A new definition of the habitable zone around planets, denoting where liquid water could exist, shifts Earth toward the very edge of the solar system's own habitable zone. CREDIT: PHL @ UPR Arecibo, Rogelio Bernal Andreo  Enlarge

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. 

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