Monday, April 27, 2020
Is There Another Earth Out There Essays - Astrobiology,
Is There Another Earth Out There? 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 Planet How 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, in turn, determines whether a planet can retain an atmosphere and sustain geologic activity. The abundance of metals is measured by metallicy-the ratio of the number of metal atoms to the number of hydrogen atoms. Particularly, the lower the metallicy of a planet, the lower is the abundance of metals. In fact, without enough metals, large planets can not form at all. On the other hand, too high of a metallicity can also be a problem. High metallicity increases the density of the protoplanetary disk and therefore induces the giant planets to shift position. The result of such shift is that such planet will throw any smaller, Earth-like bodies out of the system all together or push them into the sun. b. Metallicy is not enough to determine hospitality of a planet. To contain life, a planet must also be kept reasonable safe from outside threats, such as the impact of asteroids and comets. Blasts of radiation also present a problem. Until a certain point, a planets magnetic field can fend off most particle radiation and its ozone layer can screen out dangerous electromagnetic radiation. However, sufficiently energetic radiation can ionize the atmosphere and generate nitrogen oxides in amounts capable of wiping out the ozone layer. Energetic radiation hitting the atmosphere can also let loose a deadly rain of secondary particles. Effects on Earth In many ways, the Milky Way is unusually hospitable: a disk galaxy with orderly orbits, comparatively little dangerous activity (comets and asteroids) and plenty of metals. Earth satisfies the requirement for optimal metallicity. The mix of land and sea on Earth is important for atmospheric temperature control and other processes. In our solar system, the frequency of asteroid impact depends on the details of Jupiters orbit and formation; the rest of the galaxy has no direct effect. As far as radiation is concerned, the nucleus of the Milky Way is currently relatively 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 Planets Given the above factors, can one determine if there is life on other planets?
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