image: This artist's concept illustrates the two Saturn-sized planets discovered by NASA's Kepler mission. The star system is oriented edge-on, as seen by Kepler, such that both planets cross in front of, or transit, their star. This is the first star system found to have multiple transiting planets. view more
Credit: NASA/Ames/JPL-Caltech
NASA's Kepler Mission has discovered the first confirmed planetary system with more than one planet transiting the same star.
Today's announcement of the discovery of the two planets, Kepler 9b and 9c, is based on seven months of observations of more than 156,000 stars being monitored for subtle brightness changes as part of an ongoing search for Earth-like planets outside our solar system. Scientists designated the sun-like star Kepler-9.
The inner world, Kepler 9-b, orbits its star every 19.2 days at a distance of 13 million miles, while the outer world orbits once in 38.9 days at a distance of 21 million miles. (In comparison, Mercury has an orbital period of 88 days.) They orbit nearly in resonance, with the inner planet completing two orbits for every one of the outer planet. Both are Saturn-sized gas giants, with the inner world weighing in at 0.25 Jupiter mass (80 Earths) while the outer world is a slimmer 0.17 Jupiter mass (54 Earths).
"This is the first confirmed system of more than one planet transiting the same star," said Matthew Holman, a Kepler Mission scientist from the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. Scientists confirmed the multiple transits with radial velocity observations conducted at the W.M Keck Observatory in Hawaii.
Holman said systems with multiple transiting planets are particularly rich with information that provides clues as to the physical characteristics. Specifically, astronomers can measure how the time between successive transits changes from orbit to orbit due to mutual gravity between the two worlds. "We can actually see evidence of the gravitational interaction of these two planets through the deviations in their transit times," Holman said.
"This discovery is the first clear detection of transit timing variations," he added.
In addition to the two confirmed giant planets, Holman said Kepler scientists also identified an additional "super-Earth-size" transiting planet candidate that will need additional observations to confirm if it is indeed a planet or merely a false alarm. Current observations suggest that the planetary candidate might be about 1.5 times the size of Earth and orbits the star once every 1.6 days at a distance of only 2.5 million miles.
Kepler, a space-based observatory, looks for the signatures of planets by measuring tiny decreases in the brightness of stars when planets cross in front of, or transit them. The size of the planet can be derived from the change in the star's brightness. In June, mission scientists announced the mission has identified more than 700 planet candidates, including five systems with more than one planet candidate. This is the first of those systems to be confirmed.
The 28-member Kepler science team is using ground-based telescopes and the Hubble Space Telescope and Spitzer Space Telescope to perform follow-up observations on 400 of the planet candidates. The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in spring through early fall. The data from these observations will determine which of the candidates can be identified as planets.
Without the additional information, candidates that are actual planets cannot be distinguished from false alarms, such as binary stars -- two stars that orbit each other. The size of the planetary candidates also can be only approximated until the size of the stars they orbit is determined from additional spectroscopic observations made by ground-based telescopes. In the case of Kepler-9, the planetary nature was first confirmed by the scale of the transit timing variations and was further verified by radial velocity measurements.
Kepler will continue conducting science operations until at least November 2012, searching for planets as small as Earth, including those that orbit stars in a warm habitable zone where liquid water could exist on the surface of the planet. Since transits of planets in the habitable zone of solar-like stars occur about once a year and require three transits for verification, it is expected to take at least three years to locate and verify an Earth-size planet.
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