An effort to calculate the number of places in the galaxy that are capable of supporting complex life forms has produced an astonishing estimate of 100 million. However, while the lead author describes the paper as “The first quantitative estimate of the number of worlds in our galaxy that could harbor life above the microbial level, based on objective data,” there's still plenty of guesswork going on.

In Challenges, Dr Louis Irwin of the University of Texas, El Paso, and colleagues started with the burgeoning collection of Exoplanets and created a biological complexity index (BCI) from 0 to 1. Among the factors on which the BCI is based are temperature, chemistry, orbital characteristics, age and whether the planet is thought to be solid, liquid or gas a the surface.

Most planets don't stack up that well, but ten of the 637 for which adequate information was available (1.6%) come out better than Europa. Even if there is just one planet per star, much lower than we are now coming to suspect, this means there would be 100 million planets better suited to life than Europa. Within the sample five planets scored better than Mars.

“Other scientists have tried to make educated guesses about the frequency of life on other worlds based on hypothetical assumptions, but this is the first study that relies on observable data from actual planetary bodies beyond our solar system,” said Irwin.

There is, however, a big hole at the core of such reasoning – we don't know if there is life on Europa. Moreover, if it does exist, is it just single celled organisms, or something that would qualify as “complex” on the paper's definition, “diverse in size (including macroorganismic), form, history, and distribution". 

Any discussion of these numbers leads inevitably to the Fermi Paradox. If life is so common, why haven't we seen evidence more convincing than lights in the sky and people claiming to have been rectally probed on a dark highway at night?

Irwin notes that, given the size of the galaxy, even with a 100 million potential locations, the average distance between these locations is 24 light years, assuming random distribution. Out here where the stars have started to thin the distance is probably even greater. While 24 or even 50 light years might be a manageable distance to cross for a well established civilization, no one thinks Europa is likely to host technologically advanced life (well no one but Arthur C Clarke).

Consequently, the nearest species capable of crossing the vast gulfs between worlds may have a very large journey indeed to get to us. Besides the problem of knowing how likely life is to form where conditions are right, the study can only work with the sample we have. Since it is much harder to find small planets than large ones we are picking up a set that is not truly representative. However, this is more likely to cause an underestimation of the number of habitable worlds than an overestimation. Moreover, since it is likely that many planets would have moons, some at least as suitable as Europa, the prospective number of potential locations looks conservative. 

All five known exoplanets rated higher than Mars are around red dwarf stars.

One interesting aspect to the work is the comparison of the most promising explanets with locations in our solar system. On Irwin's methodology Europa is 0.71 and Mars 0.83, suggesting if we do not find life on either there something may be wrong with the scoring. On the other hand, while the Earth gets a score of 0.97, as it would seem to observers in another system, Irwin gives Gliese 581c a perfect score. This flies in the face of other thinking about the suitability of this “superearth” as a home for life, revealing how much is still controversial in this emerging field.