Ultraviolet Habitable Zones

Two years ago, the discovery of Earth-sized planets around M-dwarf stars generated well-deserved excitement in the astronomical community. These stars comprise more than 75 percent of all main-sequence stars (those generating energy by fusing hydrogen into helium in their cores) and they reside as single stars rather than in multi-star systems. For comparison, Sun-like G stars represent less than eight percent of all stars. The existence of planets around M dwarfs greatly expands the potential life-sites in the universe if any of the planets meet all habitability requirements. However, as I predicted, additional research affirms Earth’s unique capacity to continuously support life.

Planets are deemed life-friendly if certain features fall within a specified (and typically narrow) range known as “habitable zones”. For instance, scientists know the surface temperature of a planet must fall within a narrow range in order for liquid water to exist. This constraint allows one to define a water habitable zone around stars. Although liquid water is a necessary condition for life, it is not sufficient. The amount of ultraviolet (UV) radiation a planet receives also affects its habitability. Naturalistic models typically use UV radiation from the Sun as the energy supply required to drive the chemical reactions related to life’s origins. (Creation models don’t have this requirement.) Yet too much UV radiation destroys the DNA backbone vital to life. Thus, each star also exhibits a UV habitable zone.

A team of Chinese scientists recently determined what kind of stars show an overlap between the water and UV habitable zones.1 Their research demonstrated that the two zones coincide for only a relatively small fraction of stars similar to the Sun. For stars with temperatures below 4,600K (like all M-dwarf stars), the UV habitable zone resides closer to the star than the water habitable zone allows. For stars hotter than 7,100K, the UV habitable zone is located beyond the water habitable zone. Requiring a planet to fall within both habitable zones eliminates over 80 percent of all stars, including all M-dwarfs, as potential sites for discovering extraterrestrial life in the universe.

These results add to the growing body of evidence that Earth uniquely provides a habitat for human life. This idea aligns with the biblical description of God preparing our home planet specifically for that purpose.

Endnotes:
1. Jianpo Guo et al., “Habitable Zones and UV Habitable Zones around Host Stars,” Astrophysics and Space Science 325 (January 2010): 25–30.

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One thought on “Ultraviolet Habitable Zones

  1. I don’t think that we can discount M-type stars just yet as your article brings up that, “stellar flares of M type stars can generate adequate UV radiation, which supplies the energy source for the synthesis of many biochemical compound… Hence, M type stars with moderate flares are the best candidates to host habitable planets.”

    In addition, it has been proffered in another article that, while “In general, early K-type stars and early M stars may have XUV emissions of about 3–4 times and about 10–100 times higher, respectively, than solar-type G stars of the same age…Early M-type stars appear to stay at high activity levels until ages of about 1 Gyr, and then decrease in an analogous way to G and K-type stars.” (pg. 193) After this initial stage of heightened XUV emission, a K or G type star might be habitable for 30 billion years or more.

    Not to mention the fact that, “it has been concluded that the solar integrated XUV flux (0.1–120nm) was higher by a factor of six 3.5 Gyr ago [at the time of the earliest fossil evidence of life] than compared to the present state. Also, during the first 100Myr after the Sun arrived at the zero-age main-sequence (ZAMS), the integrated XUV flux was up to 100 times more intense than today.”(pg. 192) Which would suggest that if the chemical precursors for life could survive during a period when flux was 100 times more intense or if life could survive during a period of six times the current UV radiation level, then the UV habitable zone for life may be wider than thought, especially if the planet is within the habitable zone where there is an ocean of liquid water for organisms on that planet to hide from UV radiation.

    And even M-type stars that emit UV radiation early in their main sequence stage might not be uninhabitable for life because, “Any wholly subterranean organisms would be wholly out of reach of UV radiation from flares. Sterilization would be difficult in a natural environment, which offered deeper water, water with impurities,
    porous and creviced rocks”. The article cites that “The wide range of credible biological stratagems available to reduce the dangers of stellar flares would give opportunities for macrofauna.” (pg. 420)

    http://www.as.utexas.edu/astronomy/education/spring02/scalo/heath.pdf

    http://www.springerlink.com/content/1442151wv3pr0870/fulltext.pdf

    Planets like Earth orbiting G-type stars like our Sun need not be the only type of habitable system.

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