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 “habitability 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 presenting the index, explained in a statement, "Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme…so that as we move into a time when there are hundreds of targets available, we might be able to say, ‘OK, that’s the one we want to start with.'”
The index comes in anticipation of the 2018 launch of the James Webb Space Telescope. The telescope will help us determine 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, and one scientists will have to make use of wisely—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 star’s “habitable zone” — more informally called the “Goldilocks zone” — which is the swath of space that’s “just right” to allow an orbiting Earth-like planet to have liquid water on its surface, perhaps giving life a chance…. 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.
The index values planets that are Earth-like, but not too Earth-like. Barnes and co-authors Victoria Meadows and Nicole Evans wrote in their paper how they determined the "H," or habitability, score, of exoplanets named by the Kepler and K2 missions as likely candidates for life:
We ranked the known Kepler and K2 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 — it means that an Earth twin orbiting a solar twin that is observed by Kepler 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.
Danielle Wiener-Bronner is a news reporter.