For the first time, two giant planets have been detected crossing in front of the same star in data collected by NASA's Kepler spacecraft, according to findings published Thursday in the journal Science. The discovery is based on seven months of observations of more than 156,000 stars as part of a search for Earth-size planets outside of our solar system.

The transit signatures of two planets -- marked by tiny decreases in the star's brightness when a planet crosses in front -- were noted in the data for the star known as Kepler-9, more than 2,000 light-years away. The planets were named Kepler-9b and 9c. The timing of the drops in brightness, caught by Kepler's ultra-precise camera, help determine the distance of the planet from the star. Other data from these measurements indicate the mass of the planets.

A third, much smaller transit signature was also reported in the observations of Kepler-9. Scientists said it could be made by a planet about 11/2 times the size of Earth that moves in a 1.6 day orbit closely around the star. NASA reported that additional observations will be needed before identifying it as a planet.

Using more observations from the W.M. Keck Observatory in Hawaii, the scientists determined that Kepler-9b is the larger of the two planets, and both have masses similar to but less than Saturn. Kepler-9b lies closest to the star with an orbit of 19.2 days, while Kepler-9c has an orbit of 38.9 days.

The report comes out of a peer review of more than 700 planet candidates identified from the first 43 days of Kepler data. Kepler, now 18 million miles away, is scheduled to continue its mission for another 3 1/2 years.

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