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THE GALACTIC HABITABLE ZONE |
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by www.seds.org (Students for the Exploration and Development of Space) Our galaxy is one of the most massive galaxies in the nearby universe, which makes it unique. Our solar system seems to have some unique qualities which makes our star capable of supporting a habitable zone and thus liquid water necessary for life as we know it. Some of these unique qualities are: The metallicity of our Sun appears to be just right to support terrestrial planet formation. Old stars located near the center region of our galaxy, are metal-pool because they formed from just hydrogen, helium, and lithium. Some of the the more massive stars complete their nuclear fusion cycles and explode as supernovas. The heavy elements produced from successive cycles of nuclear burning are dispersed into the interstellar medium. Second generation stars are created out of these heavier elements, and each stellar explosion lead to a greater abundance of available metals. A star of high metallicity, therefore, has material that originated from many previous generations of stars. Our sun has an unusually high metallicity compared to other solar-like stars, and the reason for this is not yet understood. It is possible that our star formed in a part of the Milky Way galaxy that had a high abundance of metals (elements heavier than helium), and then migrated to its present location. Recent statistical studies on the metallicities of stars with extrasolar planetary candidates around them indicate that, metal-rich stars are more likely to have planets orbiting them. The most likely explanation for this is that a certain minimum threshold of metals is required to form rocky terrestrial planets and the cores of giant gaseous planets. Therefore, a star that forms from an interstellar cloud high in heavy elements is more likely to form planets than a cloud deficient in metals. The Sun's circular orbit about the galactic center (with a orbital period of about 250 million years) manages to avoid the hazardous spiral arms of the Milky Way Galaxy. Keeping away from spiral arms, where new stars are being formed, prevents our solar system from being gravitationally disrupted. Fortunately, our star orbits through the galaxy at nearly the same rate as the spiral-arm rotation. This synchronization protects our solar system from crossing a hazardous spiral arm to often. Avoiding the spiral arms prevents our planet from encountering supernovae and giant molecular clouds, which can perturb the cometary bodies out of the Oort cloud leading to a higher number of cometary showers in the inner solar system. The low eccentricity of our star's orbit (near zero) makes is unique from most other solar-type stars which have a higher eccentricity. The Sun's low eccentricity decreases the probability that it will cross a spiral arm Our solar system is located at a safe distance (about 8.5 kpc) from the Galactic Center, which provides a safe haven from disruptive gravitational effects and harmful radiation produced from clouds of ionized gas rapidly moving around a supermassive black hole. If our solar system were located close to the Galactic Center, the combined perturbing effects from all the stars would cause a high flux of comets to rain in from the Oort comet cloud. The high number of cometary impacts would create global extinction events that would prevent complex life from evolving on the inner terrestrial planets. If our solar system were located near the inner region of the galaxy, the increased exposure to gamma radiation and x-rays, in addition to cosmic rays, would be lethal to any life trying to evolve on a terrestrial planet. The effects of radiation would damage the protective ozone layers of planets. In addition, secondary particle cascades created in the planet's atmosphere would be produced by high-energy particles. This would, in turn, increase radiation levels at the surface of the planet. When combined together, these three conditions produce what is known as a Galactic Habitable Zone. The existence of life on our planet, from simple microorganisms to human beings, is a result of these unique conditions. It has been suggested that approximately 95 percent of all stars in the Galaxy wouldn't be able to support complex life from evolving. This is due to the fact that their rotational rate around the Galactic Center is not synchronized with the rotational rate of the hazardous spiral arms. When additional conditions required for keeping a solar system habitable are factored in, the probability of finding another solar system within the Galactic Habitable Zone, is extremely low.
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