Extraterrestrial intelligence, << EHKS truh tuh REHS tree uhl, >> is life with such abilities as thinking and learning originating beyond Earth. Space probes have discovered no life—let alone intelligent life—on the other planets or the moons of our solar system. But many scientists think that intelligent life could exist on worlds around other stars. Beings that originate beyond Earth are often referred to as extraterrestrials or simply aliens. See Alien life.
Scientists think that intelligent life may exist on other worlds. They think this, in part, because the universe contains a vast number of stars and planets. Our own galaxy, the Milky Way, has hundreds of billions of stars. Many scientists think that from 10 percent to more than 50 percent of these stars may have planets. Thus, our galaxy alone could contain more than a trillion planets. Furthermore, the universe has more than 100 billion galaxies. Scientists expect that many planets do not have the conditions—such as liquid water—necessary to support life as we know it. But if even a tiny fraction of planets have the right conditions, the Milky Way still might contain millions of worlds with life. Perhaps some of these worlds have intelligent life.
One effort to find extraterrestrial intelligence is called SETI. It stands for Search for Extraterrestrial Intelligence. SETI research involves looking in the vicinity of other stars for signals sent by extraterrestrials. Such signals could take the form of radio waves or other light waves. In 1960, the American astronomer Frank Drake conducted the first SETI experiment. He used a radio telescope to try to detect signals coming from around two relatively nearby stars. During the 1990’s and early 2000’s, scientists used radio telescopes to hunt for signals coming from many hundreds of stars. Also in the 2000’s, researchers began building the Allen Telescope Array in California. The array was designed to conduct SETI studies of about 1 million stars.
In the late 1990’s, astronomers also began searching for signals in the form of brief, bright flashes of visible light. Scientists think that an extraterrestrial intelligence might produce such flashes with powerful lasers. The flashes could briefly outshine the star near which they originate. But they might last only billionths of a second or less. Natural sources are not known to produce such flashes. Astronomers have used visible-light telescopes and electronic equipment to try to detect such events.
Many people wonder whether intelligent extraterrestrial life could have come to Earth. Each year, thousands of people report seeing unidentified flying objects (UFO’s). Some people believe that such objects could be spacecraft from other worlds. But scientists have found that most sightings can be explained as ordinary things, such as airplanes or balloons, or as natural astronomical objects, such as meteors or bright stars. Scientists have no evidence that Earth has ever been visited by extraterrestrials.
