For most researchers' money, an Earth-like planet is the best bet for finding alien life. But looking in such an exclusive range of possibilities might give them only half the story.

A team of scientists is now proposing an index that ranks a planet's habitability using a much wider set of criteria.

"We are trying not to be geocentric, calculating planetary habitability independent of liquid water," said physicist Abel Mendez of the University of Puerto Rico in Arecibo and one of the co-authors of the new index, published in Astrobiology on Nov. 21.

Astronomers have discovered more than 700 extrasolar planets, many of them gas giants that orbit too near or far from their parent star to be comparable to Earth. But Mendez and his group want to expand the narrow possibilities generally considered necessary for a planet to host life.

The team proposes to rank planets on both an Earth Similarity Index (ESI) and also a broader Planetary Habitability Index (PHI). The first index looks at how close a planet is to Earth in mass, temperature, and composition while the second is based on the whether or not it possesses more exotic chemistries, liquids, and energy sources than found on our planet. Alien life could be based on elements other than carbon, require liquids other than water, and gain energy through means other than sunlight.

Within our own solar system, for instance, Saturn's moon Titan is a tiny world where water is only available in frozen chunks hard as rock. But its temperature range makes possible the existence of lakes and rivers of liquid hydrocarbons, leading some researchers to speculate that native life could exist there.

As such, Titan gets a rather low ESI but a more optimistic, middle-range PHI. Mars, in contrast, is closer to Earth in size and composition, giving it a high ESI, but has no known liquid on its surface, placing it lower than Titan on the PHI range. Such rankings could allow scientists to give a more nuanced and complex view of planetary habitability when searching for life on other worlds.

While current telescopes can't detect many of the characteristics needed to assess how an exoplanet would rank on either of these indexes –- such as size or atmospheric composition –- future instruments could give astronomers these capabilities. Mendez suggested that in 10 or 20 years researchers might announce an ESI and PHI whenever a new exoplanet is discovered, refining the rankings as better data becomes available. "We are putting down the groundwork now for these indexes to evolve," he said.

But other researchers are skeptical of the claim that we might be able to detect distant habitable planets that are far different than Earth.

"Trying to argue you have life based on some completely alien biosignature is going to be hard," said planetary scientist Jim Kasting of Penn State University and author of the book How to Find a Habitable Planet.

There are good biochemical reasons for thinking that life is carbon based and relies on liquid water elsewhere, he said. Until scientists have actual data showing that life can exist in more exotic situations, it would be best to focus on Earth-like exoplanets, he added.

Image: Wikimedia/Lucianomendez

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