Two More Habitable Zones Expand the Case for Design

If you've ever visited the Dead Sea (1,412 feet below sea level) or climbed any high mountain (15,000 feet or more above sea level), you know how altitude affects breathing. Our lungs are incredibly robust, providing the highest conceivable respiratory efficiency for advanced, high-metabolism life.1 Their efficient operation, however, is possible only if the atmospheric pressure falls within a certain range.2 If Earth's air pressure were three times greater or lesser than it is at sea level, respiration would be impossible for advanced, high-metabolism organisms.

A just-right atmospheric pressure belongs to a long list of requirements for the existence of advanced life. A recent paper published by five planetary astronomers led by Edward Schwieterman of the University of California, Riverside (UCR), reveals two more additions to that list—requirements that astrobiologists had not previously considered in their quest to find habitable planets.3 This new set of findings indicates that aerobic (oxygen-­consuming) life depends on just-right atmospheric abundances of carbon dioxide and carbon monoxide.

Atmospheric Carbon Dioxide Levels & Habitability

A habitable zone is the range of orbits around a star within which a planet can support life. Most astrobiologists only consider the liquid water habitable zone and view this zone as relatively wide (see Figure) because the amount of carbon dioxide (a potent greenhouse gas) in a planet's atmosphere can take on different values to compensate for variations in stellar brightness. The Schwieterman paper explained how such zones are much narrower for complex aerobic creatures than they are for microbes.

While certain microbes can tolerate a wide variation in atmospheric carbon dioxide concentrations, complex aerobic life, and especially large animals, cannot. Photosynthesis ceases to operate when atmospheric carbon dioxide levels dip below 0.000153 bar (1 bar = 0.987 Earth's atmospheric pressure at sea level). At levels below 0.00018 bar, large-bodied mammals and birds would starve to death, due to the lack of photosynthetic food production. On the other hand, even brief exposure to levels above 0.0051 bar would prove deleterious for mammals.4 Elevated carbon dioxide levels would cause severe biological consequences, including ion buffering changes in internal body fluids, circulatory arrest, and respiratory acidosis.

Ecophysiologists Astrid Wittmann and Hans Pörtner have demonstrated that atmospheric carbon dioxide levels as low as 0.00095 bar would generate a level of ocean acidification sufficient to destroy many species of marine corals, echinoderms, mollusks, crustaceans, and fish.5 Such a scenario would upset the entire marine ecosystem and catastrophically impact the quantity of food that comes from Earth's oceans and lakes.

While most astrobiology research papers presume that atmospheric carbon dioxide levels can range from 0–15 bars, the range for an 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. It would appear that for higher life-forms, several of the other known planetary habitable zones (see Table on next page) likewise must be narrower than what astrobiologists typically report.

Atmospheric Carbon Monoxide Levels & Habitability

Complex aerobic life—specifically, life dependent on a circulatory system—requires an atmosphere with a minimum of 10 percent molecular oxygen. In a separate paper, the UCR team showed that on planets that have this much atmospheric oxygen and that orbit stars dimmer than the Sun, photochemical conditions will result in high levels of atmospheric carbon monoxide.6

Planets orbiting stars dimmer than the Sun receive less near-ultraviolet radiation. Even with low levels of carbon dioxide in their atmospheres and abundant surface liquid water, such planets would produce so little atmospheric hydroxide (OH–) as to greatly lengthen the lifetime of atmospheric carbon monoxide. That is, even for planets orbiting stars only slightly dimmer than the Sun, dangerously high levels of atmospheric carbon monoxide would be present.

Simpler life-forms belonging to the ecosystem of complex aerobic life also produce carbon monoxide. Known carbon monoxide producers include phytoplankton,7 biomass burning,8 and photolysis (decomposition or separation of molecules by light) in dissolved organic matter within the oceans' surface layers.9

Carbon monoxide is highly toxic for life-forms with circulatory systems because molecules such as hemoglobin have an affinity for carbon monoxide, an affinity 234 times greater than their affinity for molecular oxygen.10 For humans, 10 hours' exposure to carbon monoxide levels above 100 parts per million can lead to permanent organ impairment or death. Exposure to carbon monoxide levels of 10 or more parts per million for longer than 8 hours does serious damage to human health.11 Medical researchers calculate that a decade or more of continuous exposure to carbon monoxide above 1 part per million may be sufficient to result in serious health impairment. However, research affirming this calculation remains incomplete as yet.

The UCR team calculated that any planet with tropospheric water vapor equal to or greater than Earth's while orbiting a cool M-type (low mass, low luminosity) star would possess too much atmospheric carbon monoxide to sustain aerobic complex life. Planets orbiting M-type stars but having significantly less tropospheric water vapor would also possess too much atmospheric carbon monoxide. Planets with less surface water than Earth that orbit larger, brighter, K-type stars would have not only too much carbon monoxide but also too much atmospheric carbon dioxide.

