If intelligent alien life was confirmed, but not through invasion, just scientific discovery, how would the world react?
If we discovered non-intelligent alien life:
Assuming the discovery was made in our solar system (in the oceans of Europa for example). The excitement would mostly come from scientists, primarily from the field of astrobiology, and both NASA and the ESA would most likely begin planning either robotic or manned missions to whichever body in our system tested positive for life in order to gather samples for analysis.
If the detection of non-intelligent life comes from an exoplanet (a planet that's not from our solar system), then the most we'd be able to do with our current technology is observe from afar, primarily by using orbital telescopes to gather more information to not only confirm the detection, but also begin observing the distant planet for potential signs of intelligence. SETI may also start broadcasting directed radio messages at the exoplanet, hoping for some kind of response.
Society as a whole would be largely apathetic towards this discovery.
If we discovered intelligent life:
Discovering alien intelligence would be the single biggest scientific discovery in all human history and would most certainly invoke a much broader and larger reaction from society as a whole. In every country, people would be either out in the streets celebrating, or cowering out of fear in the nearest corner.
Organized Religion would suffer the most after hearing about this discovery since the three Abrahamic religions (Judaism, Christianity, and Islam) are largely geocentric in nature, viewing humanity and the Earth as unique entities that are special to God. This means that those religions would have to subject their holy scriptures to serious reinterpretation in order to accept the existence of other intelligent lifeforms. Fundamentalists may choose to deny the discovery entirely, or explain it away as the aliens being demons sent by their religion's equivalent of Satan to destroy humanity and God.
The world of science would be turned on its head by this discovery, with countless scientists from every space-related field scrambling to gather as much data as possible. If the detection came from radio waves, then they'd be working tirelessly to decode the message and, hopefully, assemble some kind of message.
Science Fiction would also go crazy, creating all kinds of new stories, ideas, designs, etc for these alien beings in a hope to better understand our newly-found cosmic neighbors.
Let's just hope they're friendly.
The world's reaction would depend on whether we thought the life was intelligent or not.
If we discovered non-intelligent life, as (IMHO) we may well do in our solar system, the reaction would be similar to what happened when we discovered planets around other stars - a lot of happy excitement among the science- and science-fiction- friendly population. There would also be a lot of interest and excitement (and a flood of speculative academic papers) among biologists et al, but not much else. Mobile life would create more excitement than sessile life, but I don't think it'd be qualitatively different.
Now, if/when we discover alien intelligence, that's when the general population will get excited. I would expect big arguments over whether to treat the life as a threat. This would be independent of whether the threat was real - we could discover transmissions of an interplanetary peace dance, and we would still get some pretty fearful arguments (note that distance makes any threat pretty remote without evidence of very fast travel). I can't say how the major religions would respond, but I suspect they would adapt pretty quickly, each in its own way.
All the reactions would be talk, though. Unless we saw signs that aliens were due to arrive within ten years, I would not expect any real actions from anyone.
Q: If intelligent alien life was confirmed, but not through invasion, just scientific discovery, how would the world react?
It'd make arguments on the internet in general, an Quora more interesting in that they'd be some real data to argue from as opposed the delusions so common.