The 'Habitable Zone,' also known as the 'Goldilocks Zone' is a specific distance to a particular star type where the conditions for liquid water exists, and thus therefore may also have the perfect conditions for life. This theory was spurred by the idea that "
our world is a typical rocky planet in a typical planetary system." Taking it further, a new concept, first proposed by
Guillermo Gonzalez in 1991, is letting scientists think in a much larger proportions; on a galactic scale.
The Milky Way Galaxy is structured similarly to billions of other spiral galaxies in the universe. There has been many revisions of this theory, all of which are less taken into account than the current goldilocks zone which only finds habitable zones from a solar system point of view. One of these revisions based a Galactic Habitable Zone (GHZ) off three factors: supernovae rates, the abundance of metal elements (which is required for the development of rocky planets), and time taken for complex life to evolve. This GHZ insists that although greater star density meant the increase in likelihood of dangerous proximity of supernovae and higher levels of radiation that could exterminate life, the possibility of finding life closer to the galactic circle is 10 times more likely than in the outer galaxy. This is because historically the greatest amount of star formation has occurred in the inner region of the galaxy, and thus has a much higher possibility of finding rocky planets in those solar systems.
However, supernovas seem to be the largest impeding factor from allowing planets from forming life and complex organisms. It has been calculated that in our lifetime, the majority of stars in the Milky Way Galaxy are blasted by radiation and supernovas and only 30% of all the stars in the galaxy remained unscathed and untouched by these deadly forces. One must keep in mind that a human lifetime is not long, and the percentage of stars untouched by deadly radiation will significantly decrease over time. This could be one of the major factors why Earth is quite far from the galactic center. Thus one can see that the GHZ doesn't show where a region of the galaxy is uninhabitable or not, but merely shows these regions to be more likely inhabitable than others. This theory contradicts the last theory of the Galactic Habitable Zone where it is insisted that the GHZ is a belt between distances 22,800 light years (7 kiloparsecs) and 29,300 light years (9 kiloparsecs) from the galactic center.
Although the newest model of the Galactic Habitable Zones proposes that life is more likely in near the galactic center and thus has directed the Search for Extraterrestial Intelligence (SETI) to target that location more thoroughly, there are still many assumptions and factors this model has not compensated for. The gravitational effects of nearby stars to solar systems within the inner galactic circle due to the close proximity of stars from each other would make star's habitable zones hectic (
creating perturbed orbits). Some scientists believe that the current Galactical Habitable Zone model is one of the most complete studies of GHZ to date. Although I believe we are far from creating a working model of our galaxy's GHZ that is as accurate and specific as the solar system habitable zones that we currently follow, it may still lead to new insight and understanding to planet forming locations.
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