First, the existence of life off the Earth is not a fact. It is still a subject of enormous debate.
We have no firm evidence, the controversial Mars fossils (above example) not withstanding, that life exists anywhere else in the universe. All we can do is speculate and make observations to see whether the evidence might be consistent or inconsistent with such an idea. There is a saying that 'the absence of evidence does not mean evidence for absence'.
We know that there are other solar systems in space because we now have firm direct evidence for this .We do not know if Earth-like planets are common or rare. But at least we know that harsh environments such as interstellar space can cook up very complex organic molecules, and precursor molecules for amino acids and familiar living systems on earth.
I think the circumstantial evidence is quite good, but we still have no direct facts that support the idea of life existing on other worlds.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.