The researchers also found evidence suggesting that two previously discovered planets have additional companions, said Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz.

Vogt and his colleagues, Geoffrey Marcy of UC Berkeley, Paul Butler of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington in Washington, D.C., and Kevin Apps of the University of Sussex, England, made the discoveries using the High Resolution Echelle Spectrograph (HIRES, designed and built by Vogt) on the Keck I Telescope in Hawaii. Their findings will be published in the Astrophysical Journal.

The researchers have been using the facilities at the W. M. Keck Observatory for the past three years to conduct a survey of 500 nearby sunlike stars in search of planets. The project is supported by the NASA Origins Program, which has provided both funding and telescope time, and by the National Science Foundation.

The six new planets increase by about 25 percent the number of known "extrasolar" planets, giving astronomers a substantial amount of additional information about planetary systems, Vogt said. One of the planets, HD 192263, was also recently detected by Nuno Santos and collaborators in Geneva, Switzerland, who reported it while Vogt and his colleagues were preparing their paper.

The new planets orbit stars that are similar in size, age, and brightness to the Sun and are at distances ranging from 65 to 192 light-years from Earth. The planets themselves range in mass from slightly smaller to several times larger than the planet Jupiter (0.8 to 6.5 times the mass of Jupiter). They are probably also similar to Jupiter in their compositions--basically giant balls of hydrogen and helium gas, Vogt said.

The presence of a planet around a star is indicated by a telltale wobble in the motion of the star as a result of the gravitational force exerted by the orbiting planet. Vogt and his coworkers recently achieved independent confirmation of this method for detecting planets when they were able to predict and measure the dimming of a star as a planet passed in front of it.

The orbits of the new planets, like those of most of the extrasolar planets discovered so far, tend to be quite eccentric, tracing paths that are oval rather than circular. One of the planets, around a star called HD 222582, has the most wildly eccentric orbit yet known, carrying it from as close as 0.39 astronomical units (AU: the distance from Earth to the Sun) to as far as 2.31 AU from its parent star in the course of its 576-day orbit.

"It is beginning to look like neatly stacked, circular orbits such as we see in our own solar system are relatively rare," Vogt said.

Interestingly, five of the six planets are located within the so-called "habitable zones" of their stars. This is the region where temperatures would allow water to exist in liquid form. Most of the extrasolar planets the researchers have studied have turned out to be outside the habitable zone, either too close to their star or too far away, and therefore too hot or too cold, Vogt said.

"These planets are at just the right distance, with temperatures in one case around 108 degrees Fahrenheit--like a hot day in Sacramento," he said.

Planetary systems with Jupiter-sized planets in oval-shaped orbits are not expected to harbor Earthlike planets, Vogt added. In fact, if an Earthlike planet were put into such a system, it would be quickly ejected by the gravitational influence of the Jupiter-mass planet. Vogt noted, however, that if these Jupiter-sized planets are similar to those in our own solar system, they probably have numerous moons associated with them.

"For a planet in the habitable zone of its star, such moons offer the possibility of liquid water and the eventual emergence of life," he said.

In addition to the discovery of six new planets, the researchers gathered new data on four previously known planets. Two of them, around the stars HD 217107 and HD 187123, showed long-term trends in their orbits indicating the presence of an additional companion. These companions, which may be planets or larger objects (e.g., brown dwarfs), appear to be orbiting their host stars in a long period, taking at least two to three years to complete an orbit, Vogt said. These findings are significant because previously only one other system of multiple planets, around the star Upsilon Andromedae, had been identified.

"It will take years of additional observations to work out the masses and orbits of these companions, but the evidence suggests there are a fair number of multiple planet systems out there," Vogt said.

Specific details about the new planets and their host stars are given below:

HD 10697 is a G5IV star, slightly cooler and a bit larger than the Sun. It lies 106 light-years away in the constellation Pisces. Its planet has a minimum mass of 6.35 Jupiter masses and a 1,072-day orbit. The radius of this orbit is about 2.13 AU, but the orbit is somewhat eccentric, so the planet's distance from its star ranges from 1.87 AU to 2.39 AU. At its average orbital distance, it lies just at the outside edge of the habitable zone of its star, and is expected to have an equilibrium temperature (due to energy received from its parent star) of about 15 degrees F.

HD 37124 is a G4V star, slightly cooler than the Sun. It lies 108 light-years away in the constellation Taurus. Its planet has a minimum mass of 1.04 Jupiter masses and a 155.7-day orbit. This orbit is also quite eccentric. At its average orbital distance of 0.55 AU, it sits just within the inner edge of the habitable zone of its star, and is expected to have an equilibrium temperature of about 130 degrees F. This is the lowest metallicity star known to have a planet.

HD 134987 is a G5V star, 83 light-years away in the constellation Libra. Its planet orbits in a 260-day eccentric orbit. This planet has a minimum mass of 1.58 Jupiter masses. At its average orbital distance of 0.81 AU, its expected equilibrium temperature is a balmy 108 degrees F. It lies well within the habitable zone of its star.

HD 177830 is a K2IV star, about 1,000 degrees Kelvin cooler than the Sun, lying about 192 light-years away in the constellation Vulpecula. It harbors a 1.22 Jupiter mass planet in a 392-day, highly eccentric orbit. This orbit carries the planet from as close as 0.63 AU from its star to as far as 1.57 AU. At its mean orbital distance of 1.10 AU its expected temperature is about 192 degrees F. The planet is probably within the habitable zone of its star.

HD 192263 is a K2V star lying 65 light-years away in the constellation Aquila. A planet around this star was first reported by Nuno Santos, a Portuguese graduate student at the University of Geneva. Vogt's team has obtained essentially the same results as Santos: a 0.78 Jupiter mass planet orbiting in a 24.36-day orbit. This orbit has a radius of only 0.15 AU, with little or no eccentricity. It orbits well outside the habitable zone of its star.

HD 222582, a G3V star, is a near solar twin, 137 light-years away in the constellation Aquarius. Its planet orbits in a widly eccentric 576-day orbit, which carries the planet from 0.39 AU to 2.31 AU from the parent star in the course of its oval orbit. This is the most eccentric extrasolar planet orbit yet known. The planet's expected temperature is about -38 degrees F. Its mean orbital distance places it squarely in the habitable zone of its star.

National Science

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EXO WORLDS

Astronomer Claims to See Shadow of Exo Planet Across Star
Berkeley - November 13, 1999 -
Astronomers have witnessed for the first time a distant planet passing in front of its star, providing direct and independent confirmation of the existence of extrasolar planets that to date have been inferred only from the wobble of their star.