The habitable zone for complex life around many stars could be much smaller than previously thought once the concentrations of carbon monoxide and carbon dioxide on planets is considered. That is the conclusion of astrobiologists in the US, who say that high concentrations of these gases could completely preclude the existence of life on planets orbiting some stars.
The search for extraterrestrial life often focuses on what is known as the “habitable zone” of stars. This is commonly defined as the range of distances from a host star warm enough for liquid water, a key requirement for life, to exist on a planet’s surface. However, according to Edward Schwieterman, at the University of California, Riverside, and his colleagues that description works for basic, single-celled microbes, but not for more complex creatures – everything from simple sponges to humans.
They point out that substantially more carbon dioxide than present in Earth’s atmosphere is needed to maintain suitable temperatures over much of the traditionally-defined habitable zone. At the outer edge of the zone, concentrations of several bars would be required, they say, yet most complex aerobic life on Earth is limited by concentrations of just fractions of a bar.
Abundance of toxins
Another issue is carbon monoxide. On Earth this toxic gas does not accumulate, because the Sun drives chemical reactions in the atmosphere that destroy it quickly. Most exoplanets in the traditional habitable zone, however, orbit red dwarfs. These are cooler, smaller stars than the Sun, and are predicted to promote greater abundances of gases like carbon monoxide in the atmospheres of orbiting planets.
“Most previous investigations have focused either on climate, particularly the maintenance of temperatures above the freezing point of water, or on potential biosignature gases like oxygen or methane rather than gases that could be toxic,” Schwieterman says. “Folding in physiological limitations of complex life is another step that requires applying an additional knowledge base.”
To define a habitable zone for complex life (HZCL), the team used computer models to predict atmospheric climate and photochemical conditions around stars with spectral characteristics ranging from those of F-type stars to red dwarfs. To help classify this zone, they also looked at known toxicity limits for a range of organisms.
Complex organisms on Earth
Upper long-term physiological carbon dioxide tolerances from a range of complex organisms on Earth suggest that complex life might be able to tolerate carbon dioxide concentrations of up to 0.05 bar – although lower concentrations would be lethal for most animals. When the researchers assumed carbon dioxide tolerances for complex life of 0.01, 0.1, and 1 bar, they found that the HZCL around a Sun-like star is only 21%, 32%, and 50% as wide as the conventional habitable zone, respectively.
This means that many planets that would normally be within the habitable zone would be unsuitable for complex life. For example, the exoplanet Kepler-62f is within the habitable zone of its star, but the researchers believe that it would need carbon dioxide concentrations of 3–5 bar to maintain surface conditions for liquid water. This is approximately 1000 times greater than has occurred during the entire history of complex life on Earth.
Earth is closer to the edge of Sun’s habitable zone
When they considered carbon monoxide, they found that no safe zone at all exists around some red dwarfs. This includes two of our closest stars, Proxima Centauri and TRAPPIST-1. This is because the type and intensity of ultraviolet radiation that these stars emit can lead to high concentrations of carbon monoxide.
Schwieterman says that these results mean that we are less likely to find complex life on planets in the middle and outer habitable zone. “Planets in the outer region of the habitable zone require either carbon dioxide levels so high they’d be toxic to life like animals as we know them on Earth – or their surfaces would be frozen,” he explains. “Alternatively, other greenhouse gases could contribute to warming, but most of these gases are incompatible with high levels of oxygen, which is required for complex life as we know it. We can’t completely rule out alternative biochemistries that could compensate for extremely high carbon dioxide, but we have no reason to think they may exist either.”
According to Schwieterman the work can be seen as both pessimistic and optimistic. “On one hand, I think we should temper our expectations for the range of planetary conditions amenable for intelligent life,” he says. “On the other hand, we can optimize our search for intelligent life and stand a better chance of finding it if it’s out there.”
The research is described in The Astrophysical Journal.