Dating habits of the earthbound
Except for the Energy Beings, most seem to be hydrocarbon life forms, but methane breathers who thrive at -200 C will sometimes turn up.
What they all have in common is that they are Really Alien.
The "science" is called Astrobiology, the famous "science in search of a subject". You can keep up on the latest news, but for now if you want aliens, you are going to have to create them yourself.
Suggested reading includes The Encyclopedia of Science Fiction's entry on "Aliens", Steve Colgan's Worlds of Possibility blog, Life Everywhere by David Darling, The Science of Aliens by Clifford Pickover and Aliens and Alien Societies by Stanley Schmidt.[first use unknown, but goes back at least to 1940s]General SF term for an extraterrestrial or alien possessing human-level intelligence (see sophont).
This is for an obvious reason: the audience wants aliens of One of the first ways in which we learn to classify objects is into two groups: 1. Nevertheless, the task of defining "life" is both difficult and subtle; something that at once becomes evident if we stop to think. The caterpillar is alive, but the rock is not; as you guess at once, since the caterpillar is moving and the rock is not.
Is it easy to tell a live seed from a dead seed, or either from a grain of sand?
For that matter, is it always easy to tell whether a man is merely unconscious or quite dead?
This is partly because it is not clear what we would fight them over, and partly because they may have an alarmingly high TECHLEVEL, making war with them a dangerously one-sided proposition. See COSMIC BACKGROUND HISTORY.2) Aliens with Forehead Ridges.
Much more common - especially in HOLLYWOOD SCIFI - than Really Aliens, these are species that look almost exactly like Earth Humans, except for some distinguishing visible feature such as, well, forehead ridges, or odd-shaped ears, or whatever.