Alien life rules

As far as I understand it, in terms of physiology, aliens could and likely are different-looking than us humanoids. But in terms of biology, our best guesses are that aliens would also be carbon-based and drink water-- not because we're arrogant enough to assume that all life must be like us, but we have no evidence in any direction to prove or disprove that. We have to start somewhere, otherwise we'd be spreading way too thin. That's why we limit our search to Earth-like planets in the Goldilocks zone that could be capable of holding water-- again, not because we know aliens need water, but because if we don't start with that assumption, we'd get absolutely nowhere.

I'm not well versed in alien evolutionary theory at all, but wouldn't differences in gravity, water, temperature, air makeup etc cause a significantly different evolutionary path? Earth is a Goldilocks planet for human life, have we considered that other planets may be a Goldilocks for something else?

And Earth is a place where pretty much everything stemmed from the same evolutionary trunk. We are far, far more closely related to octopi or centipedes than we would be to anything that evolved outside our biosphere. In fact it's pretty likely that we may not even recognize alien life as being alive, at least in the sense we are used to. A silicon based or argon based life form would exist and operate in a completely alien manner (heh) to the carbon base that we're used to seeing.

Moral of the story being, the chance of humanoid aliens existing is a miniscule fraction of the chance of alien life existing at all. Much more likely that we'd scan right past an alien lifeform and quite possibly not even notice it was there, especially if we're busy scanning for carbon-based Earth style life forms. Gotta keep your eyes and your options open, because the end result of a completely different evolutionary timeline will inevitably produce a completely different style of creature.