In 1961 Frank Drake, who conducted the very first SETI search, produced the now famous Drake Equation . This derives an estimate of the number of communicative civilisations N by multiplying together a number of factors which together determine the overall probability. It is important to note the word estimate because few of the factors are known with any degree of certainty.

NASA’s Kepler space telescope was launched in 2009 and measured the brightness of 200,000 stars at unprecedented precision for over four years, with the prime mission goal of detecting Earth-sized exoplanets. Now after another four, Kepler’s final planet catalog is complete — over 4,000 planet candidates have been found, with 50 of them possibly rocky and capable of having liquid water.

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For the first time in human history, we can calculate how common planets the same size and temperature as Earth is, a key component to SETI’s goal of figuring out how common life may be in the universe.