ely inactive. To quote the author: “If our ideas about the GHZ are correct, we live within an especially comfortable region of the Milky Way. Any civilization seeking a new world would, no doubt, place our solar system on their home-shopping list”.The Possibility of Life on Other PlanetsGiven the above factors, can one determine if there is life on other planets? According to the article, such can be determined. The outlook on the possibility of extra-terrestrial life, however, is weak. Various reasons exist for author’s opinion. The authors argue that only part of the Milky Way satisfies the requirement of optimal metallicity. Taking into account the disk metallicity, rough limits can be placed on the GHZ both in space and in time. In fact, few planets in our galaxy satisfy the requirement for optimal metallicity. The broad universe looks even less inviting than our galaxy. About 80 percent of stars in the local universe reside in galaxies that are less luminous than Milky Way. Because the average metallicity of a galaxy correlates with its luminosity, entire galaxies could be deficient in Earth-size planets. As far as cosmic threats are concerned, the cometary threat is very sensitive to the galactic environment. The frequency of comet perturbation depends on the position in the Milky Way. As one goes toward the galactic center, the density of starts increases, so there are more close encounters. Comets are thought to reside in two long-term reservoirs: the Kuiper belts and the Oort cloud. Other stars probably have similar retinues. All this suggests that complex life form is rare in the galaxy. The inner regions of our galaxy suffer from orbital instabilities, radiation bursts, and cometary perturbations. The outer regions are safer, but because of the lower metallicity, terrestrial planets are typically smaller there. Another effect concerns the dynamics of stars in a galaxy. Stars in ellipti...
