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In 1961, scientist Frank Drake wrote down a simple-looking equation for estimating the number of active, technologically-advanced, communicating civilizations in the Milky Way. From first principles, there was no good way to simply estimate a number, but Drake had the brilliant idea of writing down a large number of parameters that could be estimated, which you would then multiply together. If your numbers were accurate, you'd arrive at an accurate figure for the number of technologically advanced civilizations that humanity could communicate with, within our own galaxy, at any given moment. It's a brilliant idea in concept, but one that's become less and less useful as we've learned more about our Universe. As it stands today, the Drake equation is broken, but we know enough about the Universe to construct an even better framework. The possibilities of having another inhabited world in our Milky Way are incredible and tantalizing, but if we want to know whether it's real or not, we absolutely have to get the science right. The Drake equation, to be specific, said that the number of civilizations (N) we have at any given time within our galaxy, is equal to the product of seven different unknown quantities from astronomy, geology, biology, and anthropology, each of which build off of the previous element. They are: R∗, the average rate of star formation, fp, the fraction of stars with planets, ne, the average number stars-with-planets that have one that could support life, fl, the fraction of those planets that developed life, fi, the fraction of life-bearing planets that developed intelligent life, fc, the fraction of these intelligence-having planets that are technologically communicative across interstellar space, and L, the length of time such a civilization can broadcast-or-listen. Multiply these numbers all together, in theory, and that will give you the number of technologically advanced, broadcasting civilizations we have in the Milky Way today.