Habitable zone redefined – and we’re barely in it ourselves

We can all be picky at times, but you probably think of this planet as reasonably bearable. In a new model for what constitutes a habitable zone, however, the Earth barely scrapes over the bar.

And, it turns out, some extrasolar planets previously believed to be in habitable zones may, in fact, not be after all.

The new Penn State Department of Geosciences team work builds on a prior model to offer a more precise calculation of where habitable zones around a star can be found. And comparing these with the previous model, habitable zones turn out to be farther out from the stars than thought.

“This has implications for finding other planets with life on them,” says post-doctoral researcher Ravi Kumar Kopparapu.

The changes come thanks to updated absorption databases of greenhouse gases, which have more accurate information on water and carbon dioxide than was previously  available. In the earlier model, water and carbon dioxide weren’t being absorbed as strongly, so planets had to be closer to the star to be in the habitable zone.

The team now hopes to use the data to assist with the Habitable Zone Planet Finder, a project in Penn State’s Department of Astronomy and Astrophysics. The NSF-funded team is currently constructing a precision spectrograph aimed at finding Earth-sized planets in the Milky Way that could sustain liquid water.

Using the new model, the Earth appears to be situated at the very edge of the habitable zone. But there’s no need to pack your bags, says the team: the model doesn’t take into account feedback from clouds, which reflect radiation away from Earth and stabilize the climate.