Introduction

The discovery of possible past life on Mars (see section Hot Topics) and the new tantalizing hypotheses about the presence of water, and, perhaps, of living organisms on one of the moons of Jupiter, Europa, are signs that the emergence of life doesn't require conditions as special as it has been believed in the past.

Although we don't know whether there had been life on other bodies of the Solar System, we know that there have been no intelligence civilizations. To look for theses, we then turn our search to the spaces outside the solar system.

Two main roads that are being followed. One approach consists in the listening of electromagnetic signals, i.e. of intentional messages sent through space in form of electromagnetic waves. This program, now under the auspices of private institutes, is reviewed in section Current Serches. The other approach goes after the detection of habitable zones, places where there are conditions for life to emerge. For example, it makes sense to consider planets orbiting other stars whose lifetimes are comparable to the one of the Sun (10 billion years); these planets should not be too close to their star, or their rotation on their axes will be locked so as to present only one face to the star, as the Moon does with the Earth. This will produce extreme conditions on those planets. On the other hand, habitable planets cannot be too far away or there will no enough light to warm up the planet and for the existance of water in the liquid form.

A great morale boost in the search for places where life could emerge outside the Solar System has been given by the discovery of planets orbiting distant stars. Previous to this set of discoveries, which occured in the last ten years, there was little agreement among scientists as to the likelihood that Sun-like stars had orbiting planets.

These discoveries are discussed in next section.