San Francisco State University-based planet search has found nine of 12 extrasolar planets detected since 1995
SAN FRANCISCO, CA -- September 24, 1998 -- Deploying the massive Keck telescope in Hawaii in a new planet search, a team of astronomers has detected two planets orbiting Sun-like stars, bringing to 12 the number of distant worlds discovered beyond our solar system.
One of the new discoveries, a Jupiter-sized sphere that most likely appears deep blue-violet, barely skims the outer reaches of its yellow star, passing 25 times closer to the star than the Earth’s orbit of the Sun, and nine time closer than Mercury’s path around the Sun. Its close orbit allows it to circle its star about every three days. In contrast, the other new planet has a more Earth-like orbit. Its average distance from its star is nearly the same as the Earth-Sun distance, the first planet discovered with such a familiar distance. A year on this planet is 437 days.
"We had discovered planets that orbit much closer and much farther from their stars than the Earth-Sun distance," said Geoffrey Marcy, University Distinguished Professor of Science at San Francisco State University who, along with Paul Butler of the Anglo-Australian Observatory, has discovered nine of the dozen planets so far detected.
"We wondered if nature rarely puts planets at one Earth-Sun distance," Marcy continued. "Now we know that such planets are not rare." Marcy also holds a post as adjunct professor of astronomy at UC Berkeley.
A report on the planet with the small orbit (around star HD187123) has been accepted by Publications of the Astronomical Society of the Pacific. The paper on the planet with the more Earth-like orbit (around star HD210277) will be submitted to the Astrophysical Journal Letters. Co-authors with Marcy and Butler on both papers, and colleagues in the SF State-based discovery team, are Steve Vogt, professor of astronomy at the University of California, Santa Cruz, who developed the spectrometer needed for planet detection; Debra Fischer, post-doctoral researcher with Marcy at SF State; and Kevin Apps, an undergraduate at the University of Sussex.
Apps, a sophomore in physics and astrophysics at Sussex and an amateur astronomer since the age of seven, is intensely interested in the likelihood of planets around Sun-like stars. In 1997, he e-mailed Marcy and Butler, asking if he could see their list of candidate stars in the new planet search they were launching at the Keck Observatory in Mauna Kea, the world’s largest telescope for optical and infrared astronomy. Upon analyzing the stars’ temperature, luminosity, composition and other features usi ng new satellite data available on the Internet, Apps discovered that 30 of the stars were actually not good candidates, and he offered to supply a list of 30 "solar ringers," as he calls them, in their place.
To his surprise, Marcy and Butler agreed to substitute 30 candidate stars with Apps’ 30 solar ringers. One of the newly discovered planets orbits a star—HD187123—from Apps’ list.
"I don’t think I can put into words how I feel about Geoff and Paul finding a planet around one of my suggested targets," says Apps.
Marcy is quite impressed with the young amateur. "He used the latest satellite data, sifted out the stars that would have the best likelihood of harboring planets. He shows a fierce interest in this research. It’s great to have him as a colleague."
The two discoveries are among 430 candidates in the new planet search using the Keck Observatory. The observations were made over 12 nights during the last nine months, under the auspices of NASA and the University of California.
Marcy expects to discover "something like" two dozen Jupiter-sized planets orbiting stars within one Earth-Sun distance. "That should happen within the next three years if the law of averages applies," he says.
But a second goal of the planet search is to discover Jupiters much farther out from their stars—"like five Earth-Sun distances: the signposts of solar systems like ours," Marcy adds. "Make no mistake about it," he says. "What we’re all about is discovering (planets) where evolution might have gotten a toehold. Jupiter-sized planets at a greater distance from their star would suggest a solar system that could host a rocky Earth-like planet. And if it should turn out that out of more than 400 stars, none has a Jupiter orbiting at five Earth-Sun distances, that would be a frightening reality. It might be the first sign that Earth is truly unusual and so life may be rare."
The planets were discovered by detecting a characteristic wobble in the motion of the star, a wobble caused by the gravitational effect of the planet orbiting the star. The detection was made using the HIRES spectrometer built by Steven Vogt and deployed on the Keck telescope.
HD 187123, the star with the close orbiting planet, is a near twin of the Sun. This star is 154 light years (48 parsecs) away in the constellation Cygnus (the Swan). The star has the same size, mass, temperature and luminosity as the Sun, but is richer in heavy elements such as iron. Almost all planets found to date orbit stars that are at least as rich in heavy elements as the Sun. This trend toward heavy elements, Marcy says, may be a clue about how planets form.
The planet is closer to its host star than any planet found before. It orbits only 0.042 AU away from its sun. (An AU is the Earth-Sun distance). Indeed, it orbits only four stellar diameters from the surface of the star HD187123. Its period is 3.097 days.
HD210277, the star with the planet that has the Earth-like orbit,is slightly bigger than the Sun. It is 68 (21 parsecs) away, in the direction of the constellation Aquarius. The planet has an average orbital distance barely greater (15 percent) than that of the Earth’s. Until now, the extrasolar planet that had been closest to an orbit the size of Earth’s was 16 Cygni B, with an average distance 70 percent greater than Earth’s. The planet is about the size of Jupiter.
The technical paper and graphs on the discovery of HD187123 can be viewed at: http://cannon.sfsu.edu/~gmarcy/planetsearch/papers.html
Further information on the planet search is available at: http://www.physics.sfsu.edu/~gmarcy/planetsearch/planetsearch.html
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