Artists' conception of the 6 Earth-mass exoplanet Gliese 667 C (Graphic: NASA/JPL)

Artists' conception of the 6 Earth-mass exoplanet Gliese 667 C (Graphic: NASA/JPL)

An international team of scientists is about to embark on a unique search for life on planets far beyond our solar system.

So far, astronomers have found 702 planets outside of our solar system and that number is expected to reach several thousand or more within the next few years.

As the list of newly discovered exoplanets continues to grow, some scientists worry the search for extra-terrestrial life will narrow, focusing on just a few of these new worlds, especially those with Earth-like conditions.

That’s because the scientific community’s search for alien life is widely based on the idea that Earth serves as the best model when considering which conditions that are best suited to hold life on other planets.

In other words, many scientists tend to look at a planet that either mimics, or merely has some of the important traits of Earth, before considering a search for extra-terrestrial life.

However, other scientists see that approach as a limiting form of earthling-biased thinking.

That’s where the research team,  led by Dirk Schulze-Makuch, an astrobiologist at Washington State University, and Abel Mendez, a modeling expert from the University of Puerto Rico, comes in.

The team is proposing to narrow the search for extra-terrestrial life based on two basic questions:  whether Earth-like conditions can be found on other worlds – since its’ already known that those conditions can harbor life – and  whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not.

In a paper to be published in Astrobiology, the team suggests a new system for classifying exoplanets by using two different indices.

One would be an Earth Similarity Index (ESI) for categorizing a planet’s more earth-like features. The second is a Planetary Habitability Index (PHI), which describes a variety of chemical and physical parameters that are theoretically conducive to life in conditions unlike those on Earth.

“Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” the paper’s authors write. “For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”

Using the proposed system, researchers have already found that two planets in the Gliese 581 systemGJ 581 c and GJ 581d – may have habitats comparable to Mars, Jupiter’s moon Europa and Saturn’s moon Enceladus.