The graphic shows habitable zone distances around various types of stars, according to an updated habitable zone definition. Some of the known extrasolar planets that are considered to be in the habitable zone of their stars are also shown. On this scale, Earth-Sun distance is 1 astronomical unit, which is roughly 150 million kilometers. CREDIT: Chester Herman

‘Habitable Zone’ for Alien Planets, and Possibly Life, Redefined

by Clara Moskowitz, SPACE.com Assistant Managing Editor
Date: 29 January 2013 Time: 11:23 AM ET

One of the most important characteristics of an alien planet is whether or not it falls into what’s called the habitable zone ­— a Goldilocks-like range of not-too-close, not-too-far distances from the parent star that might allow the planet to host life.

Now scientists have redefined the boundaries of the habitable zone for alien planets, potentially kicking out some exoplanaets that were thought to fall within it, and maybe allowing a few that had been excluded to squeeze in.

“This will have a significant impact on the number of exoplanets that are within habitable zone,” said research team leader Ravi Kumar Kopparapu of Penn State University.

The habitable zone defines the region where a planet might be able to retain liquid water on its surface. Any closer to the star and water would vaporize away; any farther, and it would freeze to ice. But water in its liquid state is what scientists are after, since that is thought to be a prerequisite for life.

The new definition of the habitable zone is based on updated atmospheric databases called HITRAN (high-resolution transmission molecular absorption) and HITEMP (high-temperature spectroscopic absorption parameters), which give the absorption parameters of water and carbon dioxide — two properties that strongly influence the atmospheres of exoplanets, determining whether those planets could host liquid water.

Read more: New Alien Planet 'Habitable Zone' Rules | Extraterrestrial Life | Space.com.

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