December 6, 1999
Astronomers discover six new planets orbiting nearby stars
By Tim Stephens
The world's most prolific team of planet hunters has found six new planets orbiting
nearby stars, bringing the total number of planets astronomers have detected outside
the solar system to 28. The researchers also found evidence suggesting that two previously
discovered planets have additional companions, said Steven Vogt, UCSC professor of
astronomy and astrophysics.
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| Astronomer Steven Vogt and his collaborators reported evidence suggesting that
each of these planets depicted in the paintings has an additional companion orbiting
the same star. These planets were previously detected by Vogt and his coworkers,
who also reported the discovery of six new planets. Art: Lynette Cook For more space art by Lynette Cook visit www.spaceart.org/lcook) |
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 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 orbits of these companions, but the evidence suggests there are a fair number of multiple planet systems out there," Vogt said.
Further information about the planet search is available on the Nasa Web site.
