On Wednesday, New Scientist reported that researchers at Penn State University led by Ravi Kumar Koppaparu have refined the range of the habitability zones around stars commonly called "Goldilocks zones." This is the region surrounding a star in which expected temperatures would theoretically allow for the existence liquid water. The definition had gone without revision since 1993, only one year after the first definitive detection of a planet orbiting another star.
The redefinition is based on the results of experiments designed to show the extent to which water and carbon dioxide absorb light from different types of stars. The result is that the habitable zone has been moved farther out from stars than it was previously.
By the new standard, Kepler-22b, once considered the best candidate for a habitable planet in another solar system, is now in the "hot zone." Earth's habitability rating has also taken a penalty, as it was in the middle of the habitable zone but is now only 1 million kilometers from our Sun's "hot zone." This is partially because neither definition accounts for the reflective properties of clouds that occur on Earth (and quite probably on extrasolar planets as well) which lower the surface temperature of a planet. Atmospheric pressure is also not accounted for, which can allow for liquid water at temperatures as high as 647 K (374°C). But for now, with atmospheric details of extrasolar planets still unknown, the new definition of the habitable zone is a helpful improvement.
"I think this is going to be the new gold standard for the habitable zone. But I think we should always look at planets in the habitable zone and say, maybe. It's not that planets in the habitable zone are inhabited, it just means we can't rule them out yet," says Rory Barnes of the University of Washington in Seattle, who was not involved in the new work.