To ascertain which exoplanets are more likely to support life, scientists from the Open University are assembling a revolutionary new ‘habitability index’ that is being presented for the first time today at the European Planetary Science Congress (EPSC) in Potsdam, Germany.

Dr Axel Hagermann and Professor Charles Cockell have proposed the idea of a habitability index as a companion to the famous and somewhat controversial Drake Equation – SETI astronomer Frank Drake’s attempt to quantify how many intelligent civilisations there are in the Galaxy.

The habitability index is not an attempt to calculate how many worlds in the Galaxy are habitable; instead it will attempt to determine whether any given planet has the potential to be habitable, and then compare that to other planets.

“The best way to describe our work is that, at the moment, we’re trying to identify a suitable framework to think in,” Hagermann told Astronomy Now. Hagermann describes how it will identify various environmental factors, such as the different types of radiation incident on a world from its star, and then trying to find how life will respond to these factors. “For instance, while visible and infrared wavelengths are important for life and processes such as photosynthesis, ultraviolet and X-rays are harmful. If you can imagine a planet with a thin atmosphere that lets through some of this harmful radiation, there must be a certain depth of soil where the ‘bad’ radiation has been absorbed and the good radiation can penetrate.”

Currently, if we say that a small, rocky planet is within the ‘goldilocks zone’ of its star, where temperatures are favourable for liquid water, then we label it as habitable, but as it stands this is a vague conclusion ignoring a host of environmental factors.

“We’d like to move towards, ‘in these conditions, life is more probable than in those’. This will mean finding suitable variables and finding suitable equations,” says Hagermann. “The Drake equation is a good example of how this works.”

The difference with the Drake Equation is that a lot of the variables are nothing more than educated guesses, and the equation produces an ‘answer’ in terms of the number of predicted civilisations. The habitability index will only deal with quantifiable factors.

The equation is still in its infancy, but with the help of feedback from other planetary scientists at EPSC and elsewhere, Hagermann and Cockell hope that it will evolve into a universal tool for judging the habitability of the many Earth-like exoplanets that it is hoped NASA’s planet-finding Kepler spacecraft will discover over the next few years.