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Space is getting crowded... with new planets
May 29, 2007
Artwork by Lynette Cook; UC Berkeley
May 29, 2007
Artwork by Lynette Cook; UC Berkeley
The world's largest and most prolific team of planet hunters has announced the discovery of 28 new planets outside our solar system, raising to 236 the total number of known exoplanets.
Among the 28 new worlds are at least four new multiple-planet systems, says astronomer Professor Chris Tinney, of the UNSW School of Physics, who heads the Australian part of the team.
The planets were found by closely observing their gravitational pull on their parent stars: "The more we look, the more we find planets," said Professor Tinney, whose team used the Anglo-Australia
Professor Tinney says the growing pace of discovery makes it increasingly likely that our own Milky Way galaxy is swarming with much smaller, rocky and potentially habitable worlds too small to detect with existing technology.
The discoveries come from the combined work of the California and Carnegie Planet Search team and the Anglo-Australia
The confirmed planets are among 37 new objects, all of them orbiting a star but smaller than a star, that the team has discovered within the past year. Seven of the 37 are confirmed brown dwarfs - failed stars that are nevertheless much more massive than the largest, Jupiter-sized planets - while two are borderline and could be either a large, gas giant planet or a small brown dwarf.
"We've gotten much more sophisticated in our analysis," Wright said, which has allowed the teams to detect smaller planets and planets farther from their parent star. In both cases, the planets produce much smaller wobbles in the parent star, which makes them harder to detect.
"We've added 12 percent to the total in the last year, and we're very proud of that. 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-Australia
In addition to reporting 37 new sub-stellar objects, Wright singled out an exoplanet discovered by their team two years ago as "extraordi
After their 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 recently 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," Professor Marcy said. "So this planet has the interior structure of a hybrid super-Earth/Nep
Its short 2.6-day orbit around Gliese 436 means it is very close to the star - only 3 percent of the Sun-Earth distance - making it like a hot Neptune, Wright said. It also is weird in having an eccentric orbit, not a circular orbit like most giant planets found orbiting close to their parent star. The eccentric orbit, in fact, suggests that the star may have another planetary companion in a more distant orbit.
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 their 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 team's Web site, http://exoplane
"Retired" stars are more stable
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, with 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, effects 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 more stable, making planet-caused wobbles detectable.
So far he 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 star, Johnson said.
"Only one of the 9 planets is within 1 AU (astronomical unit, or 93 million miles), and none of them are 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 star in 2 to 100 days. Even though such planets are easier to detect, no such planets have been detected orbiting retired A stars, whose typical planet has an orbital distance about equal to Earth's orbit or greater, with a 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 the 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'
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-Australia
A high-resolution JPEG image of artist Lynette Cook's concept of the Gliese 436 system can be downloaded here
UNSW Faculty of Science media liaison Bob Beale mobile phone 0411 705 435
