SPOKANE — Thousands of planets will likely be discovered in the next few years, and a new system is needed to better classify the ability of those worlds to support life, according to a Washington State University professor.

In a paper to be published in the December issue of Astrobiology, professor Dirk Schulze-Makuch and colleagues propose a new system that classifies some planets as either similar to Earth or not similar but capable of harboring life.

That is different from the current system, which is dominated by the notion that Earth serves as the best model of conditions suited to the emergence of life on other worlds, Schulze-Makuch said.

The search for life on other planets should actually begin with two questions, said Schulze-Makuch, who is working with nine fellow researchers, representing NASA, the SETI (Search for Extraterrestrial Intelligence) Institute, the German Aerospace Center and four universities.

"The first question is whether Earthlike conditions can be found on other worlds, since we know empirically that those conditions could harbor life," he said. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."

Co-authors of the paper included David Catling, of the University of Washington; and Marina Resendes de Sousa Antonia, Carol Turse and Grayson Boyer, all of Washington State University.

About 700 exoplanets, those outside our solar system, have been discovered so far, and only a few have the potential for Earthlike life. But there are an estimated 50 billion planets in our galaxy.

The authors suggest that newly discovered planets be classified using two different indexes: an Earth Similarity Index, for those with Earthlike features; and a Planetary Habitability Index, for those that contain chemical and physical properties that are theoretically conducive to life in more extreme conditions.

Seth Shostak, of the SETI Institute in Mountain View, Calif., said the new system makes sense.

"For scientists, the scale provides a quick categorization of a planet's relevance to biology, in the same way that stellar types can immediately tell an astronomer something about the size, temperature and brightness of a star," Shostak said.

The new system also has the potential to better inform the public, he said.

"If the scale becomes widely used, it might also prove useful in conveying to the public exactly how excited they should become when a new planet discovery is made," Shostak said.

The two indexes proposed by the group mark the first attempt by scientists to categorize exoplanets and exomoons in accordance with their potential to harbor some form of life, he said.

"Interest in exoplanets is going to focus initially on the search for terrestrial, Earthlike planets," said Schulze-Makuch.

But the authors believe that focusing exclusively on Earth-based assumptions is too limiting.

"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star," the paper said. "For example, the hydrocarbon lakes on Titan (Saturn's largest moon) could host a different form of life."

"Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life," they wrote.

The major barrier to this proposal is that human imagination has difficulty conceiving of radically different life-forms, Schulze-Makuch said.

"How will it be different from us, and to what extent, and what environment does it require?" he said. "There is a lot of scientific speculation associated with it."

The paper's authors contend the alternative is to risk overlooking potentially habitable worlds.

Other co-writers of the paper were Alfonso Davila, of SETI; Alberto Fairen, of NASA; Abel Mendez, of the University of Puerto Rico at Aricebo; Philip von Paris, of the German Aerospace Center; and Louis N. Irwin, of the University of Texas-El Paso.