Extraterrestrial (also Alien Worlds in the UK) is a British-American two-part television documentary miniseries, aired in 2005 in the UK by Channel 4, by the National Geographic Channel (as Extraterrestrial) in the US on Monday, May 30, 2005 and produced by Big Wave Productions Ltd. The program focuses on the hypothetical and scientifically feasible evolution of alien life on extrasolar planets, providing model examples of two different fictional worlds, one in each of the series's two episodes.

The documentary is based on speculative collaboration of a group of American and British scientists, who were collectively commissioned by National Geographic. For the purposes of the documentary, the team of scientists divides two hypothetical examples of realistic worlds on which extraterrestrial life could evolve: A tidally locked planet (dubbed "Aurelia") orbiting a red dwarf star and a large moon (dubbed "Blue Moon") orbiting a gas giant in a binary star system. The scientific team of the series used a combination of accretion theory, climatology, and xenobiology to imagine the most likely locations for extraterrestrial life and most probable evolutionary path such life would take.

The "Aurelia" and "Blue Moon" concepts seen in the series were also featured in the touring exhibition The Science of Aliens.

At the start of the documentary, the presenter and team of scientists draw attention to their reasons for speculating about life on extrasolar planets. Discoveries regarding extrasolar planets were first published in 1989 raising the prospect of whether life (as we know it or imagine it) could be supported on other planets. It is currently believed that for this to happen a planet must orbit in a relatively narrow band around its parent star, where temperatures are suitable for water to exist as a liquid. This region is called the habitable zone.

The most Earth-like exoplanets yet found, Gliese 667 Cc and Gliese 581g (disputed), have masses larger than Earth's and orbit red dwarf stars in the habitable zone.

The sensitivity of current detection methods makes it difficult for scientists to search for terrestrial planets smaller than this. To allow smaller bodies to be detected, NASA was studying a project called the Terrestrial Planet Finder (TPF), a two-telescope concept slated to begin launching around 2014. However, Congressional spending limits under House Resolution 20 passed on January 31, 2007 by the U.S. House of Representatives and February 14 by the U.S. Senate have all but canceled the program.

Prior to the TPF's cancellation, astrophysicists had begun speculating about the best places to point the telescope in order to find Earth-like planets. Whereas life on Earth has formed around a stable yellow dwarf, solar twins are not as common in the galaxy as red dwarf stars (which have a mass of less than one-half that of the Sun and consequently emit less heat), or bigger, brighter blue giants. In addition, it is estimated that more than a quarter of all stars are at least binary systems, with as many as 10% of these systems containing more than two stars (trinary etc.)—unlike our own sun, which has no companion. Therefore, it may be prudent to consider how life might evolve in such environments. Such speculation may still be of use should a future planet-finding telescope be launched, and possibly for NASA's Kepler mission.

The first episode of the series focused on Aurelia, a hypothetical Earth-sized extrasolar planet orbiting a red dwarf star in our local area of the Milky Way.