Planet hunters have always been keen to find Earth's twin, but an astrobiology team now suggests that "superhabitable" planets may be even better places to look for alien life.

Since 1995, astronomers have detected more than 1,000 worlds orbiting nearby stars, sparking a race to find the one that most resembles Earth, blessed with oceans and an oxygen-rich atmosphere. That's because Earth is the only place in the universe where we know that life has evolved. (See: "More Than 1,000 Potential New Planets Found.")

In the journal Astrobiology, however, researchers René Heller of Canada's McMaster University and John Armstrong of Weber State University in Ogden, Utah, calls that idea too Earth-focused. "From a potpourri of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable, even bizarre from a biocentric standpoint," they write.

Instead, they suggest that astronomers should focus their planet hunting on worlds that might harbor conditions even more amenable to life. The authors dub these hypothetical worlds "superhabitable." (See "Think Outside the Box to Find Extraterrestrial Life.")

Their report adds to a chorus of voices in the planet-hunting community that have called for rethinking the idea of "habitable zones" where worlds that follow orbits friendly to oceans and life would exclusively exist.

Water Worlds

What characteristics might make a world superhabitable? Like all potentially habitable worlds, they should have water, agree Heller and Armstrong. But they list more than a dozen additional geological and atmospheric factors that could influence habitability.

For instance, older planets would presumably have had more opportunities for life to evolve. Larger worlds, ones up to three times as massive as Earth, might be more likely to have an atmosphere due to more volcanic activity, which releases gases.

Earth itself is thought to be located on the fringes of the habitable zone, they note, so maybe planets that are located nearer to the center of the habitable zone are more congenial to life.

Scientific Skepticism

Other scientists disagree about the usefulness of the concept of superhabitability. "A planet is either habitable or it's not," says atmospheric scientist Jim Kasting, who first introduced the concept of the circumstellar habitable zone, which defines a planet as habitable if it orbits its star at a distance where it's not too hot and not too cold for liquid water to form on the planet's surface.

Similarly, astrophysicist Steven Desch said that "calling a planet superhabitable is comparable to calling someone only a little bit pregnant ... Having more of what is needed for life, in my mind, doesn't make it more likely to have life."

But Ravi Kopparapu, a physicist at Penn State University, agrees with the authors that the "binary" habitable zone concept (either friendly to life or not) is too restrictive. Plenty of worlds within the habitable zone are unlikely to support life, while others—such as the icy moons of Saturn and Jupiter, which may have vast underground oceans—could potentially support life but fall outside the habitable zone. As scientists continue to discover a menagerie of exoplanets, considering more variables could help to prioritize which planets to target for follow-up.

Nevertheless, "there is a very good reason why the binary habitable zone concept is important and relevant," says Kopparapu. Currently, when astronomers discover a planet, all they can learn about it is its mass and radius, how much light it receives from its star, and occasionally the composition of its upper atmosphere. Until scientists develop the techniques to study a planet's surface features, tectonic activity, and geological composition, the habitable zone concept remains the best guess of its habitability, says Kopparapu.

Worlds Waiting

When NASA launches the James Webb Space Telescope in 2018, it may help scientists to take a closer look at a planet's atmosphere, detect if it has oceans, and analyze its chemical composition.

If superhabitable planets exist, and if we develop the means to find them, they may turn out to be more common than Earthlike planets—current studies suggest that super-Earths are more common than Earth-size planets (although those studies may be biased by the fact that it's easier to spot larger planets).

The concept of superhabitability could broaden our chances of discovering life on other worlds, Kopparapu says, "because it opens up the possibility that there may be some super-Earth planets with appropriate conditions for life ... I think it is noteworthy to consider these planets for future habitability studies."