I think you'll find that every accepted definition of the word 'alien' refers to beings coming from elsewhere, even if 'elsewhere' means just across a border.
Nothing is 'obvious' in science without confirming evidence. At present, there is no evidence of any kind to show there is life anywhere else in the entire universe but here on Earth. And while most people accept that there is a very strong likelihood that life does exist elsewhere, that is not the same as knowing and not the same as it being 'obvious'
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Aliens to whom? All life that we're aware of originates from Earth thus cannot be alien to the Earth or us.Alien just means foreign.When you speak of space aliens you're basically talking about non human foreigners that come from ANOTHER planet.We haven't found any other planets with life yet so it's not "clearly obvious" that they exist.
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It's always interesting and amusing to read the responses of this often-seen question - 'do you think intelligent aliens exist?'.
One type of answer is what I refer to as the run-for-cover response. In this kind of answer, the answerer does not deny the possibility of the existence of intelligent alien life forms, but makes a special point to say that they are all so far away and/or so rare that we will never, ever meet any of them. It's almost as if the very idea of intelligent aliens is so scary that we must all be assured that the human race will never have to meet any of them.
Yes, it's clearly obvious that aliens exist, but since we haven't met any yet, only from from a logical and statistical point of view. To believe otherwise is essentially to believe in magic; that the Earth has some magic in it that occurs nowhere else in the universe, so intelligent life can only occur on Earth. (I'm not a believer in magic, but some people apparently are.)