The planet-hunting Kepler space telescope has found a new planetary system orbiting a distant star that could include a planet nearly the same size as Earth, NASA scientists said on Thursday.
In findings to be published in the journal Science this week, the scientists report the discovery of two large planets about the size of Saturn orbiting a star similar to the sun. A third small object orbiting the star could be a much smaller planet, just a bit larger than Earth, but more work must be done to confirm it is actually a planet.
The two confirmed planets are known as Kepler 9b and Kepler 9c and quickly orbit around their sun-like star called Kepler 9 some 2,000 light years away from Earth, astronomers told reporters.
The scientists observed seven months of data from the telescope and were able to measure how the planets interact with one another to deduce their size, mass and composition. The system is the first confirmed by Kepler.
Before they can know for certain whether there is a third planet orbiting Kepler, they must rule out any other phenomena, such as another nearby star, which could cause similar observations, said Matthew Holman, an astronomer at the Harvard-Smithsonian Centre for Astrophysics.
If confirmed, it would be the smallest planet yet discovered outside of the solar system, with a radius just 1.5-times the size of Earth. But the planet is located far too close to its star to harbour life, Holman said.
The Kepler mission is designed to discover other Earth-like planets in the Milky Way galaxy, but in order for those planets to be capable of harbouring life they must be located in what scientists dub the "Goldilocks" zone - neither too hot nor too cold.
Finding such a planet will take several years because the further a planet is from its star the longer it takes for it pass in front of the star - making it more difficult to detect, said William Borucki, Kepler's chief scientist.
He urged the public to be patient. "Mankind has been asking the questions, 'Are there other planets out there? Is there other life out there?' for a couple of thousands of years," he said. "We're now in a very exciting period, in the next few years we will have the answer to these questions."
Earlier this year, NASA said Kepler had found some 700 candidates for further observation, at least five of which appeared to include more than one possible planet transiting the star.
The news follows an announcement earlier this week that European astronomers have spotted a solar system just 127 light-years from Earth with up to seven planets orbiting a star, the biggest system found since exoplanetary research began. Scientists have confirmed the existence of five of those planets and said two more are possible, including another small planet about the size of Earth.
The Kepler space telescope launched last year is finely tuned enough to detect Earth-sized planets orbiting distant stars. The $590-million telescope programme is to spend at least the next three-and-a-half years pointed at a large swath of the Milky Way galaxy containing some 4.5 million stars.
The most advanced cameras ever used in space will focus on some 100,000 to 150,000 stars deemed most likely to have orbiting planets, scientists said at a prelaunch press briefing. Data from the cameras will be used to find planets by looking for distortions in the light being emitted as an orbiting planet crosses in front of the star.