Given the size of the universe, it seems inevitable that there is life on other planets. There is a tendency to regard the origin of life here as something isolated, I'd argue it's a mistake resulting from a common habit of treating life as something apart from the physical universe. The notion that life is an anomaly is a failure of perspective that comes from an arbitrary separation between the organic and inorganic worlds. There is a continuous chain of events stretching from the first auto-catalytic chemical reactions in the primitive environment of our planet and the organized complexity of a living cell.

Life is an inevitable consequence of the laws of physics and chemistry. Since those laws are what they are, the development of life should be seen as equally natural and inevitable on other suitable worlds because natural processes are inherently repeatable. Give the right chemical reactants and energy in similar amounts under comparable conditions, and you should invariably expect the same result.

There's a good thought experiment by Edwin Jaynes imagining a laboratory with thousands of identical flasks each containing pure water, and each heated to exactly the same temperature. Such a scenario leads us to expect with near certainty that the dissolution of a given chemical compound in one of them will be repeated with all the others, being a function of stable conditions uniform throughout the whole experiment. To suppose that the substance dissolves in only one single flask is to imply that the laboratory’s conditions are not uniform. It would mean the room must be an extremely special place, with the temperature and pressure in just the right combination to sit on the solubility boundary of the compound. From the statistician’s point of view such an arrangement is a vanishingly small probability as compared with the unvaried explanation.

And our universe is just such a laboratory with each solar system being its own flask, and the world of life on our Earth establishes one data point proving that the probability of abiogenesis is not zero. If the laws of physics have permitted chemistry to pass into biology here, statistical logic suggests that they will do so elsewhere. The presence of life in a single case by no means implies a crowded universe, but it does strongly suggest that biology is no stupendous miracle.

And so the burden of proof is shifted by the fact that the essential inputs of biology are not unique to the Earth, but are shared by the universe as a whole. The latter acts as a vast spawning ground dotted with incalculable potential nurseries for life where the specific chemical threshold may be breached. And we haven't even really explored much of our own solar system, such as the icy moons of Jupiter and Saturn or even Mars.

The great variety of organisms on our planet, embodying the main streams of biological development and occupying every conceivable sort of environment, shows the plasticity of life’s basic physiological processes. Though the primary biochemical constituents be the same, their aggregation into functional systems may be accomplished in many ways, allowing life to take hold in the most extreme environments from acid pools to searing deserts.

All that said though, it’s highly unlikely we’d observe a civilization at our level of development. Life on Earth appears around 4.5 billion years ago. Humans start evolving around 2.8 million years ago. Use of language appears around 100,000 years ago. Writing is invented around 5500 years ago.

Inventions of language and writing are the landmark moment here. Before language was invented the only way information could be passed down from ancestors to offspring was via mutations in our DNA. If an individual learned some new idea it would be lost with them when they died. Language allowed humans to communicate ideas to future generations and start accumulating knowledge beyond what a single individual could hold in their head. Writing made this process even more efficient.

So, after millions of years of life on Earth no technological development happened. Then when language was invented humans started creating technology, and in a blink of an eye on cosmological scale we went from living in caves to visiting space in our rocket ships. It’s worth taking a moment to really appreciate just how fast our technology evolved once we were able to start accumulating knowledge using language and writing.

Now let’s take a look at how technology itself has been evolving. Once we discovered radio communication we went through a noisy period where we were leaking a lot of our broadcasts into space, and within a span of a 100 years we started using more efficient communication, and encryption. If somebody intercepted our broadcasts today they would look like noise because they’re designed to look like noise. Our society today is utterly and completely unrecognizable to somebody from even a 100 years ago. If we don’t go extinct, I imagine that in another thousand years future humans will be completely alien to us as well.

So the period during which intelligent life would be recognizable to us during its course of evolution is infinitesimally small. The time between creating language and becoming an advanced technological society is measured in thousands of years, while evolution of life is measured in millions of years. The chance of two different intelligences finding each other at exact same stage of development where they might be able to communicate is incredibly unlikely.

