There are a great number of theories concerning extraterrestrial life. According to the “rare Earth hypothesis”, the conditions on Earth are close to unique and the possibility of them taking place on any other planet is close to zero. However, this theory takes it for granted that the appearance of life requires conditions identical to those on Earth. But is it true? If life on other planets exists, it may be completely different from what we are used to not only in form, but in fundamental principles as well.
Of all the chemical elements only carbon and silicon seem to be suitable for being the basis of life, although silicon can form connections with far less other elements. Needless to say, we have never encountered silicon-based lifeforms, but if they exist, they may have properties that have nothing in common with what we used to associate life with.
According to another theory, life may be a rather common thing in the Universe; even in our Solar system at least several planets (Venus, Mars, Titan, Ganymede, Europa) in addition to Earth show signs of having atmosphere and in theory may host some primitive lifeforms. All in all, taking into account the vastness of Universe and the minuteness of our knowledge of it, it is very improbable that life doesn’t exist anywhere at all.
Of course, there is no definite answer to the question I used as a title. Nobody can say for...