14 6 Light Years From Earth

Found: Earth's Distant Cousin (About 15 Light-Years Away)

Published: June 14, 2005

In a discovery that they described as a milestone in the quest to find out if humans are alone in the universe, astronomers announced yesterday that they had found the smallest planet yet outside the solar system.

With a mass only seven times that of the Earth, the new planet is probably a ball of rock, its discoverers said. Orbiting closely to a dim red star in Aquarius known as Gliese 876, it is the third and innermost member of a shrunken version of our own solar system.

"This is by far the most Earth-like planet ever found," said Dr. Geoffrey Marcy of the University of California, Berkeley, a member of the team that made the discovery using one of the giant 10-meter diameter Keck telescopes in Hawaii.

Dr. David Spergel of Princeton University, who is involved in an effort to build a planet hunting satellite for NASA and who was not a member of the team, said, "This is a really cool result."

The detection, he said, suggests that Earth-like planets are common in the universe. "There are likely to be a lot of planetary systems around nearby stars," he wrote in an e-mail message.

The team announced its results in a news conference at the National Science Foundation in Washington. Besides Dr. Marcy, its members are Dr. Paul Butler of the Carnegie Institution of Washington, Dr. Jack Lissauer of NASA's Ames Research Center and Dr. Eugenio Rivera of the University of California, Santa Cruz. They and a slightly larger group have submitted a paper to The Astrophysical Journal.

The announcement continues a remarkable run of alien-planet news that has run almost nonstop since the first one was detected 10 years ago. Most of the 150 or so planets that have been discovered since are gas giants like Jupiter. But as techniques have improved and more data have accumulated, astronomers have been able to find smaller fry. Last year they announced the discovery of a few planets with masses similar to Neptune's, presumably balls of gas and ice as well as some rock.

"This is different; now we're getting down to the nitty-gritty," Dr. Marcy said, describing the planet as a ball of rock, made of the same materials as Earth, and perhaps with an atmosphere.

In the news conference Dr. Lissauer said the team did not know where the planet had come from: whether it was a big rock that had aggregated itself in its present orbit, or perhaps a gas giant that had formed farther out, migrated inward and then had been ripped apart by tidal forces.

The new planet, said Dr. Marcy in an interview, is about as small as astronomers are going to be able to detect with the present generation of instruments. Planned for the next decade are a series of spaceborne telescopes built to discover and study planets as small as Earth.

This is the 107th score for Dr. Marcy and his longtime partner, Dr. Butler. Like the others, the new planet was detected by observing a wobble in its home star caused by the gravity of the planet as it goes around.

The team had long been interested in the Gliese 876 system, about 15 light-years from here. Its two giant outer planets orbit in an unusual lockstep configuration, taking 30 and 61 days to go around, so that they line up every two months.

The new planet, according to the data, circles Gliese every 1.9 days, putting it way inside the other two planets, slightly less than two million miles from the star. By comparison, Earth is 93 million miles from the Sun. This planet is about 70 percent bigger than Earth, Dr. Marcy estimated, and is made of silicates, iron and nickel, like the terrestrial planets in our own system.

At such a close distance, the planet could be tidally locked, keeping the same face toward its star at all times. Dr. Marcy said the temperature on the side of the planet facing the star would be 400 to 700 degrees, like "chicken roasting temperatures."

That is too warm for liquid water or ice, but it could be cold on the backside of planet, which, as Dr. Marcy said, "faces the darkness of the universe."

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