Planetary Habitable Zones

"For the host stars with effective temperatures lower than 4,600 K, the ultraviolet habitable zones are closer than the habitable zones. For the host stars with effective temperatures higher than 7,137 K, the ultraviolet habitable zones are farther than the habitable zones. For a hot subdwarf as a host star, the distance of the ultraviolet habitable zone is about ten times more than that of the habitable zone, which is not suitable for the existence of life." ~ "Habitable zones and UV habitable zones around host stars" by Jianpo Guo, Fenghui Zhang, Xianfei Zhang,  and Zhanwen Han

The "habitable zone" is something that is crucial in the conversation about life on other planets. The habitable zone is the intersection of two cosmological regions that must both be favorable to life: one within a solar system (circumstellar habitable zone) and the other within the host galaxy (galactic habitable zone). The galactic habitable zone defines a region that is close enough to the galactic core to provide a sufficiently high level of heavy elements to form rocky planets (like Earth) and yet far enough away so that high-frequency radiation does not harm or destroy life. The circumstellar habitable zone is usually defined as the region around a star where liquid water can exist on a planet. Recent research, however, has added a new constraint to the circumstellar habitable zone. This new constraint is an ultraviolet (UV) habitable zone. This region is a band around a star where any planets in it will receive enough UV radiation energy to drive the chemical reactions related to life’s origins (assuming a naturalistic evolution model) and yet not too much, which would result in the destruction of DNA. DNA would never be able to survive on a planet that is too close to its host star because of too much radiation, and it would never be able to form on a planet that is too far from its host star because of not enough radiation.

The paper cited above shows that in the vast majority of stars the liquid water habitable zone and the UV habitable zone do not intersect. "Effective temperatures" that the paper refers to are the temperatures of black bodies that would emit the same total amount of electromagnetic radiation as the stars being studied. Basically around any stars with effective temperatures below 4,600 K the UV habitable zone is too close to the star for liquid water to exist (it would all evaporate), and around any stars with effective temperatures above 7,137 K the UV habitable zone is too far from the star for liquid water to exist (it would all freeze). Around such stars there is no possibility for life as we know it. In fact, requiring a planet to fall within both zones (the liquid water and the UV zone) eliminates 80% of all stars as possible candidates for life-supporting planets. This adds to the growing body of evidence that Earth is uniquely fined-tuned for life as we know it.

By His Grace,
Taylor