England - As they add another three to the list of 41 known
planets outside our solar system, a team of astronomers based
at the University of California, Berkeley, is beginning to
see patterns, including hints that many extrasolar planets
may have siblings.
only one Sun-like star has been found with multiple planets:
Upsilon Andromedae, around which the same team discovered
three planets last year.
Debra A. Fischer, a post-doctoral fellow at UC Berkeley, looked
more closely at data for 12 stars her team has been observing
long enough to betray the existence of long-period planets,
if any, and around which they had already discovered one planet.
She found that five - nearly 50 percent - exhibit unexplained
wobbles that could result from the tug of a companion, whether
another planet, an unseen star or something in between.
the first time anyone has noticed that such a high percentage
of stars with one known planet show evidence of a second companion,"
planets with a possible second planetary companion are large
gas giants very near their central star, so close that most
astronomers think such planets must have formed farther out
and later migrated inward.
to know if there are other companions out there," Fischer
said, "because anything else in the system will affect the
dynamics and theories of how the planets moved in and parked
in their current orbits."
plus data on the three new extrasolar planets, will be presented
Aug. 7 by team leaders Geoffrey Marcy, professor of astronomy
at UC Berkeley, and Paul Butler, an astronomer at the Carnegie
Institution of Washington, at the International Astronomical
Union meeting in Manchester, England. A paper describing the
results by Fischer, Marcy, Butler and their colleagues will
appear soon in the Astrophysical Journal.
new planets discovered around the stars HD 12661, HD 92788
and HD 38529 are large gas giants similar to the planet Jupiter.
All are in highly eccentric orbits that alternately bring
them close to the planet and carry them far away. This dance
alternately drags the star toward and away from Earth by a
distance about half the radius of the star, causing a Doppler
shift in the star's light that astronomers can detect.
the stars, HD 38529, has a planet that whips around in 14.3
days. This planet alone cannot explain the observed wobble
in the star, however, leading Fisher and her colleagues to
speculate that it has a second companion at a much greater
distance from the star.
indication of a second companion, but we can't be sure because
we only see a portion of the orbit," Fischer said. "The companion
could be a dim star, a brown dwarf or another planet."
are failed stars - large objects just shy of the mass necessary
to sustain hydrogen burning in their core.
on another star, 55 Cancri, around which the team found a
planet in 1996, indicates it may have a second companion too,
Fischer noted. Such a planet, if real, would be at least 3-4
Jupiter masses and have a period of 4,715 days - about 13
Earth years - in a highly eccentric orbit at an average distance
of 5.5 AU, similar to Jupiter's orbit around our Sun. One
astronomical unit, or AU, is 93 million miles, the distance
from the Earth to the Sun.
still speculative," she cautioned. Still, it led her to look
more closely at the dozen stars with known planets for which
her team had at least two years of data, enough to show evidence
of a second, more distant companion orbiting with a long period.
these planets turned out to have "residual velocities" that
could not be explained by a single planet, suggesting the
presence of a second companion. The team must collect data
for a complete orbit before concluding that another companion
noted that detecting a second companion requires an accurate
knowledge of the wobbles caused by the already known planet,
which is possible only when that planet is near the star and
has a short period. Thus, stars with more distant planets
might well have other companions that would not be detectable
using the Doppler method.
all three of the new stars are rich in the heaviest atoms,
like iron, meaning the stars were formed from dust that had
already been cycled through at least one other star. This
continues a trend of high metallicity among stars with known
Butler, Fischer and their colleagues are monitoring about
900 stars in the northern sky using the University of California's
Lick Observatory in California and the Keck Observatories
in Hawaii, plus another 200 in the southern sky with the Anglo-Australian
Telescope. They look for wobbles that indicate a planet may
be circling the distant sun. To date they have discovered
30 extrasolar planets, including the three reported today.
In all, there are now 44 known stars with planets or planetary
systems, excluding our own.
now at a stage where we are finding planets faster than we
can investigate them and write up the results," Marcy said.
"It's wonderful. Planet-hunting has morphed from the marvelous
to the mundane."
are feeding other projects, too, including a test of adaptive
optics at the Keck Telescopes. With adaptive optics, astronomers
could potentially see larger planets around nearby stars.
Such observations underway now could set upper limits on the
masses of the planets, whereas Doppler observations set only
a lower limit for the mass. The research was supported by
the National Aeronautics and Space Administration, the National
Science Foundation and Sun Microsystems.
information on extrasolar planets can be found at the team's
Web site, www.exoplanets.org/.