The Earth is old. Some stars that rage today were not even clouds of gas when it was born. The Earth has seen the rise and fall of planets and of galaxies. Its history is a long one, stretching back over a time so long that years are almost meaningless. As far as we know, it is the only world capable of supporting complex life, or indeed any kind of life at all. With the discover of hundreds of exoplanets, this may not always be the case. The possibility of extra-terrestrial life, even just bacteria, is one of the greatest concepts in all of science.