Launched last March, Kepler was designed to look for extrasolar planets, aka exoplanets, via transits—the periodic dimming of light from stars due to planets passing in front them, as seen from the telescope's vantage point. (Read about Kepler's first planet discoveries.)
After analyzing seven months' worth of data from Kepler, a team led by Matt Holman of the Harvard-Smithsonian Center for Astrophysics found two transiting exoplanets orbiting the star Kepler-9, which lies about 2,300 light-years from Earth.
One of the planets, dubbed Kepler-9b, takes just over 19 days to orbit its star. The other, Kepler-9c, takes almost 39 days to complete an orbit. (Find out how exoplanets get their names.)
The researchers noticed that both planets' orbital periods speed up and slow down at regular intervals. This means the two planets are locked in gravitational "resonance"—so that each planet's gravity is affecting the other's orbit.
Using that data, the scientists were able to calculate the masses of the planets, and they found that both worlds are slightly less massive than Saturn.
Newfound Planet System Includes a Hot Earth?
When the astronomers accounted for the amount of stellar dimming the two planets should cause, they found another, faint source of interference.
This signal could mean that a third planet, smaller and nearer to the host star, is transiting Kepler-9 every 1.6 days. The third planet would be about 1.5 times the mass of Earth and made of rocks rather than gas.
But the researchers aren't celebrating just yet—interference from background stars or stellar companions can look a lot like the signals of transiting exoplanets.
"At this point, we have a very, very interesting candidate, and I hope soon we may be able to see something more," Holman said. (Read about the search for new Earths in National Geographic magazine.)
And even if the unknown object turns out to be a super-Earth, humans won't likely be colonizing it: Based on its tight orbit, the planet's surface temperature would be about 2,200 Kelvin (3,500 degrees F, or 1,900 degrees C).
The newfound planetary system is described in this week's issue of the journal Science.