Rory Barnes/University of Washington

The cosmos are very large, but scientists think there's a good shot we'll find life out thereā€”and soon. And they'veĀ hatched a plan to narrow down the search.

Researchers at the University of Washington have put together a guide for astronomers searching for life on Earth-like planets outside of our solar system, or exoplanets. The ā€œhabitability index for transiting planets" is designed to help scientists figure out which exoplanets to examine most closely for signs of life.

Rory Barnes, lead author on a paper presentingĀ the index,Ā explained in a statement, "Basically, weā€™ve devised a way to take all the observational data that are available and develop a prioritization schemeā€¦so that as we move into a time when there are hundreds of targets available, we might be able to say, ā€˜OK, thatā€™s the one we want to start with.'ā€

The index comes in anticipation of the 2018 launch of the James Webb Space Telescope.Ā The telescope willĀ help usĀ determineĀ at the atmospheric makeup of exoplanets, which will in turn bring us closer to guessing if they could harbor the liquid water needed to sustain life. It's an exciting, but limited, resource, andĀ one scientists will have to make use of wiselyā€”not every planet will be worth examining. The university explained in a statement why a habitability index could make a difference:

Traditionally, astronomers have focused the search by looking for planets in their starā€™s ā€œhabitable zoneā€ ā€” more informally called the ā€œGoldilocks zoneā€ ā€” which is the swath of space thatā€™s ā€œjust rightā€ to allow an orbiting Earth-like planet to have liquid water on its surface, perhaps giving life a chanceā€¦.Ā The new index is more nuanced, producing a continuum of values that astronomers can punch into a Virtual Planetary Laboratory Web form to arrive at the single-number habitability index, representing the probability that a planet can maintain liquid water at its surface.

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The index values planets that are Earth-like, but notĀ tooĀ Earth-like. Barnes and co-authors Victoria Meadows and Nicole Evans wrote in their paperĀ how they determined theĀ "H," or habitability, score, of exoplanets named by the Kepler and K2 missions as likely candidates for life:

We ranked the knownĀ KeplerĀ andĀ K2Ā planets for habitability and found that several have larger values of H than Earth. This does not mean these planets are "more habitable" than Earth ā€” it means that an Earth twin orbiting a solar twin that is observed byĀ KeplerĀ would not have the highest probability of being habitable. The best candidates have incident radiation levels, assuming circular orbits, of 60-90% that of Earth's.

In other words, a habitable exoplanet that gets less radiation from its sun than Earth does is a good bet. Seems reasonable.

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Danielle Wiener-Bronner is a news reporter.