Scientists have so far detected at least 550 planets outside the solar system – and another 2,000-plus await confirmation. But how to pick out the ones that may be Earth-like havens for life? Here's what one team looks for in assessing any planet's potential habitability and its similarity to Earth's properties.
A rock originating on Mars that fell to Earth during the Stone Age contains carbon-rich spheres and tunnels, possibly suggesting the presence of microorganisms on the Red Planet millions of years ago.
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