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, SETI, the German Aerospace Center, and four universities
“The first question is whether Earth-like 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.”
About 700 exoplanets, those outside our solar system, have been discovered so far, and only a few have the potential for Earth-like life. But there are an estimated 50 billion planets in our galaxy.
The authors suggest that newly discovered planets be classified using two different indices: an Earth Similarity Index, for those with Earth-like 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 indices 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, Earth-like 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 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.
The paper was co-written by 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; David Catling of the University of Washington; Louis N. Irwin, of the University of Texas-El Paso, and Marina Resendes de Sousa Antonia, Carol Turse, Grayson Boyer and Schulze-Makuch all of Washington State University.
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