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. PHL (at) UPR Arecibo, Rogelio Bernal Andreo.
"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. 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 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. Chester Herman.
"At the time when he wrote that paper, no exoplanets were discovered," Kopparapu told Space.com. "In 20 years, hundreds, maybe thousands, have been discovered." 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. M'endez manages a list, called the Habitable Exoplanet Catalog, of all the known planets beyond our solar system that could be habitable to life. The new study will necessitate some adjustments to the catalog, he said. "Right now as I see it as a significant change," M'endez said. "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." 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. "That will be a big change for that particular planet," M'endez said. "That means the prospects for life on the planet will be much better."The researchers detail their new habitable zone definition in a paper to be published in an upcoming issue of the Astrophysical Journal.
To explore the Habitable Planet Catalog directly, visit here. -NBC NEWS.
