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Argument for extraterrestrial intelligence The argument for the existence of extraterrestrial intelligence is based on the so-called principle of mediocrity. Widely believed by astronomers since the work of Nicolaus Copernicus, this principle states that the properties and evolution of the solar system are not unusual in any important way. Consequently, the processes on Earth that led to life, and eventually to thinking beings, could have occurred throughout the cosmos. The most important assumptions in this argument are that (1) planets capable of spawning life are common, (2) biota will spring up on such worlds, and (3) the workings of natural selection on planets with life will at least occasionally produce intelligent species. To date, only the first of these assumptions has been proven. However, astronomers have found several small rocky planets that, like Earth, are the right distance from their stars to have atmospheres and oceans able to support life. Unlike the efforts that have detected massive, Jupiter-size planets by measuring the wobble they induce in their parent stars, the search for smaller worlds involves looking for the slight dimming of a star that occurs if an Earth-size planet passes in front of it. The U.S. satellite Kepler, launched in 2009, found thousands of planets, more than 20 of which are Earth-sized planets in the habitable zone where liquid water can survive on the surface, by observing such transits. Another approach is to construct space-based telescopes that can analyze the light reflected from the atmospheres of planets around other stars, in a search for gases such as oxygen or methane that are indicators of biological activity. In addition, space probes are trying to find evidence that the conditions for life might have emerged on Mars or other worlds in the solar system, thus addressing assumption 2. Proof of assumption 3, that thinking beings will evolve on some of the worlds with life, requires finding direct evidence. This evidence might be encounters, discovery of physical artifacts, or the detection of signals. Claims of encounters are problematic. Despite decades of reports involving unidentified flying objects, crashed spacecraft, crop circles, and abductions, most scientists remain unconvinced that any of these are adequate proof of visiting aliens.