Scientists should take the conservative approach when searching for habitable zones where life-sustaining planets might exist, according to James Kasting, Evan Pugh Professor of Geosciences at Penn State, including when building Terrestrial Planet Finders.
That conservative approach means looking for planets that have liquid water and solid or liquid surfaces, as opposed to gas giants like Jupiter or Saturn. The habitable zone in a solar system is the area where liquid water, and by extension life, could exist. Defining the habitable zone is key to the search for life sustaining planets in part because the idea of a habitable zone is used in designing the space-based telescopes that scientists would use to find planets where metabolism -- and potentially life -- life might exist.
"It's one of the biggest and oldest questions that science has tried to investigate: is there life off the earth?" Kasting said. "NASA is pursuing the search for life elsewhere in the Solar System, but some of us think that looking for life on planets around other stars may actually be the best way to answer this question."
Recent research by Ravi Kopparapu, a post-doctoral researcher working with Kasting, suggests that the frequency of Earth-like planets in the habitable zones of stars known as M-dwarfs is 0.4 to 0.5. To find four potential Earth-like candidates, scientists would need to survey the habitable zones of about 10 cool stars. This data came from NASA's Kepler Space Telescope, which collected information on transiting exoplanets for almost four years before being partially disabled. Previous estimates put this frequency at 0.1, which would have forced scientists using planet finders to survey more stars, searching farther away from our Solar System.
An even more recent estimate of the frequency of Earth-like planets was announced by Eric Petigura and colleagues at the Kepler Science Conference in early November. They calculated the figure at 0.22 around stars more similar to the Sun. But Kopparapu and Kasting think Petigura and colleagues' estimate could be too high by a factor of two because they used an overly optimistic estimate for the width of the habitable zone. If so, then the old value of 0.1 may be closer to the truth.
The ability of a planet to sustain liquid water is traditionally part of the criteria when searching for life-sustaining planets. While some have argued that subsurface water would be enough to sustain life, testing that hypothesis remotely would be virtually impossible, so the focus for astronomers should remain on surface water, Kopparapu and Kasting note in a special issue of the Proceedings of the National Academy of Sciences.