Way is a member of a binary or
multiple star system, in which the stars orbit around each other with
separations that can range from being nearly in contact (close binaries)
to thousands of light-years or more (wide binaries). Most binaries have
separations similar to the distance from the Sun to Neptune (~30 AU,
where 1 AU = 1 astronomical unit = 150 million kilometers -- the
distance from the Earth to the Sun).

It has not been clear whether planetary system formation could occur in
typical binary star systems, where the strong gravitational forces from
one star might interfere with the planet formation processes around the
other star, and vice versa. Previous theoretical work had suggested, in
fact, that typical binary stars would not be able to form planetary
systems. However, planet hunters have recently found a number of
gas-giant planets in orbit around binary stars with a range of separations.
Boss found that if the shock heating resulting from the gravitational
forces from the companion star is weak, then gas-giant planets are able
to form in planet-forming disks in much the same way as they do around
single stars. The planet-forming disk would remain cool enough for ice
grains to stay solid and thus permit the growth of the solid cores that
must reach multiple-Earth-mass size for the conventional mechanism of
gas-giant planet formation (core accretion) to succeed.

Boss' models show even more directly that the alternative mechanism for
gas-giant planet formation (disk instability) can proceed just as well
in binary star systems as around single stars, and in fact may even be
encouraged by the gravitational forces of the other star. In Boss' new
models, the planet-forming disk in orbit around one of the stars is
quickly driven to form dense spiral arms, within which self-gravitating
clumps of gas and dust form and begin the process of contracting down to
planetary sizes. The process is amazingly rapid, requiring less than
1,000 years for dense clumps to form in an otherwise featureless disk.
There would be plenty of room for Earth-like planets to form closer to
the central star after the gas-giant planets have formed, in much the
same way our own planetary system is thought to have formed.

Boss points out, "This result may have profound implications in that it
increases the likelihood of the formation of planetary systems
resembling our own, because binary stars are the rule in our galaxy, not
the exception." If binary stars can shelter planetary systems composed
of outer gas-giant planets and inner Earth-like planets, then the
likelihood of other habitable worlds suddenly becomes roughly three
times more probable -- up to three times as many stars could be possible
hosts for planetary systems similar to our own. NASA's plans to search
for and characterize Earth-like planets in the next decade would then be
that much more likely to succeed.

One of the key remaining questions about the theoretical models is the
correct amount of shock heating inside the planet-forming disk, as well
as the more general question of how rapidly the disk is able to cool.
Boss and other researchers are actively working to better understand
these heating and cooling processes. Given the growing observational
evidence for gas-giant planets in binary star systems, the new results
suggest that shock heating in binary disks cannot be too large, or it
would prevent gas-giant planet formation.

The calculations were performed on the Carnegie Alpha Cluster of
workstations at DTM, the purchase of which was supported in part by a
grant from the National Science Foundation's Major Research
Instrumentation program. Boss' research on planet formation is supported
in part by NASA's Planetary Geology & Geophysics and Origins of Solar
Systems Programs, and by the NASA Astrobiology Institute (NAI). The NAI,
founded in 1997, is a partnership between NASA, 16 major U.S. teams and
six international consortia. NAI's goal is to promote, conduct, and lead
integrated multidisciplinary astrobiology research and to train a new
generation of astrobiology researchers. For more information about the
NAI on the Internet, visit:
http://nai.nasa.gov/

For images go to:
http://www.dtm.ciw.edu/boss/ftp/binary/

Two sets of images with different formats and sizes show the enhanced
formation of Jupiter-mass clumps in a disk in a binary star system
(binary images) compared with a disk orbiting a single star (single
images). Dense clumps capable of forming gas-giant planets form faster
in the binary star disk than in the single star disk.

Unless otherwise attributed, images on this web site are copyright
Carnegie Institution and its licensors.

The Department of Terrestrial Magnetism is part of the Carnegie
Institution of Washington (www.CarnegieInstitution.org), which has been
a pioneering force in basic scientific research since 1902. It is a
private, nonprofit organization with six research departments throughout
the U.S. Carnegie scientists are leaders in plant biology, developmental
biology, astronomy, materials science, global ecology, and Earth and
planetary science.