Astronomers said yesterday they have discovered what could be the first "rocky" Earth-like planet found outside the solar system. It is a lifeless, oven-like world about 71/2 times the size of Earth, orbiting a small star in the constellation Aquarius.
The team detected the new "extra-solar" planet by observing a tiny wobble induced in the star by the planet's gravity. The team made more than 150 observations over three years with precision measuring instruments before announcing its findings.
"This new technology has revealed the most terrestrial planet ever found," said team leader Geoffrey W. Marcy, an astronomer at the University of California at Berkeley. "For the first time, we are finding our planetary kin among the stars."
Paul Butler of the Carnegie Institution in Washington said he, Marcy and other team members began close observations of the star Gliese 876 several years ago. Gliese 876 is 15 light-years from Earth, an "M-dwarf" star about one-third the size of the sun. M-dwarves are the most common stars in the galaxy.
In 1998, Marcy and Butler reported detecting a "gas giant" planet about twice the size of Jupiter orbiting Gliese 876. Three years later, they reported a second gas giant about half the size of Jupiter. Astronomers do not "see" extra-solar planets; they deduce their presence by measuring the wobble.
The two large planets at Gliese 876 "dance in resonance," Butler said during a telephone news conference from the National Science Foundation. As they studied this interaction, "there was a clear signal of a third planet," he added. The foundation funded part of the research.
Over time, the team deduced that the third planet was less than half the size of any previously detected extra-solar planet, and thus was probably "rocky," or "terrestrial," to distinguish it from the Jupiter-like giants that have enough gravity to retain a massive gas envelope. Improvements in the instruments at Hawaii's W.M. Keck Observatory have enabled the team to detect smaller and smaller wobbles in stars -- signaling the presence of smaller planets.
"Over the years, we've announced the discoveries of 107 extra-solar planets, and we consider this the most exciting," Marcy said. So extraordinary was it, he added, that it became impossible to keep their work secret, so the team decided to announce the find. The research has been submitted for publication to the Astrophysical Journal.
While they had detected "the most Earth-like world ever discovered," team member Jack Lissauer said, "it's a very unearthly world." Lissauer, of NASA's Ames Research Center, said the planet is about 71/2 times the size of Earth and only 2 million miles from Gliese 876. It zoomed around the star in an orbit that lasted just 1.94 days.
"The surface of the planet is very warm -- between 400 and 700 degrees Fahrenheit," Marcy said, making it untenable for life. It has no solar system analogue, falling in size between Earth, the largest of the rocky planets, and Uranus, the smallest of the gas giants.
Still, the new research "demonstrates that current instruments and telescopes can spot almost Earth-like objects," said astronomer Kevin L. Luhman of the Harvard-Smithsonian Center for Astrophysics. "This is only the tip of the iceberg."
Since scientists began hunting for extra-solar planets about a decade ago, they have found more than 150. The overwhelming majority of them are Jupiter-like giants close to their host stars, enabling them to impart a significant wobble with relative frequency.
"But there's this huge range of sizes and distances that we haven't seen yet, and with the upgrades at Keck, this team is probably going to find a large number of them," Luhman said in a telephone interview. "They're going to have to go back and do the same stars all over again. But I bet they won't mind that."