Two billion planets in our galaxy may be suitable for life
Alok Jha, science correspondent
Our galaxy probably contains at least two billion planets that, like Earth, have liquid water on their surfaces and orbit around their parent stars in the "habitable zone" for life. The nearest, according to astronomers, could be a mere 12 light years away.
A new study, published today in the Proceedings of the National Academy of Sciences, suggests that Earth-like planets capable of supporting life are far more common than previously thought. Using measurements from Nasa's Kepler space observatory, scientists led by Erik Petigura the University of California, Berkely, estimated that 22% of our galaxy's sun-like stars had rocky planets circling them that were within the zone that meant they got roughly the same amount of light energy as Earth gets from the sun. There are around 100bn stars in our galaxy, of which 10% are like the sun.
So far Kepler has studied more than 150,000 stars and identified more than 3,000 candidate planets, but many of these are "gas giants", similar to Jupiter, that orbit close to their parent stars. If there is life out there, it is far more likely to have evolved on rocky planets with liquid water on their surfaces, similar to Earth.
To get their results, Petigura's team looked for planets in Kepler data that had a radius up to double that of Earth. They searched for planets that orbited far enough from their star that liquid water would not evaporate, but not so far that the water would all freeze.
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http://www.theguardian.com/science/2013/nov/04/planets-galaxy-life-kepler
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