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For Release: Monday, January 30, 2006

Release No.: 06-11

Most Milky Way Stars Are Single

Cambridge, MA -- Common wisdom among astronomers holds that most star
systems in the Milky Way are multiple, consisting of two or more stars in
orbit around each other. Common wisdom is wrong. A new study by Charles
Lada of the Harvard-Smithsonian Center for Astrophysics (CfA) demonstrates
that most star systems are made up of single stars. Since planets probably
are easier to form around single stars, planets also may be more common
than previously suspected.

Astronomers have long known that massive, bright stars, including stars
like the sun, are most often found to be in multiple star systems. This
fact led to the notion that most stars in the universe are multiples.
However, more recent studies targeted at low-mass stars have found that
these fainter objects rarely occur in multiple systems. Astronomers have
known for some time that such low-mass stars, also known as red dwarfs or
M stars, are considerably more abundant in space than high-mass stars.

By combining these two facts, Lada came to the realization that most star
systems in the Galaxy are composed of solitary red dwarfs.

"By assembling these pieces of the puzzle, the picture that emerged was
the complete opposite of what most astronomers have believed," said Lada.

Among very massive stars, known as O- and B-type stars, 80 percent of the
systems are thought to be multiple, but these very bright stars are
exceedingly rare. Slightly more than half of all the fainter, sun-like
stars are multiples. However, only about 25 percent of red dwarf stars
have companions. Combined with the fact that about 85 percent of all stars
that exist in the Milky Way are red dwarfs, the inescapable conclusion is
that upwards of two-thirds of all star systems in the Galaxy consist of
single, red dwarf stars.

The high frequency of lone stars suggests that most stars are single from
the moment of their birth. If supported by further investigation, this
finding may increase the overall applicability of theories that explain
the formation of single, sun-like stars. Correspondingly, other
star-formation theories that call for most or all stars to begin their
lives in multiple-star systems may be less relevant than previously
thought.

"It's certainly possible for binary star systems to 'dissolve' into two
single stars through stellar encounters," said astronomer Frank Shu of
National Tsing Hua University in Taiwan, who was not involved with this
discovery. "However, suggesting that mechanism as the dominant method of
single-star formation is unlikely to explain Lada's results."

Lada's finding implies that planets also may be more abundant than
astronomers realized. Planet formation is difficult in binary star systems
where gravitational forces disrupt protoplanetary disks. Although a few
planets have been found in binaries, they must orbit far from a close
binary pair, or hug one member of a wide binary system, in order to
survive. Disks around single stars avoid gravitational disruption and
therefore are more likely to form planets.

Interestingly, astronomers recently announced the discovery of a rocky
planet only five times more massive than Earth. This is the closest to an
Earth-size world yet found, and it is in orbit around a single red dwarf
star.

"This new planet may just be the tip of the iceberg," said Lada. "Red
dwarfs may be a fertile new hunting ground for finding planets, including
ones similar in mass to the earth."

"There could be many planets around red dwarf stars," stated astronomer
Dimitar Sasselov of CfA. "It's all in the numbers, and single red dwarfs
clearly exist in great numbers."

"This discovery is particularly exciting because the habitable zone for
these stars -- the region where a planet would be the right temperature
for liquid water -- is close to the star. Planets that are close to their
stars are easier to find. The first truly Earth-like planet we discover
might be a world orbiting a red dwarf," added Sasselov.

This research has been submitted to The Astrophysical Journal Letters for
publication and is available online at
http://arxiv.org/abs/astro-ph/0601375

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

Note to editors: An image to accompany this release is online at
http://www.cfa.harvard.edu/press/pr0611image.html