Me, I believe that we will discover extra-terrestrial life in our lifetime. I'm a Muslim, and the Koran suggests that Allah created and spread life on earth and in the heavens, although scholars have differing interpretations. OK I'm also a Battlestar Galactica fan, and although I don't believe we will be abandoning Earth any time soon, but it's an interesting thought experiment nonetheless.
Opponents say there are just too many prerequisites for life; too many reasons why Earth is so perfect to enable complex life forms. The so called "rare earth hypothesis" conjectures that in order for complex, multi cellular life to exist, there must be at least the following: a right-sized terrestrial planet (i.e. not a gaseous planet like Jupiter or Saturn) in the habitable zone (i.e. not too hot and not too cold), a central star and planetary system, the advantage of a gas giant guardian (i.e. Jupiter and Saturn's gravity protecting the Earth from stray asteroid impacts), large satellite, plate tectonics, the right atmosphere, and oceans. Not many, if any at all, planets have the advantage of all of these characteristics all at once. Earth, they argue, is so special because it ticks all of these boxes.
But in my opinion, here are some reasons why I am convinced that we will find evidence of extra-terrestrial life:
1. Looking at the signs on Earth, life has been discovered in the most extreme habitats. Bacteria lives in the upper atmosphere, where temperatures are beyond freezing, oxygen is lacking and UV radiation is plentiful. Sea cucumbers live 7 miles (!!!) deep in the Mariana Trench, where it never sees sunlight and pressures are off the charts. Mountain stone insects can be frozen solid at -10C for months in the winter and come back to life come spring. Hydrothermal vents in the deep sea are home to dense fauna feeding off hydrogen sulfide, a highly toxic chemical, and geothermal instead of sunlight as the primary energy source. What we now assume as inhabitable zone at some planet out there, may actually turn out to be suitable homes to some yet-to-be-identified creatures.
2. Historically, life on Earth has survived five major extinction events, each of which eradicated >70% of all known species, and that's before mentioning "smaller" events like the Mt. Toba eruption that covered half of the Earth in plume and greenhouse gases. The tardigrade, able to livewithstand temperatures from just above 1 K (that's -450 F or -272 C, yes that is cold) up to well above 150 C, pressures 6x greater than those found in the deepest ocean trenches, radiations at hundreds of times higher than the lethal dose for a human, and the vacuum of outer space. They can go without food or water for 150 years, drying out to the point where they are 3% water, only to rehydrate, forage, and reproduce afterwards. Who's to say that we won't find extreme survivalists at another star system in another galaxy?
3. The pace of progress in human intelligence is exponential. In only 10,000 years -- just a tiny part of the history of Earth -- humans have advanced from solely hunter-gatherers to observing space, actually setting foot on the moon, sending a probe to Saturn and even beyond the Solar System, and building a massive space telescope that can observe events happening at the beginning of time. We did all of that. If there's someone/-thing out there, it's only a matter of time before we find it.
4. It's just more fun that way. Saying that we are alone, or that our Earth is somehow so special, seems so self-centered and close-minded. Curiosity and innovation are fostered when we believe things are possible. Can you imagine the world if the Wright Brothers gave up believing humans could fly like birds?
Will we be punching aliens ID4-style and screaming "hasta la vista baby" ? Probably not. Even assuming that complex, multi-cellular, maybe intelligent life exists at other planets, consider this: the universe is 13 billion years old, and the Earth is much younger at 4.5 billion years old. Human civilization is around half a million years old, and this whole entire human history is only possible because we are experiencing a prolonged "warm" period in between ice ages. What are the chances that humanity's time, which will only last until the next glacial period or extinction event (another 50,000 years, scientists predict), will coincide with the story of another intelligent alien being? Probably slim. And that's only considering the time dimension. What about the space? The universe, in case you haven't heard, is a pretty big place (and is continuously expanding too!). What are the chances that not only humans and aliens coincide in terms of time, but also get to meet IRL and hang out at Starbucks? I say non-existent. What is possible is that we will find undisputed evidence, probably in the form of fossils, of simple life forms (some form of bacteria?) that lived half a billion years ago at some terrestrial body somewhere in space.
And I do believe this will happen in our lifetime. I'll bet money on it.