Scientists Call for a Conservative Approach in Looking for Earth-like Planets

A study published this week in the Proceedings of the National Academy of the Sciences suggests that astronomers, in their quest to find planets that could possibly sustain life, should take a more conservative approach. By that the researchers mean the search for planets with a star's habitable zone should be restricted to those that have liquid water or solid or liquid surfaces, rather than gaseous planets such as Jupiter or Saturn.

"It's one of the biggest and oldest questions that science has tried to investigate: Is there life off the Earth?" said James Kasting, a geoscientist at Penn State University. "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."

Ravi Kopparapu, a postdoctoral researcher working with Kasting, said that the frequency of Earth-like planets in the habitable zone - defined as a region where liquid water could exist - is 0.4 to 0.5 for M-dwarf-class stars. So to find four potential Earth-like candidates, astronomers would need to survey the habitable zones of about 10 cool stars. The process may tedious, but previous estimates placed the frequency of finding such planets at 0.1.

Other estimates about the frequency of Earth-like planets are higher, including work presented by Eric Petigura and colleagues at the Kepler Science Conference in November, which calculated the frequency at 0.22. But Kasting and Kopparapu contend that calculation used a too-optimistic width of the habitable zone.

Regardless of frequency, the search of other planets with the potential to sustain life centers around water.

"All life that we know of is carbon-based and depends on the presence of liquid water during at least part of its life cycle," Kasting said. "Hence, if we see a planet that shows evidence for liquid water, we can immediately think about the possible presence of carbon-based life."

Kasting suggested that it's possible that every planet out there that has the right conditions could develop life.

"We don't really know the answer to that. But, it could be. If you're an optimist, you think it just takes the right conditions. It happened on Earth, why wouldn't it happen somewhere else?" he said. But he also noted the potential for ambiguity to arise as we learn more about distant worlds, giving the example of detecting oxygen on an Earth-like planet, but not methane.

But Kasting notes the search for life on distant worlds is something scientists have been concerned with for centuries.

"Did it make any difference when we figured out that the Earth was going around the sun rather than vice versa? If you're just a practical-minded person, it made absolutely no difference to your life because life goes on Earth just the way it did," Kasting said.

"But if you expand your mind a little bit, it helped us figure out our place in the universe -- that we're actually on a little planet going around a rather normal star amongst many other stars in the galaxy, and there are many galaxies out there. It's been one of the most profound changes ever in human thought."