California, Carnegie Team Reports 28 New Exoplanets, 7 New Brown Dwarfs
- From: baalke@xxxxxxxxxxxxx
- Date: 29 May 2007 15:23:51 -0700
http://www.berkeley.edu/news/media/releases/2007/05/29_exoplanets.shtml
UC Berkeley Press Release
California, Carnegie team reports 28 new exoplanets, 7 new brown
dwarfs
By Robert Sanders
29 May 2007
BERKELEY - The world's largest and most prolific team of planet
hunters
announced Monday, May 28, the discovery of 28 new planets outside our
solar system, increasing to 236 the total number of known exoplanets.
exoplanet orbiting dwarf star
University of California, Berkeley, post-doctoral fellow Jason T.
Wright
and newly minted Ph.D. John Asher Johnson reported the new exoplanets
at
a media briefing at the semi-annual meeting of the American
Astronomical
Society (AAS) in Honolulu. The findings, also reported in poster
sessions at the meeting, are a result of the combined work of the
California and Carnegie Planet Search team and the Anglo-Australian
Planet Search team.
The planets are among 37 new objects - each orbiting a star, but
smaller
than a star -discovered by the teams within the past year. Seven of
the
37 are confirmed brown dwarfs, which are failed stars that
nevertheless
are much more massive than the largest, Jupiter-sized planets. Two
others are borderline and could be either large, gas giant planets or
small brown dwarfs.
Wright said the research teams have become much more sophisticated in
their analyses of the stellar wobbles caused by orbiting planets,
enabling them to detect the weaker wobbles caused by smaller planets
as
well as planets farther from their parent stars.
"We've added 12 percent to the total in the last year, and we're very
proud of that," said Wright of the 28 new exoplanets. "This provides
new
planetary systems so that we can study their properties as an
ensemble."
The California and Carnegie Planet Search team is headed by Geoffrey
Marcy, professor of astronomy at UC Berkeley; Paul Butler of the
Carnegie Institution of Washington; Debra Fischer of San Francisco
State
University; and Steve Vogt, professor of astronomy at UC Santa Cruz.
The
Anglo-Australian Planet Search team is headed by Chris Tinney of the
University of New South Wales and Hugh Jones of the University of
Hertfordshire. They and colleagues Shannon Patel of UC Santa Cruz and
Simon O'Toole of the Anglo-Australian Observatory have published their
exoplanet results in papers over the past year, but the AAS meeting is
the first time the teams have presented the past year's findings in
their entirety. exoplanet in transit around star
The ice giant planet GJ436b is like a hot-Neptune that orbits Gliese
436
every 2.6 days. Because we view its orbit edge-on, the planet often
transits the star, as in this artist's rendering.
In addition to reporting 37 new substellar objects, Wright singled out
an exoplanet discovered by their teams two years ago as
"extraordinarily
rich." Circling the star Gliese 436 (GJ 436), a red M dwarf only 30
light years from Earth, was an ice-giant planet the teams calculated
to
be at least 22 Earth masses, slightly larger than the mass of Neptune
(17 Earth masses). After the discovery in 2004 and publication of the
exoplanet's orbit earlier this year, a Belgian astronomer, Michael
Gillon at Liege University, observed the planet crossing in front of
the
star - the first Neptune-sized planet observed to transit a star.
Gillon
and colleagues reported two weeks ago how this transiting planet
allowed
them to precisely pin down the mass, 22.4 Earth masses, and to
calculate
the planet's radius and density, which turns out to be similar to
Neptune's.
"From the density of two grams per cubic centimeter - twice that of
water - it must be 50 percent rock and about 50 percent water, with
perhaps small amounts of hydrogen and helium," Marcy said. "So this
planet has the interior structure of a hybrid super-Earth/Neptune,
with
a rocky core surrounded by a significant amount of water compressed
into
solid form at high pressures and temperatures."
