The Kepler space telescope has mapped more than 1,200 planets in one tiny corner of our Milky Way Galaxy. Based on that sample, scientists say that there are approximately 50 billion planets in the entire galaxy based on a conservative estimate of one planet per star in the galaxy, including 500 million that are theoretically capable of sustaining life. In astronomer Milan Cirkovic's view, truly advanced technological civilizations (ATCs: those who survive the bottleneck presented by the threat of self-destruction through warfare or asteroid impact or other accidents) will tend to be located at the outskirts of the Milky Way. The very traits that make ATCs capable of migrating and utilizing resources with high efficiency will tend to make them systematically hard to detect from afar.
Jamin Zuckerman proposed in 1985 that stellar evolution of stars far older than our Sun is an important motivation for civilizations to undertake interstellar migrations. It seems implausible that any but the most extreme conservative societies would opt to wait to be forced to migration by slow and easily predictable process like their star leaving the Main Sequence..
Kepler has discovered some 1,300 exoplanets that revolve around a star, in an area that represents around 1/400th of the Milky Way. By extrapolating these numbers, the Kepler team has estimated that there are at least 50 billion exoplanets in our galaxy — 500 million of which sit inside the habitable "Goldilocks" zones of their suns, the area that it is neither too hot nor too cold to support life.
Astronomers estimate that there are 100 billion galaxies in the universe. If you want to extrapolate those numbers, that means there are around 50,000,000,000,000,000,000 (50 quintillion) potentially habitable planets in the universe.
As Arthur C. Clarke, physicist and author of 2001: A Space Odyssey wrote, "The idea that we are the only intelligent creatures in a cosmos of a hundred billion galaxies is so preposterous that there are very few astronomers today who would take it seriously. It is safest to assume therefore, that they are out there and to consider the manner in which this may impinge upon human society."
The Daily Galaxy via NASAJPL
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