The two Voyager spacecraft travelling through the Solar System will soon leave it behind and enter interstellar space – space between the stars. Each spacecraft carries a disc on which images, natural sounds, speech, and music have been recorded. It is hoped that, one day, intelligent beings from another world may find a disc and get a picture of life on Earth.

Jupiter’s large moon Europa has a flat surface of pink ice. It is criss-crossed with cracks, which may have been caused by the movement of a liquid ocean beneath the surface, melted by energy caused by the powerful tidal effects of Jupiter’s gravity. This has led to speculation that life may exist on Europa.

The radio telescope at Arecibo in Puerto Rico is one of several that has been used to listen for signals that might be from other civilizations in space. As part of a SETI programme called Project Phoenix, millions of radio channels have been scanned simultaneously. Radio waves are studied because they are able to travel a long distance without interference.

The earliest forms of life on Earth appeared nearly four billion years ago in the form of cyanobacteria (blue-green algae). These organisms are still found today in Western Australia, where they form distinctive mounds called stromatolites. The evolution of life began from these single-celled organisms. On other worlds life may exist in a similar, primitive form.

Looking for a signal in space that may have been sent by intelligent extraterrestrials is like looking for a needle in a haystack. Huge quantities of data are received, which must then be processed. Through the SETI@Home project, ordinary computer users can help with this task.

A planet similar to Jupiter has been found circling star HD 187123. It is known as an extrasolar planet – a planet beyond our Solar System. A SETI project directs its radio telescopes at stars known to have planets, and stars like our Sun, as these are the most likely to have planets capable of supporting life.

American radio astronomer Frank Drake, a SETI pioneer, drew up a list of key factors necessary for intelligent life to evolve on a planet. The list, named the Drake Equation, is a basis for calculating the number of possible civilizations in our galaxy. However, the factors are based on only one example of life on a planet, our own. We do not know if these factors will apply to life forms on other planets. Optimists estimate that there are millions of civilizations in the Milky Way. Pessimists estimate that there is just one – our own.