Could there be life on other planets? The ongoing search continues to yield data showing how rare, if not unique, our planet is. Scientists have identified multiple “habitable zones,” including the astrosphere habitable zone, that must be fine-tuned in order to support complex life.
Two separate studies conducted by astrobiology research teams have determined the number of potential habitable planets in the Milky Way Galaxy. One study said 45.5 billion planets exist;1 the other put the number at 40 billion.2 These large calculations presume that planetary systems throughout our galaxy are just as abundant as they are in our solar neighborhood. Also, both studies consider only the liquid water habitable zone—just one of eight habitable zones. For advanced life to possibly exist on a planet it must reside within all eight of the known habitable zones:
- water habitable zone
- ultraviolet habitable zone
- photosynthetic habitable zone
- ozone habitable zone
- planetary rotation rate habitable zone
- planetary obliquity habitable zone
- tidal habitable zone
- astrosphere habitable zone
Astrosphere habitable zones are the plasma “cocoons” carved out of the interstellar medium by a star’s wind, creating a space in which a planet could conceivably support life. These cocoons act as a buffer to screen planetary atmospheres and surfaces from the intensity of high-energy cosmic radiation, which would be deadly for a planet’s surface life.
The buffer of a star’s wind, however, must be just-right. A powerful stellar wind will generate a large plasma cocoon, but if too large, that life-supportable planet will be blasted with enough stellar radiation particles to seriously limit the life spans of advanced species. On the other hand, if stellar winds produce a plasma cocoon that is too small, planetary life may be exposed to deadly cosmic radiation.
The size of a star’s astrosphere depends on the star’s mass, age, and the density of the interstellar medium in which the star resides. Since stars are on orbital paths about the galactic center, the interstellar medium density in a given star’s vicinity will vary over the course of its orbit. The question for habitability is whether or not, at any time in a star’s burning history, the star’s astrosphere is small enough to limit stellar radiation damage to life and yet extends far enough to include the region where the liquid water, ultraviolet, photosynthetic, and tidal habitable zones overlap.
For any life to exist, other than the most primitive unicellular forms, the liquid water, ultraviolet, photosynthetic, ozone, and tidal habitable zones must overlap one another. This is possible only within a tiny region of stars that have an identical mass to the Sun. As two astronomers from the University of Arizona demonstrated, for a solar-mass star, only a close encounter with a dense molecular cloud will collapse that star’s astrosphere to a size smaller than the overlapping set of habitable zones.3 They determined that solar-mass stars will encounter these descreening events about 1–10 times per billion years. With Earth’s fossil record revealing that mass extinction events happened, on average, every 27 million years within the past 600 million years,4astrosphere descreening events may account for a few of these episodes.
Humans have the “good fortune” of living on Earth at the ideal astrosphere moment in Earth’s history. We are also extremely fortunate to be living on this planet at a time in which all eight of the known habitable zones overlap. By contrast, not one of the 1,792 planets discovered and measured outside of our solar system5 shows all eight known habitable zones overlapping, which means none are possible habitats for life more advanced than primitive bacteria. Accumulating evidence increasingly points to not 40–46 billion “habitable” planets, but just one. Thus, instead of good fortune explaining our existence, a Designer who knows all about habitable zones (including ones yet to be discovered) and who knows about the changing physics of the solar system must have designed Earth and its long history of life so that humans can thrive during this brief epoch of time.
- Jianpo Guo et al., “Probability Distribution of Terrestrial Planets in Habitable Zones Around Host Stars,”Astrophysics and Space Science 323 (October 2009): 367–73.
- Erik A. Petigura, Andrew W. Howard, and Geoffrey W. Marcy, “Prevalence of Earth-Size Planets Orbiting Sun-Like Stars,” Proceedings of the National Academy of Sciences, USA 110 (November 2013): 19273–278; Dennis Overbye, “Far-Off Planets Like the Earth Dot the Galaxy,” Space and Cosmos, New York Times, November 4, 2013, https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html?pagewanted=all&_r=0.
- David S. Smith and John M. Scalo, “Habitable Zones Exposed: Astrosphere Collapse Frequency as a Function of Stellar Mass,” Astrobiology 9 (September 2009): 673–81.
- Adrian L. Melott and Richard K. Bambach, “Do Periodicities in Extinction—With Possible Astronomical Connections—Survive a Revision of the Geological Timescale?” Astrophysical Journal 773 (August 10, 2013): id. 6; Adrian L. Melott and Richard K. Bambach, “Nemesis Reconsidered,” Monthly Notices of the Royal Astronomical Society Letters 407 (September 2010): L99–L102.
- Françoise Roques and Jean Schneider, The Extrasolar Planets Encyclopaedia Catalog, June 2, 2014,https://exoplanet.eu/catalog/.