Since 1992 astronomers have discovered more than 5000 planets around other stars. Meanwhile, the field of astrobiology has been steadily evolving, and scientists are getting closer to understanding the possibilities of life beyond Earth. The recent National Academies decadal surveys on astronomy and astrophysics and on planetary science and astrobiology highlight the importance of habitability studies to the search for life on the planets of our solar system as well as on exoplanets. Unfortunately, the scientific community is not yet taking advantage of decades of knowledge gained in habitability assessments.
Habitability is generally defined as the suitability for harboring life, usually referring to human occupancy. In the context of planetary science and astrobiology, it has come to refer to a set of environmental conditions capable of supporting any life. One of the major standing problems facing astrobiologists is how to measure the right conditions for life—or more formally, how to quantify the habitability of Earth and compare it with everything else in the universe.
Addressing that problem is essential because the study of planetary habitability guides both where to search for life and what those searches should look for. After identifying planets that are potentially habitable, researchers would use the recently launched James Webb Space Telescope (JWST) and other instruments to help determine if some of those worlds also have the right atmospheres for simple microbial life such as bacteria or archaea, or even complex life such as plants and animals. Another approach could be to search for indicators of intelligent life, or technosignatures.
The traditional astrobiology approach for determining habitability on an extraterrestrial world is to examine a set of factors such as temperature and the availability of water. One of the main habitability guidelines is the habitable zone. Broadly defined as any spatial region capable of supporting life, for exoplanets it is considered the region around a star where Earth-like planets might support long-standing surface oceans. Planets in the habitable zone are not necessarily viewed as being habitable in the broadest sense of the word, but rather as being capable of supporting one crucial ingredient for life: surface liquid water.
However, the definition of the habitable zone around stars has limitations (see the commentary by David J. Stevenson, Physics Today, November 2018, page 10). Planets with subsurface oceans or other exotic conditions may have their own habitable zones, even if they are unlikely to have liquid water on the surface. Similar zones featuring potentially life-friendly conditions can be defined at the planetary, stellar, and even galactic scales. In addition, it is a binary assessment—either a planet is in its star’s habitable zone or it’s not. There is no way that such a test, or any other habitability assessment, would definitively answer the question of whether a planet is capable of supporting life. The only way to determine that is by finding life on the planet or introducing life to the planet. When you try to force a habitability yes or no answer, you risk being very wrong and putting too much confidence in the results.
Instead of asking, “Which worlds are habitable?” we should ask, “How suitable are these worlds for life?” It is a subtle difference, but it implies very different research approaches. Fortunately, astrobiologists do not need to invent new definitions of habitability. They just need to implement decades of knowledge about the right conditions for life on good old Earth.
More than 40 years ago, ecologists developed a method to define and quantify habitability known as habitat suitability, the friendliness of an environment to life. It is measured as a quantity proportional to the carrying capacity of a system, usually on a simple 0-to-1 scale, relative to some standard of comparison. Why do ecologists talk about the habitability of ecosystems, or the whole Earth, if we already know they are habitable? Again, that is because habitability is not about finding out if environments are habitable or not, a simple binary logic; rather it’s about how suitable a set of environmental variables are for life, a spectrum of possibilities.
For example, the most habitable environments on our planet are the rainforests. They have a large carrying capacity, producing more biomass per unit of space and time (known as biological productivity) than any other terrestrial biome. A mere two variables, precipitation and temperature, can explain most of the global patterns of terrestrial productivity. They are not the only variables that come into play. But the fact is that we can simplify the problem, identifying the main factors that control and limit biological productivity with just a few variables. That should be the primary goal of habitability assessments.
The concept of habitat suitability provides a general framework for astrobiologists to construct a standard library of habitability models for specific environments, variables, and types of life. For example, researchers could analyze the habitability of Venusian clouds for microbial life as a function of altitude, temperature, and humidity. The standard for comparison on the 0-to-1 scale would be clouds on Earth. (That approach may look pretty Earth-centric, but it is the only way to recognize that we’re applying our understanding of terrestrial habitability to environments that are not necessarily Earth-like.) Researchers could then couple multiple models to study increasingly complex systems. The inclusion of different variables in additional models would enable a more complete picture of habitability.
Improving habitability studies would also provide context for the results of subsequent life-detection searches. For example, it would be amazing to use the JWST to identify an exoplanet with both oxygen and methane in its atmosphere, which is thought to be a strong biosignature. More amazing would be to also determine that a large biosphere would not be compatible with the surface temperatures of the exoplanet. Would such seemingly contradictory findings mean we need to question our observations or our habitability assessments? Or might we be looking at life as we do not know it? Thus, we use habitability assessments to identify where to look and to make sense of results after the observations.
Astrobiologists should take advantage of all the work theoretical ecologists have done to develop mechanistic and empirical models for habitability. In fact, we should collaborate with ecologists to understand the potential for life on exoplanets. It is noteworthy that physicists can write equations about atomic and cosmological scales, far from our living experiences, yet cannot write the equations of life right under our noses. Biology is harder than physics.
Abel Méndez is a planetary astrobiologist and the director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.
