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28 New Exoplanets & 4 Multi-Planet Systems and Gliese 436 w/2 grams/cc Indicates Water
RELEASE - 28 New Exoplanets and Four Multi-Planet Systems
Gliese 436: A Density of 2 grams/cc, Indicating Water
"Bounty of 28 new planets, 7 new brown dwarfs reported.
By California, Carnegie Team
Honolulu, Hawaii - The world's largest and most prolific team of
planet hunters announced today (Monday, May 28) the discovery of 28
new planets outside our solar system, increasing to 236 the total
number of known exoplanets.
University of California, Berkeley, post-doctoral fellow Jason
T. Wright and newly minted Ph.D. John Asher Johnson reported the new
exoplanets at a noon media briefing at the semi-annual meeting of the
American Astronomical Society (AAS) in Honolulu. The findings, also
reported today 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.
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.
"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."
Keep searching, Jason H.
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