… 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. [9 Exoplanets That Could Host Alien Life]
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.
“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.
“It’s a surprise that Earth is so close to the inner edge of the habitable zone,” said astronomer Abel Méndez of the University of Puerto Rico at Arecibo, who was not part of the team behind the redefinition.
Méndez manages a list, called the Habitable Exoplanet Catalog, off 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.