The sentinel hypothesis is the suggestion that if advanced alien civilizations exist they might place intelligent monitoring devices on or near the worlds of other evolving species to track their progress. A robot sentinel might establish contact with a developing race once that race had reached a certain technological threshold, such as large-scale radio communication or interplanetary flight.
Already, scientists are beginning to understand the basic conditions necessary for carbon-based life to develop. They can identify those stars in the solar neighborhood most like the Sun, and can delineate approximately the habitable zone around any given star in which complex biochemistry might be expected to flourish. Scientists are also starting to acquire the ability to detect exoplanets. More advanced species would presumably be able to stake out with reasonable accuracy those stars and their associated worlds upon which intelligent life had a good chance of eventually developing, particularly if scout ships had revealed the presence of proto-intelligence. Such promising locations might then be expected to come under increasingly intense scrutiny by automatic sentinels. This idea was used by Arthur C. Clarke in his short story "The Sentinel"1 which subsequently formed the basis for the MGM motion picture 2001: A Space Odyssey and its novelization.2
Ronald Bracewell has discussed a specific strategy by which devices could be used to monitor star systems for evolving technological races and also how we might go about trying to find sentinels that had been placed in the solar system (see Bracewell probes).
1. Clarke, Arthur C. "The Sentinel." In Expedition to Earth.
New York: Harcourt, Brace and World (1970).
2. Christian, James, ed. Extraterrestrial Intelligence: The First Encounter. Buffalo, N.Y.: Prometheus Books (1976).