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. 

2. If the planet (or a moon) isn't similar to Earth, could it still host life?

To answer this question, the team looks at four broad requirements. The first is a platform or substrate on which life can gain a toehold. It could be rock or ice. Either provides a stable platform, can corral chemical elements that can serve as food, and provides a surface on which chemical reactions can take place.

An atmosphere and a planetary magnetic field would be big pluses as shields from excessive radiation from space. Plate tectonics also would be a big plus because it recycles elements on the surface and helps stabilize a planet's climate.

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