Scientists’ quest to find a habitable planet like Earth continues to drive extraterrestrial research. However, new studies show that planets capable of sustaining complex life must be extremely rare. Astronomers have discovered eleven different planetary habitability zones—locations relative to the distance from the host star where life could conceivably exist. See table 1 for a list of these habitability zones.
Table 1: Known Habitable Zones
This list includes planetary habitable zones that have been discovered as of February 2019. In addition to the planetary habitable zones, scientists have identified galactic habitable zones and extragalactic habitable zones; that is, regions within a galaxy and within a galaxy cluster and supergalaxy cluster where life conceivably could exist. The citation attached to each planetary habitable zone directs readers to a definition for the zone and details on how it operates.
- liquid water (temperature)1
- tropospheric ozone4
- planet rotation rate5
- planet rotation axis tilt6
- atmospheric electric field9
- Milankovitch cycles10
- stellar magnetic wind11
Narrowing the Habitability Zone
The most well-known of these planetary habitable zones is the liquid water habitable zone: a planet’s distance range from its host star within which liquid water could conceivably exist somewhere on its surface. Of all the known habitable zones, the liquid water habitable zone is the widest. For life to truly exist on a planet, however, liquid water must exist on a large fraction of the planet’s surface for long periods of time. This more practical definition significantly narrows the width of the liquid water habitable zone.
Now, a research paper by five astrobiologists demonstrates that the liquid water habitable zone “for complex aerobic life is significantly limited relative to that for simple microbial life.”12 The five astrobiologists define complex aerobic life as “large (mm- to m-scale), tissue-grade aerobic heterotrophs with blood vascular (circulatory) systems.”13 This definition would include all animals possessing internal organs.
The researchers show that near the inner edge of the liquid water habitable zone (that part of the zone that is closest to the host star), clement surface temperatures for complex life can be maintained where the life-essential greenhouse gas, carbon dioxide, exists in the atmosphere at concentrations similar to that of modern Earth, namely 0.0002–0.0004 bars. (A bar is a unit of atmospheric pressure. One bar is very slightly less than the average atmospheric pressure on Earth at sea level.) However, for the middle and outer parts of the liquid water habitable zone, atmospheric carbon dioxide concentrations need to be much higher—higher than 1 bar.
Severe Health Consequences for Complex Life
High carbon dioxide concentrations in the atmosphere pose catastrophic problems for complex life. Such high concentrations not only produce acidosis (low pH) in oceans, lakes, and rivers but also in aquatic animal tissues and body fluids. This acidosis is detrimental to the long-term health of all aquatic animals and to the short-term health of high-metabolic-rate aquatic animals.14
Elevated carbon dioxide levels impact air-breathing animals as well. They disrupt respiration, pH buffering, and the autoregulation of the blood supply.15 For large high-metabolic-rate animals like humans, elevated carbon dioxide levels bring on “acute reduced cognitive performance, respiratory failure, and circulatory arrest.”16
For the equivalent of human beings, carbon dioxide concentrations that are only a small fraction of a bar can be devastating. Deleterious health consequences occur at a carbon dioxide level of 0.005 bar.17 A carbon dioxide level of 0.05 bar causes serious respiratory acidosis, resulting in depression of respiration and circulation. Carbon dioxide at 0.10 bar, if sustained, causes convulsions, coma, and death.18 At 0.30 bar, loss of consciousness occurs in seconds. These consequences are for young, healthy humans. Carbon dioxide tolerance decreases with age.
Lungs surpass all other known and conceivable respiratory organs in respiratory efficiency. They function best at 1 bar of atmospheric pressure. At 3 bars they cease to function, resulting in the rapid death of the organism. Whatever greenhouse gases that a planet’s atmosphere needs to compensate for its host star’s luminosity—where the planet orbits farther out from the host star than the inner edge of the liquid water habitable zone—will raise the atmospheric pressure beyond the limit for effective lung function.
Advanced life on planets orbiting stars less massive than the Sun face a catastrophic carbon monoxide problem. Stars less massive than the Sun promote photochemical conditions that lead to relatively high levels of carbon monoxide in the atmospheres of planets orbiting them.19 Carbon monoxide is extremely toxic for organisms with circulatory systems since oxygen-carrying molecules like hemoglobin possess an affinity for carbon monoxide that is thousands of times greater than for free oxygen (O2).20
Stringent Habitability Requirements Point to Design
The team of five astrobiologists drew the following conclusions. First, complex aerobic life is ruled out for any planet that needs substantially more (50 times more) atmospheric greenhouse gases than Earth to possess liquid water on its surface. Second, conceivably habitable planets orbiting M- and K-dwarf stars (88 percent of all stars) will possess atmospheric carbon monoxide levels that will be lethal for aerobic complex life. Third, limitations on aerobic complex life by carbon dioxide and carbon monoxide explain why we find ourselves on a planet that is very near the inner edge of the liquid water habitable zone of a G-dwarf star.
One can reasonably draw two more conclusions implied by the five astrobiologists’ research paper, but not presented. First, planets capable of sustaining complex life must be exceedingly rare. Second, when one takes into account that the existence of aerobic complex life requires a planet that simultaneously resides in all eleven habitable zones, the number of planets in the universe capable of sustaining such life most probably is just one. In other words, given the enormous degree of fine-tuning needed to make a planet habitable for aerobic advanced life, that single planet cannot be a cosmic accident. Its features testify of divine design and divine love for aerobic advanced life. I leave it to readers to determine the identity of that single planet.