Habitability & 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 eliminates nearly all such planets as plausible hosts for 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. However, they burn up too quickly and their luminosities increase too rapidly to be candidates for hosting advanced aerobic life.

The Sun is a G-type star. Stars of this type make up about 7 percent of all stars. Planets drier than Earth orbiting G-type stars would likely possess too much atmospheric carbon monoxide for advanced life. Even planets equally wet or wetter, but without the many other fine-tuned conditions described previously, would likely possess either too much or too little carbon dioxide to be habitable by advanced life.

In the context of complex aerobic life, the latest research demonstrates that the potential habitability of planets beyond Earth is much lower than astrobiologists previously thought. Now, two more habitable zones—the carbon dioxide zone and the carbon monoxide zone—have been added to the still growing list of known planetary habitable zones—regions around a host star in which an orbiting planet could conceivably host advanced life (see Table).

To date, astronomers have identified thirteen planetary habitable zones that must converge for any planet to serve as a possible host for advanced life. In other words, all thirteen zones must simultaneously overlap. As of July 11, 2019, astronomers had discovered and measured the properties of 4,106 planets.12 Among these 4,106 planets, only one exists where all thirteen habitable zones "just happen" to come together in the same place and time.

::: Table :::
Known Planetary Habitable Zones

This list includes planetary habitable zones that have been discovered as of July 2019. In addition to the planetary habitable zones, scientists have identified galactic habitable zones and extragalactic habitable zones—regions within a galaxy, galaxy cluster, and supergalaxy cluster where life could conceivably exist. (Primary literature citations and links to definitions for each of the first eleven zones, and details on how each one operates, may be found in this open-access online article:

When combined with literally hundreds of other planetary features that must fall within precise ranges for the sake of our existence,* these research findings provide ever more abundant evidence that Earth is no mere cosmic accident. The weight of physical evidence testifies of a supernatural, super-intelligent Creator who shaped and crafted a home where billions of humans could exist and thrive, recognize the source and meaning of their existence, and ultimately accept the invitation to enter into an eternal, loving relationship with him.

*Hugh Ross, "RTB Design Compendium (2009)," Today's New Reason to Believe (blog), Reasons to Believe (Nov. 16, 2010):

1. Michael J. Denton, Nature's Destiny: How the Laws of Biology Reveal Purpose in the Universe (The Free Press, 1998), 127–129.
2. Ibid.
3. Edward W. Schwieterman et al., "A Limited Habitable Zone for Complex Life," Astrophysical Journal 878, no. 1 (June 10, 2019). The entirety of this paper is free to the public. Also, a free preprint of the paper is available at
4. Occupational Safety and Health Administration, United States Department of Labor, "Permissible Exposure Limits/OSHA Annotated Table Z-1" (March 29, 2019):
5. Astrid C. Wittmann and Hans-O. Pörtner, "Sensitivities of Extant Animal Taxa to Ocean Acidification," Nature Climate Change 3 (August 2013), 995–1001: doi:10.1038/nclimate1982.
6. Edward W. Schwieterman et al., "Rethinking CO Antibiosignatures in the Search for Life beyond the Solar System," Astrophysical Journal 874, no. 1 (March 20, 2019): doi:10.3847/1538-4357/ab05e1.
7. Cédric G. Fichot and William L. Miller, "An Approach to Quantify Depth-Resolved Marine Photochemical Fluxes Using Remote Sensing: Application to Carbon Monoxide (CO) Photoproduction," Remote Sensing of Environment 114 (July 15, 2010), 1363–1377: doi:10.1016/j.rse.2010.01.019.
8. M. O. Andreae and P. Merlet, "Emission of Trace Gases and Aerosols from Biomass Burning," Global Geochemical Cycles 15 (December 2001), 955–966: doi:10.1029/2000GB001382.
9. Ludivine Conte et al., "The Oceanic Cycle of Carbon Monoxide and Its Emissions to the Atmosphere," Biogeosciences 16 (February 2019), 881–902: doi:10.5194/bg-16-881-2019.
10. C. L. Townsend and R. L. Maynard, "Effects on Health of Prolonged Exposure to Low Concentrations of Carbon Monoxide," Occupational and Environmental Medicine 59 (October 2002), 708–711: doi:10.1136/oem.59.10.708; D. Nicholas Bateman, "Carbon Monoxide," Medicine 31, issue 10 (Oct. 1, 2003), 41–42: doi:10.1383/medc.
11. Ibid., Townsend and Maynard, "Effects on Health."
12. Exoplanet TEAM, The Extrasolar Planets Encyclopaedia, The Catalog (July 10, 2019):

is an astrophysicist and the founder and president of the science-faith think tank Reasons to Believe (RTB).

This article originally appeared in Salvo, Issue #51, Winter 2019 Copyright © 2019 Salvo |