NASA's Kepler telescope has for the first time observed a planetary system with two planets passing in front of their star.
The planets, now named named Kepler-9b and 9c, were seen transiting the sun-like star called Kepler-9. The announcement comes just days after ESO released data on a planetary system believed to have five orbiting planets, discovered with the HARP ground-based telescope.
Kepler can measure the tiny drop in brightness that occurs when a planet transits a star; the size of the planet can be worked out from the scale of the dips.
The time between dips gives the distance of the planet from the star, and variations in their regularity can be used to work out the planets' masses and detect other non-transiting planets in the system.
Kepler found more than 700 possible planets in its first 43 days, and has discovered five more systems that appear to have more than one transiting planet.
"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, NASA Kepler program scientist.
Scientists refined the estimates of the masses of the planets using observations from the WM Keck Observatory in Hawaii. They believe Kepler-9b is the larger of the two, with both having masses a bit 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.
"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."
As well as the two confirmed giant planets, the scientists believe they've found a third, much smaller transit signature. It would correspond to a planet about 1.5 times the radius of Earth in a near-sun 1.6 day-orbit.
The team plans further observations to determine whether this really is a planet or some other astronomical phenomenon.