Its short, 2.6-day orbit around Gliese 436 means the exoplanet is very
close to the star - only 3 percent of the sun-Earth distance - making
it
a hot Neptune, Wright said. It also has an eccentric orbit, not a
circular orbit like most giant planets found orbiting close to their
parent stars. This orbit, in fact, suggests that the star may have
another planetary companion in a more distant orbit.
drawing of exoplanet interior
Drawing of the predicted interior of an ice-giant planet like GJ436b
or
Neptune. (Credit: Jason Wright/UC Berkeley)
"I'm sure people will immediately follow up and try to measure the
atmospheric composition of this planet." Wright predicted.
Also among the 28 new exoplanets are at least four new multiple-planet
systems, plus three stars that probably contain a brown dwarf as well
as
a planet. Wright said that at least 30 percent of all stars known to
have planets have more than one. Because smaller planets and outer
planets of a star are harder to detect, he predicts that the
percentage
will continue to rise as detection methods improve.
"We're just now getting to the point where, if we were observing our
own
solar system from afar, we would be seeing Jupiter," he said, pointing
out that the teams' Doppler technique is now sensitive to stellar
wobbles of a meter per second, much less than the 10-meter per second
limit they started out with 15 years ago.
Wright keeps track of all known exoplanets for the California and
Carnegie Planet Search team's Web site, http://exoplanets.org, which
hosts the only peer-reviewed catalog of exoplanets within 200 parsecs
(652 light years) of Earth. This includes "everything that is close
enough to study and possibly follow up with imaging," he said.
Three of the newly reported planets are around large stars between 1.6
and 1.9 times the mass of our sun. Johnson has focused on exoplanets
around massive stars, known as A and F stars, which have masses
between
1.5 and 2.5 solar masses. Planets around these massive stars are
normally very hard to detect because they typically rotate fast and
have
pulsating atmospheres, traits that can hide or mimic the signal from
an
orbiting planet. He discovered, however, that cooler "retired" A stars
-
"subgiant" stars that have nearly completed hydrogen burning and have
stabilized for a short period of time - are quiet enough to make
planet-caused wobbles detectable.
So far, Johnson has tracked down six previously discovered exoplanets
around retired A stars, and by combining this set with the three newly
discovered exoplanets, has been able to draw preliminary conclusions.
For one, planets around more massive stars seem to be farther from
their
host stars, Johnson said.
"Only one of the 9 planets is within 1 AU (astronomical unit, or 93
million miles), and none of them is within 0.8 AU, of their host
stars,
which is very different than the distribution around sun-like stars,"
he
said, noting that many sun-like stars harbor hot gas giants that whip
around their host stars in two to 100 days. Even though short-period
planets are easier to detect, no such planets have been detected
orbiting retired A stars, whose typical planets have an orbital
distance
about equal to Earth's orbit or greater, with an orbital period of a
few
years.
Based on the results of his search for planets around retired A stars,
Johnson has discovered that massive stars are more likely to harbor
Jupiter-sized planets than are lower-mass stars. The chance of having
a
Jupiter-like, giant planet orbiting within 2 AU is 8.7 percent for
stars
between 1.3 and 2 solar masses, versus 4 percent for sun-like stars
with
masses ranging from 0.7 solar masses to 1.3 solar masses, and 1.2
percent for M stars with less than 0.7 solar masses. As would be
expected from the core accretion model of planet formation, large
planets are more often observed around massive stars, probably because
these stars start out with more material in their disks during the
early
formation period.
Johnson will continue to focus on retired A stars, 450 of which have
been added to the teams' target list. As more planets are discovered
around subgiants, it should become clearer whether larger orbits are
"a
result of different formation and migration mechanisms in the disks of
A-type stars, or simply a consequence of the small number of massive
subgiants currently surveyed," he and colleagues wrote in a paper
submitted in April to the Astrophysical Journal.
The California and Carnegie Planet Search team uses telescopes at the
University of California's Lick Observatory and the W. M. Keck
Observatory in Hawaii. The Anglo-Australian Planet Search team uses
the
Anglo-Australian Observatory. Together, these teams have discovered
more
than half of all known exoplanets.
The work is funded by the National Aeronautics and Space
Administration,
the National Science Foundation, the W. M. Keck Observatory, the
Carnegie Institution of Washington, the Anglo-Australian Observatory
and
the UC Observatories.
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