New 'habitability index' to help find alien life

The hunt for alien life may get easier, thanks to a 'habitability index' created by scientists that can point out which of the thousands of exoplanets discovered so far have a better chance of hosting life.
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.
But so far that has been just a sort of binary designation, indicating only whether a planet is, or is not, within that area considered right for life.

The new metric, called the "habitability index for transiting planets," 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.
In creating the index, University of Washington's professors Rory Barnes and Victoria Meadows, with research assistant and co-author Nicole Evans factored in estimates of a planet's rockiness, rocky planets being the more Earth-like.
They also accounted for a phenomenon called "eccentricity-albedo degeneracy," which comments on a sort of balancing act between a planet's albedo - 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. The two counteract each other.

The higher a planet's albedo, the more light and energy are reflected off to space, leaving less at the surface to warm the world and aid possible life.

But the more noncircular or eccentric a planet's orbit, the more intense is the energy it gets when passing close to its star in its elliptic journey.
A life-friendly energy equilibrium for a planet near the inner edge of the habitable zone - in danger of being too hot for life - would be a higher albedo, to cool the world by reflecting some of that heat into space, Barnes said.
Conversely, a planet near the cool outer edge of the habitable zone would perhaps need a higher level of orbital eccentricity to provide the energy needed for life.

Barnes, Meadows and Evans ranked in this way planets so far found by NASA's Kepler Space Telescope, in its original mission as well as its "K2" follow-up mission.
They found that the best candidates for habitability and life are those planets that get about 60 per cent to 90 per cent of the solar radiation that the Earth receives from the sun, which is in keeping with current thinking about a star's habitable zone.
The research was published in the Astrophysical Journal.