'Habitable Zone' for alien planets redefined

"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.

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.

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 vaporise away; any farther, and it would freeze to ice, Space.com reported.

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.

The scientists cautioned that the habitable zone definition still does not take into account feedback effects from clouds, which will also affect a planet's habitability.

The previous habitable zone definitions were derived about 20 years ago by Penn State researcher James Kasting, who was also part of the team behind the updates.

The new definition isn't radically different from the old one. For example, in our own solar system, the boundaries of the habitable zone have shifted from between 0.95 astronomical units (AU, or the distance between Earth and the Sun) and 1.67 AU, to the new range of 0.99 AU to 1.7 AU.

"It's a surprise that Earth is so close to the inner edge of the habitable zone," said astronomer Abel Mendez of the University of Puerto Rico at Arecibo, who was not part of the team behind the redefinition.

"Right now as I see it as a significant change. Many of those planets that we believe were inside are now outside. But on the other side, it extends the habitable zone's outer edge, so a few planets that are farther away might fall inside the habitable zone now," Mendez said.

He mentioned one planet in particular, Gliese 581d, was thought to lie at the outer edge of its star's habitable zone. With the new definition, though, it falls almost smack in the middle, making it perhaps a better candidate for extraterrestrial life.

The new habitable zone definition will be published in the Astrophysical Journal.

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