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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. 

5. What about liquids?

  • By Pete Spotts Staff writer

The fourth trait is the presence of liquids, ideally water, on the planet's surface or below it. Temperature and atmospheric pressure play important roles in retaining surface water.

Some information on whether there are liquids on a planet can be gleaned indirectly. The team points out that the presence of lakes filled with liquid hydrocarbons on Saturn's frosty moon Titan was first proposed in 1983. NASA's Cassini mission confirmed their existence 22 years later. The potential for hosting lakes was inferred from Titan's temperature, estimates of atmospheric pressure, and chemistry.

The research team devising the rating system suggests that current or proposed space-based observatories – NASA's James Webb Space Telescope and the European Space Agency's Gaia mission – should be able to pin down conditions affecting each of these four key categories for some exo-planets, especially those closest to Earth. Answers for others may remain well out of reach.

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Mark Sappenfield
Editor

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