A survey conducted by astronomers at Cornell University has taken into account the characteristics of 637 known exoplanets and elaborated a Biological Complexity Index (BCI) to assess the relative probability of finding complex life on them. Their data supports the view that as many as one hundred million planets scattered around the Milky Way, and perhaps more, could support life beyond the microbial stage.

The Biological Complexity Index

We know that organic molecules are present in star-forming regions, protoplanetary disks, meteorites, comets, and even deep space; moreover, water is among the most common molecules in the Universe, and energy is available in many forms both on the surface and deep within a planet. These reasons lead us to believe that may be other forms of life even within our galaxy; however, the question of just how many might be there has been a topic of speculation for decades.

In 1961, American astronomer Frank Drake proposed a formula that could be used to roughly estimate of the number of intelligent and technologically advanced extraterrestrial civilizations in the Milky Way. However, we have so little data on the world outside our solar system that estimating the parameters of this formula accurately is next to impossible. Depending on your initial assumptions, the number of advanced civilizations in our galaxy according to Drake's equation could range from virtually zero to a whopping 36.4 million.

Now, a group of scientists at Cornell University led by associate professor Alberto Fairén have proposed a formula that takes into account the characteristics of over 600 known exoplanets to help estimate how likely we are to find complex life on them. By "complex life," the researchers don't mean necessarily technologically advanced or even highly intelligent life, but rather life forms that are above the microbial level and form stable food chains like those found in ecosystems on Earth.

The researchers took into account the density, temperature, substrate, chemistry, distance from its star and age of a given exoplanet, combining these parameters into a unique metric that they call the Biological Complexity Index (BCI).

The BCI can use the limited information in our possession to assess the relative likelihood of different planets to host complex life (Image: Cornell University)

The index doesn't represent an absolute statistical prediction of whether complex life could be present on a planet; rather, it can be used to estimate the relative likelihood of life having evolved there, based on the conditions that we know are compatible with the evolution of complex life forms on a planet, and assuming that no further information is available.

In essence, an outside observer could use the index to compare two planets or moons which are light-years away and with only limited, easily detectable information at his disposal, tell which one is the most likely to harbor life.

Life on other planets

Prof. Fairén and colleagues have used the BCI index to assess the habitability of 637 known exoplanets for which they had access to all the necessary parameters. According to their report, 11 of those exoplanets (1.7 percent of the sample) have a BCI above that of Europa, and five (0.8 percent of sample) have a score higher than Mars. Although that number might seem small, when extrapolating it to the entire galaxy this means there may be north of 100 million planets in our Milky Way alone on which complex life has plausibly evolved.

Of course, the accuracy of this estimation is constrained by the limited amount of data that we have on those planets. For instance, our instruments aren't currently powerful enough to detect Earth-size planets that are very far away, and this might mean that the estimate is actually a conservative one. On the other hand, some planets that might look hospitable from light-years away may not look as good after a closer look.

According to some astronomers, worlds that are larger, warmer, and older than Earth, orbiting dwarf stars, are probably the most likely candidates for hosting complex life. The results from Fairén team's survey are in accordance with this theory, as all five exoplanets detected with a BCI value higher than Mars have exactly these characteristics.

Curiously enough one of the planets, Gliese 581c, has an even higher BCI value than Earth. Again, this is not a comment on the absolute likelihood of finding complex life there; rather, it means that if an external observer (such as a technically advanced alien civilization) were to observe both Earth and Gliese 581c from light-years away, with only limited information at their disposal, they might be led to conclude that Gliese 581c is the more likely candidate for hosting life – at least, if they were using the same formula.

The Biological Complexity Index plotted against the Earth Similarity Index (Image: Cornell University)

We know quite well from looking at the fossil record that life appeared on Earth very soon after the environmental conditions were favorable on the surface. Therefore, a further refinement might be to combine the BCI with a second metric that takes into account how similar a planet is compared to Earth. The researchers have therefore proposed an "Earth Similarity Index" (ESI) rates the similarity of extrasolar planets to Earth on the basis of mass, size, and temperature.

Overall, the data produced by the researchers supports the idea that the evolution of complex life on other worlds is relatively rare across our galaxy, but still extremely large in terms of absolute numbers. So, even though they may very well be countless other advanced forms of life in the Milky Way, we are so far from one another that we are unlikely to make the trip there in the foreseeable future.

When the James Webb Space Telescope – an instrument so powerful that it could easily detect a firefly from a distance of 240 thousand miles (385,000 km) – launches in 2018, we will be able to gather much more accurate data on which to base our estimations.

The researchers describe their findings in an open-access paper published on the journal Challenges.

View gallery - 3 images