There is currently no evidence that passes scientific scrutiny indicating that alien civilizations exist. If they are common, we might expect to have been contacted or visited by them. This is known as the Fermi Paradox, and can be paraphrased as “Where is everyone?” Some scientists think that this lack of contact in itself is evidence that intelligent life is rare; others counter that we have searched a very small region of space: akin to dipping a glass in the ocean, seeing no fish in the glass, and concluding that the ocean is devoid of life.

Several attempts have been made to estimate the prevalence of extraterrestrial life by using the best available information from astronomical observations. This includes estimating the number of habitable planets as discussed earlier, as well as estimating less well-known parameters like the fraction of habitable planets that give rise to life, the chance that life develops intelligence, the chance that intelligent life can communicate across space, the average lifetime of a communicating civilization, and other variables. Direct searches have also been made, including efforts to scan nearby stars for signs of radio signals from intelligent civilizations. The Drake Equation is one attempt to estimate the number of detectable extraterrestrial civilizations in the Milky Way, but measurements of some of the critical parameters are still poor, and research and debate continue.