NASA announced its Kepler spacecraft, launched in March 2009, has discovered the first confirmed planetary system with more than one planet crossing in front of, or transiting, the same star. The transit signatures of two distinct planets, named Kepler-9b and 9c, were seen in the data for the sun-like star designated Kepler-9, the space agency confirmed.
Scientists refined the estimates of the masses of the planets using observations from the W.M. Keck Observatory in Hawaii. The observations show 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 about 19 days, while Kepler-9c has an orbit of about 38 days. By observing several transits by each planet over the seven months of data, the time between successive transits could be analyzed, NASA documents explained.
"This discovery is the first clear detection of significant changes in the intervals from one planetary transit to the next, what we call transit timing variations," said Matthew Holman, a Kepler mission scientist from the Harvard-Smithsonian Center for Astrophysics. "This is evidence of the gravitational interaction between the two planets as seen by the Kepler spacecraft."
The distance of the planet from the star can be calculated by measuring the time between successive dips as the planet orbits the star. Small variations in the regularity of these dips can be used to determine the masses of planets and detect other non-transiting planets in the system. NASA said Kepler's "ultra-precise" camera measured tiny decreases in the stars' brightness that occur when a planet transits them: The size of the planet can be derived from these temporary dips.
"Kepler's high quality data and round-the-clock coverage of transiting objects enable a whole host of unique measurements to be made of the parent stars and their planetary systems," said Doug Hudgins, the Kepler program scientist at NASA's headquarters in Washington.
In June, mission scientists submitted findings for peer review that identified more than 700 planet candidates in the first 43 days of Kepler data. The data included five additional candidate systems that appear to exhibit more than one transiting planet. The space agency noted the Kepler team recently identified a sixth target exhibiting multiple transits and accumulated enough follow-up data to confirm this multiplanet system.
In addition to the two confirmed giant planets, Kepler scientists also have identified what appears to be a third, much smaller transit signature in the observations of Kepler-9, according to NASA. "That signature is consistent with the transits of a super-Earth-size planet about 1.5 times the radius of Earth in a scorching, near-sun 1.6 day-orbit," an agency press release explained. "Additional observations are required to determine whether this signal is indeed a planet or an astronomical phenomenon that mimics the appearance of a transit."