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What is the Galactic Habitable Zone (GHZ)?

In addition to a habitability zone around a star (circumstellar habitable zone, or CHZ), a habitability zone for our galaxy has also been suggested (uninspiringly called the “Galactic Habitable Zone,” or GHZ). The basic idea behind the concept is that certain times and positions within our galaxy (the Milky Way) are favorable to the existence of Earthlike planets and life. Factors that may affect the GHZ are, for example, the relative abundances of heavy metals (i.e., elements heavier than Helium) as a function of position and time in a galaxy, and the proximity of potentially habitable planets to dangerous cosmic explosions (such as supernovae, stars that explode after a certain stage in their evolution). However, the problem is that the factors that affect the GHZ are not well understood and none of the relevant factors can be quantified in a meaningful way.

Indeed, the GHZ may be yet another unnecessary level of “worry,” given that we don't really understand the exact conditions necessary for life anyway. In a paper by N. Prantzos (2008) that questions the usefulness of the GHZ, the author performed some calculations on various constraints but had to make many assumptions that he said, “...are far from being well founded at present.” The author concludes: “Thus, the concept of a GHZ may have little or no significance at all.”

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File under: Galactic habitable zone concept and definition; Habitable zones in the Galaxy; Habitable zones in the Milkyway.

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