Welcome to Earth. The Blue Planet. Sol 3. Gaia. Terra. Whatever other name you might think of. Earth, for anyone who isn't aware, is the only life sustaining planet we know exists, a smallish piece of rock that managed to develop a stabilised atmosphere and liquid water in its 4.54 Billion year life. The Earth also has a Moon, which it is not widely known, actually came from another planet, commonly referred to as Theia, that collided with the Earth, breaking up, one bit fusing with the Earth, the other forming the Moon.
Now that we worked out where we are, who lives here? Well an unknown number of species, of varying physiology, with the exception that it's all water based. The largest number of one particular group of species is the beetle variety, 1 in every 5 living things is a beetle. The 'dominant' species on the planet are Humans, we're doing pretty well, all 6.7 Billion of us. Now one asks the obvious question, if we have life, then doesn't that mean life can develop on other planets?
The Drake Formula stipulates that there are 10,000 extraterrestrial species we might communicate within our Galaxy. This is countered by the Fermi Paradox, which basically asks why, if the Universe is so old, have no space civilizations been detected. Humans, who haven't even travelled to another planet, make a lot of noise for a planet. We send messages and other junk all over the place, any highly advanced civilizations would have at least picked up some of our racket, and dropped by to tell us to keep the noise down. Of course, the other argument is, because we haven't been to another planet, we really haven't backed out the driveway yet, we don't even have our car keys.
And since we haven't backed out yet, the chance of us being in a car accident (Finding another civilization) are remarkably low. We can actually say, with almost complete certainty, there are alien civilizations out there. In the Milky Way Galaxy alone there are 200 Billion stars, if we assume 50% of those stars have planets, that's 100 Billion stars with planets, with an average number of 7, that's 700 Billion planets, each one capable of supporting some form of life, although only a small percentage would support water based life (Like us). Chances are good that there's other life out there, some of that has to be intelligent.
When someone says 'Alien', people fall into two categories, the monster variety, hell bent on eating or destroying humanity (Like the aliens from the 'Alien' movies, sorry, bad example) or the aliens that might be like us, empire building, intelligent species. Of course, encountering the monster variety is probably more preferable, as they tend to be stupid or unable to adapt. A larger civilization might be hostile, and we would seriously not win (We got lucky in Independence Day, the entire Alien population was on the ship we blew up, they kinda failed to mention the genocide of an entire species in that movie). Of course big plus if they're a benevolent happy-go-lucky civilization handing out free technologies willy-nilly. Then they could be like us, and have the standard reaction of; "Oh, that's nice, a race of semi-intelligent monkeys, how cute-wait, O.M.G. Neutrino in the Flock Sector! ROAD TRIP!!!" and off they fly, to get whacked off stardust. But these are all on the assumption that they find us, what if we find each other, or we find them?
Say humanity expands over a few Solar Systems, maybe quite a few, and we encounter a race of equal power, say similar psychology, first and foremost, absolute fear on both sides, not wanting to instigate conflict would be priority. Conflict itself would be unlikely, as chances are, we wouldn't meeting them personally. By the point of contact, Humans would have machines capable of travelling lightyears to scout out new territory, and then make judgements themselves on a course of action. If another spacefaring civilization is out there, it's doing exactly the same thing. So our robots meet theirs, there's an "Aww Shucks" moment on the part of Earth's space agencies, and the politicians set about policy making. Of course they could hide it from the public, or not, by then in an increasingly media dominated world, combined with computers in our freaking skin, nothing is secret, like one giant Big Brother...except watchable. The aliens are probably going through the same thing, inevitably a lot more robots will rock up, from both sides, equipped with more instruments than you can poke a stick at, and set about figuring out who the other guy is.
