We know life as it is on Earth. Can we transpose this unique example to space?
Can we reasonably think about a life based on silicon, the element that is close to carbon and shows similar properties? (1)

Outside scientific speculations about the validity of such an hypothesis, we have to stay pragmatic. If we take 110 different molecules that have been identified in interstellar clouds, 83 have carbon (approximately 60 are of organic type) (3) and 7 are made with silicon. We see that organic chemistry is widly spread and that the few number of silicon molecules doesn't speak for this kind of chemistry.

Even if we have seen that some creatures on our planet can live in hydrogen sulphide (9), they are made of carbonaceous molecules. Till now, nothing did challenge the uniformity of life based on organic chemistry.

That's why scientists are looking for tracks of organic life in the cosmos. We know the molecules that reveal a potential organic activity thanks to the study of the terrestrial example.

Where to look for other forms of life ?

• Into the Solar System (12):
  • Mercury and Venus are too close to the Sun.
  • Martians samples show that there's no life on Mars, but that it could have existed in the past.
  • There is an activ organic chemistry on Jupiter, Saturn, Uranus ans Neptune but they are very improbable candidates for life.
  • Europa and Titan are the best candidates for the presence of life and for space projects.

• Out of the Solar System (12):
  We have seen that interstellar clouds are rich with organic molecules. it's interesting to look beyond our Solar System. This search can only be made indirectly via the study of radio signal, infrared and spectrographic analysis of distant objects.