Five Criteria for Assessing the Possibility of Life on Other Planets

Scientists want to answer the question, is there life out there in the universe? Do you think astronomers want detailed measurements of eight solar system planets or limited measurements of 1,000 exoplanets?

extrasolarFortunately, they have both; unfortunately, those limited measurements don’t yet provide a definitive answer regarding the presence of life—at least not for all the exoplanets. However, as the number of known exoplanets grows, so do the number of tools scientists develop to assess the likelihood of extraterrestrial life.

One new tool combines the limited information gleaned from exoplanet data and the detailed information from our solar system to assess the relative probability of life existing on various planets. Five categories of information—geophysics, substrate, energy, temperature, and age—accessible by current technology (or by straightforward inference from that data) provide the basis for evaluating an exoplanet’s potential capacity to host life.

Geophysics and substrate overlap at some level (for example, a planet’s density will influence whether it is a gas giant or rocky planet), but they also contain independent information. Thus, proper characterization of an exoplanet’s relative capacity to host life must include both categories. Geophysics relates directly to an exoplanet’s density and eccentricity, two observable properties. Substrate refers to an exoplanet’s differentiation. Gas giants (all atmosphere) and small rocky planets (with no atmosphere) represent the least differentiated class. Planets with a solid surface and simple atmosphere reside further up the scale. The most differentiated bodies resemble Earth—a rocky surface, standing bodies of liquid, and a complex atmosphere.

Energy accounts for the two quantities available to drive the chemical reactions required by life. On Earth, many organisms ultimately derive energy from the Sun’s radiation via photosynthesis. However, others extract energy from chemical gradients present in the rocks composing Earth’s crust. So, energy includes the amount of stellar flux received by the planet and the “redox” molecules available.

Temperature characterizes where the temperature of the planet’s surface and subsurface falls in the range dictated by liquid water. Age accounts for the fact that the development of life here on Earth required a vast amount of time until the planet could support human (or even multicellular) life.

Depending on starting assumptions about the optimal values for these categories, one can calculate a biological complexity index (BCI) where higher BCI values indicate greater likelihood of supporting life. For example, one group of researchers found, based on the assumptions they made, that around 1.5% of exoplanets have a higher BCI than Europa and just under 1% exceed the value for Mars.1 Remember though, despite scientists’ optimism that one or both of these bodies might have hosted life at some point, no evidence exists to indicates that either one ever did.

High BCI values will help focus future searches for life’s signatures by identifying the most likely candidates on which to expend precious telescope resources. More important, in my opinion, is the potential to evaluate different assumptions about the how common life is in the universe. Changing the starting assumptions will lead to different BCI values so measurements of any life signatures will validate or falsify different rankings. I plan to show some specific examples of how this might work in a future article.

The importance of this research is not primarily the BCI values associated with a given exoplanet but the framework it develops to guide future research. The framework includes the currently available information from exoplanet searches but is flexible enough to incorporate data that future projects expect to measure—like atmospheric composition, cloud cover, surface chemistry, rotation rate, etc. If God truly designed Earth to support human life (and I believe He did), then a framework like these BCI calculations will help provide scientific evidence in support of that assertion.


  1. Louis N. Irwin et al., “Assessing the Possibility of Biological Complexity on Other Worlds, with an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy,” Challenges 5 (June 2014): 159–74.
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