Toxic Atmospheres Could Halve the Number of Planets in the "Habitable Zone"

New study highlights a new and critical variable in our search for complex extraterrestrial life

National Radio Astronomy Observatory

In a new study undertaken by the University of California in Riverside, researchers suggest that previous estimates about the number of planets that could support complex life might be significantly too high. The real number is much lower.

Traditionally, much of the search for extraterrestrial life has focused on what scientists call the “habitable zone.” Sometimes called the Goldilocks zone, it gets this name because it’s an area that’s not so close to the star that water just boils away, or too far so that everything’s frozen, but in the middle, where it’s just right.

That description works for basic, single-celled microbes, but not for more complex lifeforms…and therein lies the importance of this claim.

The team’s work, published in The Astrophysical Journal shows that accounting for predicted levels of certain toxic gases narrows the habitable zone for complex life by at least half — and in some instances eliminates it altogether.

“Imagine a ‘habitable zone for complex life’ defined as a safe zone where it would be plausible to support rich ecosystems like we find on Earth today,” says Timothy Lyons, professor of biogeochemistry in UCR’s Department of Earth and Planetary Sciences and one of the study’s co-authors. “Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”

Using computer models to study atmospheric climate and photochemistry on a variety of planets, the team first considered carbon dioxide. Planets too far from their host star require carbon dioxide — a potent greenhouse gas — to maintain temperatures above freezing. Earth included.

The study concludes that carbon dioxide toxicity alone restricts simple animal life to no more than half of the traditional habitable zone. For humans and other higher-order animals, which are more sensitive, the safe zone shrinks to less than a third of that area.

What’s more, no safe zone at all exists for certain stars, including two of the sun’s nearest neighbors, Proxima Centauri and TRAPPIST-1.

The type and intensity of ultraviolet radiation that these cooler, dimmer stars emit can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide binds to hemoglobin in animal blood — the compound that transports oxygen through the body. Even small amounts of it can cause the death of body cells due to lack of oxygen.

"Showing how rare and special our planet is only enhances the case for protecting it. As far as we know, Earth is the only planet in the universe that can sustain human life."

Carbon monoxide cannot accumulate on Earth because our hotter, brighter sun drives chemical reactions in the atmosphere that destroy it quickly. Although, the team concluded recently that microbial biospheres may be able to thrive on a planet with abundant carbon monoxide.

Artist's rendition of the TRAPPIST-1 exoplanets


Scientists have confirmed nearly 4,000 planets are orbiting stars other than the sun, but none of them will be possible to visit in person. They are simply too far away. Closest is Proxima Centauri b, which would take 54,400 years for current spacecraft to reach. Using telescopes to detect abundances of certain gases in their atmospheres is one of the only ways to study these so-called exoplanets.

So, why hasn’t this been noticed before?

Speaking to Inverse, Edward Schwieterman, the study’s lead author and a NASA Postdoctoral Program fellow, put it this way:

“The real difference between what’s been done before and what we’re putting forward with our study is the difference between what are the minimum conditions for a simple life, and what are the conditions for more complex forms of life.
“The definition of the habitable zone is focused on one thing, which is the potential for liquid water, but the greenhouse warming necessary to maintain liquid water in the outer regions of the habitable zone requires a lot of CO2.
“If you’re imagining what does habitable mean? Who is the habitable zone for would be a way to frame this. Is it for microbes? Or is it for humans? Or other forms of advanced life? A lot of people imagine that these are the same thing, and they’re not the same thing.”

What NASA's Habitable Exoplanet Observatory might look like


Findings from the team’s previous work is already informing next-generation space missions such as NASA’s proposed Habitable Exoplanet Observatory. For example, because oxygen is essential to complex life on Earth and can be detected remotely, the team has been studying how common it may be in different planets’ atmospheres.

Other than Earth, no planet in our solar system hosts life that can be characterized from a distance. If life exists elsewhere in the solar system, Schwieterman explains, it is deep below a rocky or icy surface. So, exoplanets may be our best hope for finding habitable worlds more like our own.

“I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman says. “As far as we know, Earth is the only planet in the universe that can sustain human life.”

The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet’s surface. Substantially more CO2 than present in Earth’s modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.
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