'Habitability index' to reveal where to look for alien life

Astronomers from University of Washington's virtual planetary laboratory have created a way to compare and rank exoplanets to help prioritise which of the thousands discovered warrant close inspection in the search for life beyond Earth.

The new metric, called the "habitability index for transiting planets", produces 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.

"Basically, we have devised a way to take all the observational data that are available and develop a prioritisation 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'," explained astronomy professor Rory Barnes.

The index is a tool to help fellow astronomers decide which worlds might have the better chance of hosting life, and so are worthy of focusing limited resources on.

Traditionally, astronomers have focused the search by looking for planets in their star's "habitable zone" -- more informally called the "Goldilocks zone".

In creating the index, the researchers factored in estimates of a planet's rockiness, rocky planets being the more Earth-like.

They also accounted for the energy reflected back to space from its surface and the circularity of its orbit which affects how much energy it receives from its host star.

"This innovative step allows us to move beyond the two-dimensional habitable zone concept to generate a flexible framework for prioritisation that can include multiple observable characteristics and factors that affect planetary habitability," said astronomy professor Victoria Meadows.

The Kepler Space Telescope has enabled astronomers to detect thousands of exoplanets, including those beyond our solar system.

The James Webb Space Telescope, set for launch in 2018, will be the first able to actually measure the atmospheric composition of a rocky, possibly Earth-like planet far off in space, and so vastly enhance the search for life.

The paper is forthcoming in the Astrophysical Journal.