Can an equation determine potential habitats?

Researchers are laying the groundwork for a new equation that could mathematically quantify a habitat’s potential for hosting life, in a similar way to how the Drake equation estimates the number of intelligent extraterrestrial civilizations in the Milky Way.

Microbes living in sandstone in Antarctica – but what determines how far down they live? Credit: C Cockell

Dr Axel Hagermann of the Open University proposed a method to find this ‘habitability index’ at the European Planetary Science Congress in Germany last month.

“At present, there is no easy way of directly comparing the suitability of different environments as a habitat for life. The classical definition of a habitable environment is one that has the presence of a solvent - for example water - availability of the raw materials for life, clement conditions and some kind of energy source, so we tend to define a place as ‘habitable’ if it falls into the area where these criteria overlap on a Venn diagram.

“This is fine for specific instances, but it gives us no quantifiable way of comparing exactly how habitable one environment is in comparison with another, which I think is very important,” explained Dr Hagermann.

Dr Hagermann and colleague Professor Charles Cockell have the ambitious aim of developing a single, normalised indicator of habitability, mathematically describing all the variables of each of the four habitability criteria. Initially, they are focusing on describing all the qualities of an energy source that may help or hinder the development of life.

“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 in the soil where the ‘bad’ radiation has been absorbed but the ‘good’ radiation can penetrate.  We are looking to be able to define this optimal habitable region in a way that we can say that it is ‘as habitable’ or ‘less habitable’ than a desert in Morocco, for example,” said Dr Hagermann.