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little green men [Q&A]


By far the most common question I get asked on astronomy outreach visits is: are there aliens like us somewhere else in the Universe?

There are ten billion stars in our Galaxy, and a hundred billion galaxies in the Universe that we can see, and the Universe itself is apparently a couple of hundred times bigger than what we can see. That’s a lot of stars, with a great potential for planets or moons that can sustain life. It seems quite plausible that somewhere in that multitude of sites there is another Earth-like planet, with animals at least as intelligent as us, wondering if they are alone in the Universe or is there someone Out There.

Right now, we have only one known case where life has arisen, taken hold, grown to be self-aware, built a civilization, and reached for the stars. We don’t really have a good idea what factors are important to estimate the number of habitable worlds, let alone what defines the rise (and fall) of civilizations. A crude attempt to figure this out was made in the `60’s by Frank Drake, who wrote down the so-called Drake Equation (which btw is a great example of a back-of-the-envelope order-of-magnitude calculation) to estimate the number of civilizations in our Galaxy with whom we can communicate right now. Depending on what numbers one plugs into the factors that go into the estimate, that can be anything between 1e-6 to 10,000, or ranging from “yes, we are and have always been alone,” to “there is a teeming Galactic Empire just waiting for us to discover warp drive!” In other words, we don’t know.