Although no conclusive evidence of life has yet been found on any planet or moon beyond Earth, many people believe that life exists on other worlds and that such beings are more advanced than humans. In ancient folklore, the arrival on Earth of various creatures from other planets and stars was treated as an entirely natural occurrence. Those imaginative stories have been recast with scientific elements in modern science fiction. Many people, however, regard the aliens of science fiction as real. Although the literal meaning of the word “alien” is “a person from another country,” in common usage it has come to mean a living being from another planet.
