Carbon 14 radioactive dating done
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).
For that matter, is it always easy to tell whether a man is merely unconscious or quite dead?More specific and etymologically correct than sentient.Still less common than that term, but has been used by multiple writers..Only rarely will anybody get to know one on a personal level.TRADE with them is sporadic, and even WARFARE seems less frequent than it used to be in the GOLDEN AGE.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.
(Or else Earth Humans are descended from them, though this raises troublesome questions about chimpanzees.)Perhaps because of these awkward issues, Aliens with Forehead Ridges have become much less common in written SF (save for media tie-ins) than they were some decades ago.
In written SF, the KNOWN GALAXY seems increasingly to be inhabited only by Earth Humans.
A particular race of Aliens with Forehead Ridges may all have a culture like that of medieval Japan, or one based entirely on music, but you will very rarely find more than one culture per species.
(The Vulcans and Romulans of Trek fame are a rare exception.)Because of the similarity (or at least comprehensibility) of cultures, Earth Humans can have far more complex and intimate relations with Aliens with Forehead Ridges than with Really Aliens.
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.