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New York. Scientists working with NASA’s Kepler satellite have reported that they might have spotted a planet just 1.5 times the diameter of Earth around a Sun-like star 2,000 light-years away.

“We’re still in the process of confirming this candidate is a planet,” Matthew Holman, from the Harvard-Smithsonian Center for Astrophysics, said this week.

Holman is the lead author of an article describing the discovery that the journal Science published on its Web site.

This is the first announcement of a candidate Earth-size planet by the Kepler mission, which in March 2009 launched a one-ton spacecraft to search for planets like ours that just might harbor life.

The planet was among more than 700 candidate planets that the team announced in June. If it is made of similar stuff to Earth, its mass would be three to four times as much.

Astronomers are quickly closing in on Earth-size planets elsewhere in the galaxy as they find planetary systems that look more and more like our solar system.

“This represents a significant step toward Kepler’s goal of determining the frequency of Earth-size planets in the habitable zones of Sun-like stars,” Holman said.

Earth-size planets in orbits that are not too hot or too cold are considered the most likely places to look for life elsewhere in the universe.

The Kepler team also observed, more definitively, two giant, Saturn-size gas planets around the same star, known as Kepler-9.

On Tuesday, a European team reported what may be a smaller planet, with mass as little as 1.4 times that of Earth, around a star 127 light-years away.

In the 15 years since the first extrasolar planet orbiting a Sun-like star was discovered, astronomers have found close to 500 more. The first were huge gas planets — composed mostly of hydrogen, similar to Jupiter — that orbited extremely close to their stars.

But as detection methods improved, astronomers began to find planets closer in size to Earth and planetary systems that contain nearly as many planets as our solar system.

The Earth-size planet seen by the European astronomers appears to be one of seven circling the star, HD 10180, located in the constellation Hydrus.

Christophe Lovis, from the University of Geneva, who led the observations, said the group was certain about the existence of five of the planets, all about the mass of Neptune, but squeezed into orbits closer to the star than Mars is to the Sun.

They are less certain about the smallest planet. “For this one, we have about 1 percent false alarm possibility,” Lovis said. “For us, 99 percent is just not enough to be completely sure.”

The team also tentatively detected a larger, Saturn-size planet farther from the star. Neither of the slightly-bigger-than-Earth planets is Earth-like or has much chance of life.

Both have orbits very close to their stars that would sear the surfaces.

The small Kepler-9 planet completes an orbit in just over a day-and-a-half at a distance of 2.5 million miles from the star.

 The small HD 10180 planet is even closer and faster, less than two million miles from the star and completing an orbit in 28 hours.

Earth, by contrast, is 93 million miles from the Sun and its orbit takes 365 days.



The New York Times