The habitable zoneThere are currently approximately seven billion people living on Earth. Who can imagine 1 million people, let alone 7 billion people? But his total incomprehension does not detract from reality. No one has ever seen an alien or organism that isn't native to Earth, but that doesn't make the prospect any less valid. Most of Earth's citizens can't imagine living anywhere other than this blue planet, but that shouldn't make the possibility any less real. Space, like the future, is always ahead of us, and no one can predict what it will bring or what ideas we will find. Outside of Earth, life in the universe is not as crazy a thought as was once believed. First, a number of variables affect the habitable zone of stars. There are also an insurmountable number of potentially habitable planets that could support life. Finally, evidence of life in space has been found, although the evidence is slim. IN 1953, Hubertus Strughold and Harlow Shapley were the first to mention the habitable zone, although the term they used was "liquid water belt" (transfer). Simply put, the definition of a habitable zone is how far from a host star a planet must be to maintain liquid water (Palmer). To calculate the HZ length of a star, variables such as a planet's luminosity, temperature, solar irradiance, and distance from the sun are needed. (“Habitable Zone”). Since Earth has such large amounts of H2O, it is not unreasonable to think that everywhere else is the same, but in reality this is not the case. Unbeknownst to some, water is actually a highly prized commodity in the universe. A planet must be in an HZ to support water and subjectively to support complex life. If a planet is... middle of paper ......es September 4, 2003. Web April 1, 2014. .Townsend, Rich. “Life in the Solar System: Mars, Europa and Titan.” » Mad Star - February 27, 2014. “Water and climate”. Bohm, Sven. “An early Christmas present for the Kepler mission: the first Earth-sized planet in a habitable zone around a Sun-like star is now confirmed.” - Institute for Theoretical Astrophysics, December 9, 2011. Web. Böhm., Sven. “Life on Gliese 667Cc?” - Institute of Theoretical Astrophysics August 28, 2013. Web February 27.. 2014. .