Is There Another Earth Out There? (adopted from an article by Guillermo Gonzalez, Donald Brownlee and Peter D. Ward, Refugees for Life in a Hostile Universe, Scientific American, October 2001) Is there life on other planets? Science has been trying to answer this question for a long time. In the article, the authors argue that it is highly improbable that complex life forms exist in our galaxy. To quote the authors: “Researchers are now casting a skeptical eye on musings about the prevalence of intelligent life throughout the Milky Way”. Furthermore, according to the article, not only may most of the solar system be unfriendly to multi-cellular or complex organisms, the same may be true of much of the galaxy. In fact, in recent years, astronomers have gained an appreciation of just how deadly our galaxy can be, filled as it is with exploding stars, radiation, and stellar close encounters.Creating a Hospitable PlanetHow does one determine the hospitality of a planet? Within a given planetary system, astronomers describe the optimal locations for life in terms of the cicumstellar habitable zone (CHZ). The CHZ is generally considered to be a region around a star where liquid water can persist on the surface of an Earth-like planet for at least a few billion years. In 1999, there was a proposed concept of a galactic equivalent to the CHZ: the galactic habitable zone (GHZ). The GHZ defines the most hospitable places in the Milky Way-those that are neither too close nor too far from the galactic center. The boundaries of the galactic habitable zone are set by two requirements: a). the availability of material to build a habitable planet and b). adequate seclusion from cosmic threats. a. In order for a planet to be created, sufficient building material must exist. Interestingly, metals are the building blocks of Earth-like planets. In fact, the abundance of metals affects the size of the planets that can form. Size...
