I have visited the Dead Sea, 1,412 feet below sea level, and have climbed 14,000-foot-high mountains. In both instances I noticed a decrease in lung function. For advanced life, lungs provide the highest possible respiration efficiency.1 That efficiency, however, requires a narrow air pressure range.2
Air pressure fine-tuning is not the only requirement for advanced life. A new paper published by five planetary astronomers led by Edward Schwieterman reveals two more requirements that astrobiologists had not previously considered in their quest to find planetary habitable zones.3 The two requirements are very narrow carbon dioxide and carbon monoxide ranges for aerobic (oxygen-consuming) life.
Atmospheric Carbon Dioxide Levels and Habitability
The primary theme of the paper is that the planetary zones for habitability are much narrower for complex aerobic life than they are for microbial life. Most astrobiologists consider the habitable zone for liquid water to be relatively wide because the amount of carbon dioxide (a powerful greenhouse gas) in the planet’s atmosphere can be adjusted to compensate for a range of host star luminosities. (The liquid water habitable zone is the planet’s range of distances from its host star where liquid water could conceivably exist on its surface.)
Certain microbes can tolerate a wide range of atmospheric carbon dioxide concentrations. However, as the paper’s authors point out, complex aerobic life, and especially large animals, cannot. Photosynthesis ceases to operate for atmospheric carbon dioxide levels below 0.000153 bar. (1 bar = 0.987 the atmospheric pressure of Earth at sea level.) Large-bodied mammals and birds will starve to death at atmospheric carbon dioxide levels below 0.00018 bar. On the other hand, exposure to atmospheric carbon dioxide levels above 0.0051 bar, even for just half a day, prove deleterious for mammals.4 Respiratory acidosis, ion buffering changes in internal body fluids, and circulatory arrest are just three of the biological consequences of elevated carbon dioxide levels.
Ecophysiologists Astrid Wittmann and Hans Pörtner demonstrated that atmospheric carbon dioxide levels as low as 0.00095 will generate ocean acidification that will cause many species of marine corals, echinoderms, mollusks, crustaceans, and fish to disappear.5 Such a scenario would upset the entire marine ecosystem and catastrophically impact the quantity of food humans can garner from Earth’s oceans and lakes.
The wide liquid water habitable zones predominantly adopted in astrobiology research papers presume that atmospheric carbon dioxide levels can range from 0–15 bars. However, the range for an effective aerobic animal ecosystem is limited to 0.00018–0.00095 bars, meaning that for such life the liquid water habitable zone must be extremely narrow.
Schwieterman and his colleagues considered the impact of atmospheric carbon dioxide levels only on the liquid water habitable zone. I believe their report implies that several of the other ten known planetary habitable zones6 likewise must be narrower to accommodate higher life-forms than what is predominantly adopted in the astrobiology research literature.
Atmospheric Carbon Monoxide Levels and Habitability
Complex aerobic life possessing a circulatory system requires a minimum of 10 percent molecular oxygen in the atmosphere. In a separate paper, Schwieterman and his colleagues showed that for planets with this much oxygen that orbit stars much dimmer than the Sun, certain photochemical conditions will result in high atmospheric carbon monoxide levels.7
Planets orbiting stars dimmer than the Sun will receive less near-ultraviolet radiation. With this deficit, even with no more than 10 percent oxygen, even with low carbon dioxide levels in the atmosphere, and even with abundant surface liquid water, so little OH (hydroxide) is produced in the atmosphere that the lifetime of carbon monoxide is greatly lengthened. That is, even for planets orbiting stars that are only slightly dimmer than the Sun there will be higher levels of carbon monoxide in the atmosphere than is the case for Earth.
The simpler life-forms that must accompany complex aerobic life also produce carbon monoxide. Known examples of such production include phytoplankton,8 biomass burning,9 and photolysis of dissolved organic matter in surface layers of oceans.10
Carbon monoxide is highly toxic for life-forms with circulatory systems because molecules like hemoglobin have an affinity for carbon monoxide that is 234 times greater than it is for molecular oxygen.11 For humans, exposure to carbon monoxide levels above 100 parts per million for 10 or more hours can lead to permanent organ impairment and death. Exposure to carbon monoxide levels above 9 parts per million for longer than 8 hours impairs human health.12 Medical researchers calculate that continuous exposure to carbon monoxide above 1 part per million for a decade or more likewise brings on serious human health impairment (though adequate research affirming this calculation is lacking).
Schwieterman’s team calculated that all planets orbiting cool M-type (low mass, low luminosity) stars containing as much or more concentration of tropospheric water vapor as Earth will possess too much atmospheric carbon monoxide to sustain aerobic complex life. Planets orbiting all M-type stars that possess significantly less tropospheric water vapor by volume than Earth likewise will possess too much atmospheric carbon monoxide. Planets with a much lower surface area covered by liquid water that orbit larger, brighter, K-type stars also will possess too much atmospheric carbon dioxide.
Habitability and Design Implications
M-type and K-type stars make up about 88 percent of all existing stars. Carbon monoxide production on planets orbiting these stars will eliminate nearly all such planets from being able to host complex aerobic life. O-type, B-type, A-type, and F-type stars make up about 5 percent of all stars. These stars are more massive and much brighter than the Sun. But they burn up too quickly and their luminosity increases too rapidly for any of them to be possible candidates to host advanced aerobic life.
The Sun is a G-type star. These make up about 7 percent of all stars. Planets orbiting G-type stars that are drier than Earth may possess too much atmospheric carbon monoxide for advanced life. All planets orbiting G-type stars likely will possess either too much or too little carbon dioxide for advanced life.
In the context of complex aerobic life, Schwieterman and his colleagues have demonstrated that potential habitability on planets beyond Earth is much less likely than what astrobiologists had previously thought. When combined with hundreds of other planetary features that must be fine-tuned to extraordinary degrees,13 these research findings provide yet more evidence that Earth is no cosmic accident. The weight of physical evidence testifies of a supernatural, super-intelligent Creator who intervened to manufacture a home where billions of humans could thrive and discover the means by which they can enter into an eternal, loving relationship with him.
Featured image: Artist’s Rendition of a Possible Earth-Like Exoplanet