The first five years of this decade have been, without a doubt, a turning point in exoplanetary discoveries. Before the launch of the Kepler Space Observatory, only a handful of planets outside our Solar System were known. Now the number of confirmed planets is close to 2,000, with thousands more candidates waiting for confirmation from subsequent observations. 

The latter part of this decade will focus on the better understanding of the planets we have found so far. The goal is simple: to find habitable planets. With this objective in mind, astronomers from the University of Washington's Virtual Planetary Laboratory have created a habitability index to rank exoplanets so that follow-up explorations in the next few years can prioritize the most promising candidates. Their work has been accepted for publication in the Astrophysical Journal.

The "habitability index for transiting planets" is constructed exclusively with available observational data. As long as the planet's orbital period, the duration of transit across its host star, star surface gravity, radius and temperature are all known, it is possible to calculate the index – which quantifies the likelihood that a transiting planet may possess liquid water on its surface. The index also depends on several factors such as the rockiness of the planet, its mass and albedo. The albedo quantifies the amount of starlight reflected back into space by the planet. 

Traditionally, scientists determine whether an exoplanet is possibly habitable by seeing whether it sits in its host star's habitable zone, otherwise known as its "Goldilocks zone." This is when the region around a star is not too hot or too cold and has the right conditions for liquid water to form on the planet's surface. So, once the distance of a planet from its star is calculated, the planet is either in or out of the habitable zone. This is logically correct; a planet is either habitable or inhabitable, but this information can only tell us so much.

"That was a great first step," study author Rory Barnes said in a statement, "but it doesn’t make any distinctions within the habitable zone." On the other hand, the habitability index is a continuous variable rather than a yes or no category, creating more subtle distinctions and including some important factors that make a planet truly habitable.

While the habitability index is an important new tool, it has limitations. Some of these are theoretical (the complex properties of rocky planets, eccentric orbit complications), while others are due to the constraints on the data. More work is necessary, but with the Transiting Exoplanet Survey Satellite scheduled to launch in 2017 and Hubble's successor, the James Webb Space Telescope, scheduled for 2018, it is important to possess a statistical tool that allows us to quantify where to best point our instruments.

Top image credit: James Webb Space Telescope Artist Conception by Northrop Grumman, via Flickr. CC BY-NC-ND 2.0

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