Extraterrestrial life is any life that does not originate from Earth. None has so far been discovered. Alien or extraterrestrial life includes anything from simple bacteria to intelligent, complex life-forms—perhaps even more advanced than humans. The search for other worlds where life could exist covers our neighbouring planet Mars, more distant moons such as Enceladus, Europa or Titan, and planets in other planetary systems many light years away.

Conditions for life

For living things to exist, there must be liquid water and a source of food. Some very simple life forms can live on chemicals in rocks. They can also survive in very hot or cold conditions. Some scientists think that life might be common in other worlds besides Earth. The discovery that warm-water oceans may exist beneath the crusts of moons like Enceladus and Europa has raised the possibility that alien life could even be found in our own Solar System.

Exoplanets

Astronomers have now discovered thousands of planets belonging to planetary systems other than our own. Called extrasolar planets or exoplanets, they orbit around other stars in the Galaxy. Some lie many light years from Earth. A number of stars have exoplanets orbiting in what is known as their “habitable zone”. This is the distance from the star which has just the right temperature for water to exist in its liquid state. If liquid water exists, then there is always a chance life might exist, too.