U.S. astronomers say they have discovered what might be the most Earth-like, rocky planet ever detected outside our solar system. But it appears to be too hot to be habitable.
Since 1995, planet hunters have found 155 celestial bodies in orbit around other stars in our Milky Way Galaxy. They have been getting much better at finding ever-smaller ones as telescope technology and search methods have improved. The first to be found were gas giants like Jupiter and Saturn. Last year, smaller, Neptune size planets first appeared to astronomers.
Now, the most productive of all the searchers, Geoffrey Marcy of the University of California at Berkeley and Paul Butler of the Carnegie Institution of Washington, report finding their 107th planet and the most similar yet in mass and possibly rocky composition to Earth.
"This is truly an extraordinary announcement," Mr. Marcy says.
Geoff Marcy speaks at a Washington news briefing.
"We knew about it three years ago. We've been following it quietly, carefully guarding the secret, while we double and triple checked," Mr. Marcy says.
The newly discovered planet is relatively near us, about 15 light years from Earth. It is not precisely like our home planet, but a bigger cousin, estimated to be six to eight times Earth's mass, twice the diameter, and so close to its star that it takes less than two days to orbit, making its temperatures oven-like. The star itself is only one-third the mass of our Sun.
But Mr. Marcy says it all bears a resemblance to our solar system, especially since his team had already discovered two gas giants there.
"The whole planetary system is sort of a miniature of our solar system," Mr. Marcy says. "The star is small, the orbits are small, and in close is the smallest of them, just as the architecture is in our own solar system, with the smallest planets orbiting inward of the giants. For the first time, we are beginning to find our planetary kin among the stars."
Mr.. Marcy and his colleagues have no proof the planet is rocky like Earth, but believe its low mass prevents it from retaining gas like Jupiter does.
The astronomers found it the same way most of the other planets outside our solar system were detected. They did not see it directly, but measured the varying intensity of the starlight, which suggests the star is wobbling because a planet's gravity is pulling on it. The new planet caused a much smaller flicker in its star than all the previous planets caused in their stars, leading the astronomers to conclude it is a smaller planet.
U.S. space agency astronomy theorist Jack Lissauer suggests astronomers will find even smaller planets as telescope technology improves to detect less conspicuous star wobbles. He points out that a U.S. planet-searching satellite to be launched in three years will use another technique that measures momentary decreases in a star's brightness, suggesting a planet is passing by.
"The planet that we've announced today is probably the most Earth-like world to have been discovered since the dawn of history, but it's not likely to hold that title for very long," Mr. Lissauer says.
Any smaller planets could well come from the catalog of 15 hundred stars astronomers Marcy and Butler are currently observing.