“Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying.” – Arthur C. Clarke
Ever since humans turned their collective eye to the heavens we have pondered the question “are we alone in the universe?” The existence of extraterrestrial life has long been a subject of human fiction and a subject of great debate among scientists. Proponents who argue for the existence of extraterrestrial life usually take the massive size of the observable universe as more or less proof that life exists elsewhere.
The universe is so large and has been around for so long, it is said, that it is essentially guaranteed that life exists elsewhere, dappled among the stars like oases of complexity in the infinite desert void of space. Simply taking the size and time-scale of the universe into account, it seems almost certain that the exact conditions that gave rise to life on Earth have occurred elsewhere in the universe, presumably many time.
Generally, discussions on the possibility of the existence of extraterrestrial life are focused on examining the “habitable zone” of planets —the range of distances from the parent star where it is warm enough that liquid water could exist on the surface of the planet. To date, over 3,900 exoplanets have been discovered, many of them falling within the proverbial “Goldilocks zone;” regions where the conditions for life are just right. However, a new study may require scientists to rethink their ideas about how many of these planets could theoretically support complex life.
The study, published in The Astrophysics Journal, shows that toxic levels of carbon dioxide and other gases in the atmospheres of these exoplanets drastically constrain the size of the traditionally conceived habitable zones by as much as 50%, eliminating them entirely in some cases. The study seems to imply that the conditions to support complex life cannot exist in most regions of the habitable zone as traditionally defined. By using computer simulated models of planetary atmosphere formation and photochemistry, the scientists demonstrated that the safe zone for many known exoplanets is actually about a third of the originally assumed size.
The new study may make scientists have to reconsider their notions of when complex life is possible and leads to the conclusion that the emergence of life as we know it may actually be a much rarer occurrence than previously thought.
Habitable Zones And Complex Life
Traditionally, the habitable zone surrounding a star is understood as the range of distances from the parent star in which it is possible that liquid water exists on the surface, given the right atmospheric pressure. The exact range of the habitable zone is determined by the radiative intensity of the particular star and the composition of the planet’s atmosphere. Specifically, the planet needs to be the right distance from the star and have the right atmospheric concentration of greenhouse gases to both trap heat on the surface and not be too hot. The inner edge of the habitable zone is defined as the area before which greenhouse gases would vaporize the liquid water on the surface and the outer edge of the habitable zone is defined as the point where adding more carbon dioxide to the atmosphere would not be able to keep temperatures above freezing.
The search for extraterrestrial life is almost always predicated on the discovery of liquid water because water is essential for life as we know it. We also know that in order for carbon-based complex multi-cellular life to exist there needs to be oxygen. It is assumed oxygen is required for life because no other known chemical substance is capable of producing the high levels of free energy that oxygen does during cellular respiration.
So basically, in order for complex life like ours to exist on a planet, it seems that the determinate factors, aside from distance from the parent star, are both the levels of carbon dioxide and oxygen in the planet’s atmosphere.
Habitable Zones For Complex Life Are Smaller Than Previously Thought
And so lie the motivation for the considered study: a desire to see how atmospheric concentrations of carbon dioxide and oxygen could affect the development of complex life on a planet within the traditionally conceived habitable zone.
The team first considered carbon dioxide in their simulations. Everyone knows that too much carbon dioxide is bad for humans. However, planets need a certain level to trap enough heat to keep the surface at habitable temperatures. By estimating the levels of carbon dioxide present in the atmosphere of exoplanets located around certain classes of stars, the researchers could determine whether these planets would be capable of supporting complex life. These simulations also took into account the theoretical concentrations of dissolved carbon in the planet’s biosphere.
Their simulations show that, for planets near the edge of the traditionally-defined habitable zone, atmospheric levels of carbon dioxide would have to be 1000 to 10,000 times greater than the highest estimated values of CO2 concentrations on Earth in the past 500 million years. Moreover, they found that these levels were almost 2 orders of magnitude larger than the lethal levels of CO2 for the most CO2 tolerant known organisms.
Essentially, these results seem to imply there is a hard upper limit on how high levels of CO2 can get before any complex life would be poisoned and this upper limit constrains the habitable zone for relatively simple microbial life such as bacteria and archaea by almost 50%. For more complex animal life, the habitable zone range shrinks to almost a third of the original size.
Additionally, the results effectively eliminate the habitable zone for some stars including our closest stellar neighbors Proxima Centauri and TRAPPIST-1. The lower temperatures of these stars emit less UV radiation which can result in high atmospheric concentrations of carbon monoxide (CO). Carbon monoxide is extremely poisonous for oxygen-based life as it prevents the mechanisms responsible for cellular respiration. These result from the researchers simulations imply that planets orbiting near the inner boundary of lower temperature stars would also have an atmosphere inhospitable for complex life, although the researchers admit the possibility of simple microbial life in CO-rich atmospheres.
The main point of the study is in how it shows the habitable zone is a much narrower portion of the area around a star than previously thought.
So Where Can Complex Life Exist?
Even assuming an abnormally high resilience to CO2, the range of possible atmospheric concentrations for a planet shrinks the habitable zone. According to the study, physiological CO2 thresholds of 0.01, 0.1, and 1 bar correspond to habitable zones that are 21%, 32%, and 50% as wide as the traditionally conceived habitable zone. Further, the researchers take their findings as precluding the existence of complex life on any planet orbiting a parent star with an effective temperature of less than 3200 K due to carbon monoxide.
Additionally, the results are taken as implying that complex life is most likely to form on planets that show a decline in CO2 concentrations over a geological time scale. This is because as stars get older, they burn hotter, bringing the habitable zone close to the star. This increase in stellar luminous intensity must be matched by decreasing levels of CO2 in order to maintain the temperatures and gas concentrations necessary for complex life.
The practical takeaway from the study is that the results should direct the future search for extraterrestrial life by considering only those planets in the much more constrained habitable zone. The researchers state that they hope their results will direct further research from scientists at SETI.
Of course, no study is without limitations. Primary among these is the assumption that complex life can only take a for similar to ours. While there are good theoretical reasons for assuming this, it is unknown whether complex life can exist based on a biochemistry radically different from our own. It is possible some forms of complex life may not require liquid water, carbon, or oxygen, so any conclusions drawn from the study are limited in virtue of the researchers’ conception of what the necessary conditions for complex life are.
The researchers also point out the limitations of their 1-D computational models, pointing out the need for future 3-D simulations of planetary atmospheric conditions. Specifically, more complex models can account for factors like continent size, surface albedo (reflection) atmospheric mass, gravitational interactions, and orbital parameters like axial tilt.
Conclusions: Is Earth Rare?
Proponents of the “rare Earth” hypothesis claim that the development of complex sexually reproducing life on Earth is an exceedingly rare and astronomically improbable event. According to this hypothesis, the existence of complex extraterrestrial life is improbable and very rare.
According to the team’s findings. the rare Earth hypothesis may have more evidential backing than previously assumed. If the habitable zone for complex life is much smaller than previously assumed then that eliminates from consideration many exoplanets that were previously thought to be capable of harboring complex life. The study seems to imply that life on class M and K stars is unlikely, and M and K class stars make up almost 82% of known stars.
Regardless of what the study implies about the broader search for extraterrestrial life, it at least implies that complex life as we know it requires a much more specific set of circumstances than previously thought.