Alien life, what an exciting topic! Let's investigate some principles of life which might help illuminate what life forms there are on other planets. One of these principles states that "not all life that can exist does, but all life that cannot exist does not". What this means is that, just because we have discovered certain chemical and physical environments that suggest a life form (for example an earth worm), doesn't mean that an earth worm exists there, or that it will ever exist. However, if the environmental properties are analysed accurately, then it is possible to make an assumption that it is likely for a certain life form to have existed there, or for it to be likely to sometime develop there. The reason why "all life that cannot exist does not" is fairly obvious, either because life will not develop in unfavourable conditions, or because life in unfavourable conditions dies.
Another result of that principle is that evolution never reaches a dead-end (which it would, if all life that could exist did). Evolution is the driving property of life, analogous to the property of atoms to seek a stable electron configuration. Evolution is divisible in several other principles, the Principles of Evolution, which go more in-depth to explain the behaviour of life forms.
Therefore, the fundamental principle of life, which applies to extraterrestrial life forms as well as terrestrial life froms, states that life is a self-supporting, self-propagating reflection of its past, present and future environment. Future, how come? Did you think that evolution only works backwards? Well, find out more in the next Post, Principles of Evolution.