I don't see good evidence to believe in aliens. There are apocryphal anecdotes, films and pictures; ancient intuitions and concepts like Leibniz's possible worlds; Hoyle's panspermia hypothesis; tenuous theories like Everett's multiverse, but no hard proof. The 'WOW signal' of 1977 might be as close as we come to empirical signs of alien life, though I share Einstein's scepticism.
Ancient philosophers and cosmologists believed in alien life. According to the 5th century BC philosopher Anaxagoras, the elements and processes which created the biosphere are universal, so other forms of life and humanoid species must exist elsewhere. The 16th century philosopher Giordano Bruno also believed in other Suns and worlds with life; Thomas Paine said the same in the 18th century; and modern cosmologist Brian Cox echoes Anaxagoras' belief that boundless space and universal processes make alien life inevitable.
I challenge the assumptions which underlie these claims, even if they prove true in future. While humans can be solipsistic and anthropocentric, we cannot be sure that the processes which gave rise to life 3.5 billion years ago are universal and not unique. We don't know enough about the ancient Earth to place a figure on the emergence of life. There are mathematicians who calculate the probability of abiogenesis to be close to impossible, and the panspermia ideas just drive things a step back. We know even less about the likelihood of life on other planets than we do of life on Earth.
The American radio astronomer Francis Drake developed an equation - the Drake equation - which predicts the likelihood of other civilizations in our galaxy capable of interstellar communication. The equation sifts the planets in our galaxy through the probability of the emergence of life, intelligent life, high civilization, etc, concluding that there should be 900 civilizations in 100,000 light years of space.
Unfortunately, even if Drake came close, the probability of these civilizations within range of us must be miniscule. If they are within communication range, then it is more likely he would either have underestimated the probability of advanced life, or these civilizations would have advanced far beyond our own, which would be still less likely by his logic. How many of 900 civilizations like ours would you expect to evolve technology to travel as fast as light for hundreds and thousands of years? Suppose we assume 100 futuristic civilizations which are evenly placed around 10,000 light years apart. They would need technology which could transmit messages 50 times the speed of light to get one message to us in 200 years.
Although alien life seems possible and I enjoy imaginative sci-fi, I want symmetry between reasons and belief, so I suspend judgement on aliens. I think belief without evidence is a sign of a corrupt or credulous mind. If you believe things like that without strong evidence, then I have less reason to trust you on others things. You confuse what you want to believe with reality.
Why would aliens be our friends when most of the life on Earth is either psychopathically indifferent to us, a mortal threat, would hunts us if it could, or actively hunts us when we encroach? I see scant evidence of collaboration when even two human species like Neanderthals and Denisovans encounter Homo sapiens. The history of our species is largely a history of tribal, ethnic and civilizational conflict. In light of that, why would we assume that highly advanced alien life, which came from a wholly foreign biosphere, would be our friends? That seems incredibly stupid and suicidal. And yet we have sent probes - like the Voyager 1 & 2 and the Parker Solar probes - with our culture, languages, technology, genetics and even the layout of our cities tens of billions of miles into space.
