Redefining the habitable zone

Although finding exoplanets is interesting (and nearly a thousand have been confirmed to date), the really exciting news would be to find another planet capable of sustaining life. To do so, such a planet would almost certainly orbit within a certain radius from its star: the ‘habitable zone’ (HZ). The trouble is, astronomers aren’t entirely sure just what the boundaries of a star’s HZ are. Just recently, a team of Pennsylvania State University cosmologists, led by Ravi Kopparapu, have redefined the borders of the HZ.

The region within an HZ is largely determined by the presence of two molecules: liquid water and carbon dioxide. If a planet is too close to its star, any water will boil off into space. Too far, and the water will remain permanently frozen. Because the outer edge of the HZ can be extended if the planet has a carbon dioxide atmosphere to warm it, researchers take the presence of CO₂ into account when predicting the outermost limits of an HZ.

While Kopparapu and his colleagues still use H₂0 and CO₂ to define where an HZ will fall, they believe that the criteria used to detect those molecules needs to be adjusted. When they used updated data, they found that HZs tend to be a bit further out than previously thought. This means that some planets that were written off may in fact be within the HZ of their star, but also that some hopeful candidates aren’t in the HZ after all.

Caption: The graphic shows habitable zone distances around various types of stars. 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 one astronomical unit, which is roughly 150 million kilometers.

Credit: Chester Herman

Notice that, according to the new standards, Earth (seen near the top right) barely falls into the HZ of our sun. No doubt this new model has some more tweaking in store.

Ravi kumar Kopparapu, Ramses Ramirez, James F. Kasting, Vincent Eymet, Tyler D. Robinson, Suvrath Mahadevan, Ryan C. Terrien, Shawn Domagal-Goldman, Victoria Meadows, & Rohit Deshpande (2013). Habitable Zones Around Main-Sequence Stars: New Estimates Astrophysical Journal : arXiv:1301.6674.