For life to be possible on a planet, the planet must simultaneously reside in all the known habitable zones. So far, astronomers have identified nine distinct habitable zones. I described eight of these zones in my book Improbable Planet.1 The ninth, the electric field habitable zone, was discovered just several months ago.
The two habitable zones that have been the subject of the most research by astronomers are the liquid water habitable zone and the ultraviolet habitable zone. All physical life-forms need liquid water to exist. The liquid water habitable zone marks where liquid water conceivably could exist on a planet, depending on its distance from a star.
Ultraviolet radiation is needed for the synthesis of many biochemical compounds that are essential for physical life. Therefore, if the ultraviolet radiation from a host star is too weak, no life is possible on that planet. On the other hand, if the ultraviolet radiation falling upon a planet’s surface is too strong, DNA and other life-critical biomolecules will be damaged to a degree that wipes out all life. The ultraviolet habitable zone is the area where the ultraviolet radiation from a star is neither too weak nor too strong for the existence of life.
Other astronomical sources of ultraviolet radiation besides the host star can complicate the ultraviolet habitable zone. Such possible sources include:
- nearby supergiant stars;
- nearby supernova eruption events;
- gamma ray bursts;
- flares from nearby magnetically active stars; and
- exposure to radiation from the galactic core during those parts of the host star’s orbit about the galactic center when it is significantly above or below the plane of the galaxy.
The ultraviolet habitable zone is not the same width for all forms of life. It is widest for a few species of especially radiation-resistant bacteria. These are bacteria that devote most of their metabolic energy to repairing the damage caused by exposure to ultraviolet radiation. The ultraviolet habitable zone is much narrower for ordinary microbes. It is much narrower still for megaflora and megafauna. It is extremely narrow for the photosynthetic plants on which humans and their domesticated animals depend upon for food.
The ultraviolet habitable zone is also very narrow for humans or any life-form equivalent to human beings. Humans need a minimum level of exposure to ultraviolet radiation to synthesize vitamin D, stimulate the pineal gland, and prevent psoriasis and eczema. Only slightly more exposure to ultraviolet radiation, however, causes skin cancer, melanoma, and loss of eyesight.
For host stars with an effective temperature more than 7,100 K (7,100 °C above absolute zero) or less than 4,600 K, even for just microbes, a team of four Chinese astronomers showed that the liquid water and ultraviolet habitable zones will not overlap.1 This may seem like a fairly wide effective temperature range, but it is narrow enough to eliminate all but 3 percent of the Milky Way Galaxy’s stars.
Now, a paper recently published by Japanese astronomers Midori Oishi and Hideyuki Kamaya establishes that the zone of overlap is even narrower.2 For the first time, Oishi and Kamaya took into account the effect of the host star’s metallicity on the position and width of the star’s ultraviolet habitable zone. Metallicity is a measure of the fraction of a star’s mass that is comprised of elements heavier than helium. Because the Sun is a relatively late-born star (it is a third generation star), its metallicity = 0.02. Most of the universe’s stars are second generation stars and possess metallicity values substantially less than the Sun’s.
Oishi and Kamaya first noted that a star’s spectrum changes as its metallicity value declines from that of the Sun. They then established that the region where the liquid water and ultraviolet habitable zones overlap gets both smaller and lasts for shorter time periods as the metallicity of the host star decreases from the Sun’s value.
Oishi and Kamaya also demonstrated that for all metallicity values of host stars, a region of overlap of the liquid water and ultraviolet habitable zones exists only for stars as massive as or more massive than the Sun. The more massive a star the higher its effective temperature. The Sun’s effective temperature = 5,772 K. This new lower effective temperature limit—for the liquid water and ultraviolet habitable zones to overlap for a significant time period—leaves only about 1.5 percent of the Milky Way Galaxy’s stars as candidates for habitability. Also including the metallicity requirements leaves less than 1 percent of our galaxy stars as candidates.
These candidate limits for possible habitability are for microbes only. For plants and animals to possibly exist on a planet the ultraviolet habitable zone is much narrower and there is much less possibility of overlap with the liquid water habitable zone. The constraints are even more confining yet for advanced life and especially so for advanced life maintaining a high-technology civilization.
For plants, animals, and advanced life to possibly exist, the liquid water and ultraviolet habitable zones must sustain their region of overlap for at least a few billion years. This longevity requirement creates a problem for all stars more massive than the Sun. Such stars burn up much faster than the Sun and their luminosities change much more radically than does the Sun’s. The faster and more dramatic burn-up histories of stars more massive than the Sun eliminates the planets orbiting such stars from possibly possessing plants, animals, or advanced life.
As noted earlier, for a planet to remain habitable it must avoid at least five different kinds of dangerous ultraviolet sources beyond its host star. As I describe in some detail in Improbable Planet, one of the more remarkable and very improbable features of our planet Earth is that it has indeed avoided sterilization from these sources.
Past and recent research on the ultraviolet habitable zone gives us more reasons to thank God for the many, many ways he designed our planet and our Sun so that we can enjoy life and achieve the purpose for why God created us. It also yields yet another demonstration that the more we learn about the nature of the heavens and the Earth the more evidence we uncover for the supernatural handiwork of God.