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. "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."
Via Dr. Stefan Gruenwald