Now that hundreds of extrasolar planets are known, how do they compare to ours?  The Kepler spacecraft has found a varied assortment of all sizes and distances away from their parent stars.  Only a few reside in their star’s habitable zones.  But that’s only the first of many requirements for life.  Two recent studies indicate that Earth remains a rare bird in the celestial aviary.

Moon requirement:  Earth has a large moon that stabilizes its axis and raises ocean tides.  How rare is that?  PhysOrg reported on a study led by a scientist at University of Zurich who figures that 1 in 12 Earth-like planets has a companion moon.  “Since the Moon might have played an important role in the history of life on Earth, this estimate is important concerning the search for habitable planets.”  The study, however, was a computer simulation, not an actual observation.  It’s not clear how useful such a study can be if “Uncertainties in the study result in a range of 1 in 4 to 1 in 45.”  Even taking their best estimate from their theory-dependent, demolition-derby model, though, it cuts down on life-permitting planets to 8% of Earth-like planets (not all planets).  Not all stars have those, and those that do probably have only one.

Galaxy requirement:  Bulletin!  Life found in Milky Way!  Astrobio.Net reported the finding: “We know for certain that life exists in the Milky Way galaxy: that life is us.”  OK, maybe that is not news, but the article did confirm the idea of a Galactic Habitable Zone (GHZ) outside of which life is unlikely.  Author Gemma Lavender reminded readers of other requirements “such as atmospheric composition, a carbon cycle and the existence of water” that must also be satisfied.  Then she briefly revisited the debate between the Copernican Principle (championed by Carl Sagan and Frank Drake) and the “Rare Earth” hypothesis, advanced by Peter Ward and Donald Brownlee, who first proposed the Galactic Habitable Zone in their book Rare Earth.  But how wide is the GHZ?

Lavender entertained a new model that proposes more habitability in the inner zones than Ward and Brownlee described, despite the increased danger of supernovae, because of higher concentrations of heavy elements there.  A supernova can quickly sterilize a planet, the team led by Michael Gowanlock (NASA Astrobiology Institute) admitted, but life in the fast lane near the galactic nucleus also has benefits—more raw material for rocky planets.  Other astrobiologists are not so sure.  Regardless of who’s right, one item stood out in the study: “The team discovered that at some time in their lives, the majority of stars in our Galaxy will be bathed in the radiation from a nearby supernova, whereas around 30% of stars remain untouched or unsterilized.”  Artwork of an unlucky planet getting sterilized by its star going boom served as a reminder that not every star in a galaxy can be counted on to provide a stable habitable zone….

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