Life on other planets? Evidence is shaky, at best

Earlier this month, a report based on findings by NASA’s Kesler spacecraft made news when it claimed that Earth-like, habitable planets are surprisingly common. But closer examination reveals the report used assumptions, broad criteria, and extrapolated data to arrive at its questionable conclusion.

“These scientists use very broad, loose definitions for earth-like and sun-like,” says Dr. Jay Richards, co-author of Privileged Planet. The report inflates the definition of “earth-like” by using vague, general standards for things like size and orbit, and it extrapolates data from a very limited number of planets that revolve around stars less massive than the sun.

For example, the researchers examined 42,000 stars and found 603 planet candidates, only 10 of which the scientists deemed earth-size and within the habitable zone. The team broadly defined earth-size planets as those having a radius up to two times that of earth. The habitable zone is described as the zone in which liquid water could possibly exist and included planets receiving up to four times as much light from their stars as the earth does from the sun. The data say nothing about whether these planets can actually support life, only that they meet some of the known criteria for habitability.

The Kepler spacecraft's field of view includes about 150,000 stars, but many of the planets orbiting them escape detection. To make corrections for the missed planets, researchers inserted fake planets into the data so they could see how many their software would miss. They also extrapolated the number of planets with orbits longer than 200 days, because no such planets were observed in the Kepler data. Based on extrapolations from only the 10 planets found orbiting 42,000 stars they contend that about 22 percent of sun-like stars observed by Kepler have earth-size, potentially habitable planets.

Intelligent design, the scientific belief that nature displays evidence of having been designed, does not preclude the possibility of a few, rare extra-solar earth-like planets, according to Richards.

“It is not so much the uniqueness of earth that is evidence of intelligent design, it is the fact that the most habitable place also happens to be the place best suited for discovery.” For example, Richards points out that to make observations, we need a planet with an atmosphere that protects from bombardment yet is also transparent. The planet also couldn’t be too close to the center of the galaxy or else background radiation would skew the data. It just so happens that earth has all the factors necessary for life, which are also the same factors most conducive for discovery. “That points to intelligent design,” says Richards.

The possible existence of planets similar to earth does not mean there are other habitable planets, Richards cautions. “The most earth-like planet we know is Mars and it is lifeless. We should not be optimistic that we will find life on other planets.”