What makes a planet livable? Five things scientists look for.

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. 

By , Staff writer

4. What kind of chemistry takes place?

An environment that supports the formation of long chains of molecules, with carbon as a backbone, is preferred. In some cases, silicon might replace carbon. Carbon is particularly adept at bonding with nitrogen, sulfur, and phosphorous, forming a range of biologically important molecules. The abundance of these molecules and the basic elements that build them wafting through a planet's atmosphere would provide clues as to the planet's hospitality.

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