I would also imagine that the biological phase for intelligent life is rather short. We’re likely to develop human style AIs within a century, and they will be the ones to go out and explore the universe. Meat did not evolve to live in space, we’re adapted to gravity wells. An artificial life form could be engineered to thrive in space without ever needing to visit planets. This is the kind of life that’s most likely to be prolific in space. Furthermore, post biological intelligences would likely be running at much faster speeds than our mental processes operate on. What we consider real-time would be might we consider to be geological scales. Such beings might consider what we view as real time akin to the way we look at continental drift. We’re aware that it’s happening, but it’s of little interest to use on day to day basis. It’s quite possible that advanced civilizations become solipsistic and care little for the outside universe.

For all we know the Universe may be teeming with intelligent life and we just don’t recognize it as such. We might be like an ant hill next to a highway looking to see if there are other ant hills around.

What humanity has dubbed "aliens"? No.

Extraterrestrial creatures/other lifeforms outside our galaxy? I don't know. We don't have enough data to prove it true or false yet.

No but i want to. I just havent seen any credible evidence. shrug

I personally believe we actually live in the era after abundant life and we're one of the late starters. I don't think it'll happen but if humans could traverse the universe quickly I think we'd find a lot of long gone life and a small fraction of lost civilizations maybe a few still surviving pools of life.

I also believe life is actually pretty common. I think Mars and moons of Jupiter or Saturn have had life on them at a time

It's not a belief for me, rather knowledge that there's a high probability.

I believe they're out there. I dont believe theyre cool Star Trek aliens flying around in spaceships and shit.

Its fun to think about though, and could be real.

Yes, I believe they exist. Do I believe they've visited Earth? No.

Yes, the universe is too large and we find life everywhere we look on earth (extremeophiles) and have tantalizing evidence of life in the solar system. I even think that we don't understand intelligence, and that a non terrestrial intelligence has developed somewhere.

I don't think they are abducting humans or making crystal skulls

I 100 percent believe there are aliens out there, o don't believe they've ever come even close to the Earth

Here and now? Doubt it. Out there at some time. Seems likely.

That depends. Yes, I believe it is probable that life exists elsewhere in the universe. But no, none of that life is building pyramids in ancient Egypt, abducting cows, or anal probing hillbillies.

Yes/no is less interesting than your reasons for your answer. And most importantly, what impact does your answer have?

For me, it's a no. Because there's no hard evidence. And by not believing, I save a lot of time and lose absolutely nothing.

That isn't to say it's impossible. It's to say nobody has show me where it's probable, based on hard evidence.

And to those here posting that animals wouldn't count because they're not intelligent: you're wrong.

We are the aliens. We conquered the planet, destroyed the biosphere, and will soon be gone.

He likelyhood of this planet being the only place in the universe some complex proteins folded a billion years ago is zero.

Life on other planets? Seems likely.

Life on earth started pretty soon once the conditions were met.

Intelligent life (humanlike), maybe not. Most probably not. Out of millions of species through billions of years only humans have evolved to have this kind of thinking machine during a relatively short period of time.

Yes but I don't believe they figured out how to cross the distance/time vastness of space.

Depends what you class as "alien".

Given that there are literally billions of galaxies out there and each one contains billions of stars with each one having the possibility of its own multi planetary system, it would be rather silly to assume that on our random planet of literally trillions of others, it's the only one that life has or will evolve on. It might be not be life that has the ability to read a post on a smartphone but it might be more simple. It might be more advanced but I'm sure there's probably some form of life.

Whether any have been able to build inter-planetary/or inter-galactic vessels and travelled to Earth...highly unlikely. And we shouldn't be so conceited that if they did come that they would come for us! We've only been around for a million years. We're relative newcomers on the block.

They are hiding in Antarctica under the ice.

Giving the size of universe, there is problably life on other planets, too.

I'm an alien. I'm a legal alien.

I think it’s likely aliens exist, but unlikely any alien species will be what we expect. I highly doubt we will see humanoid “little green men” or anything like the majority of science fiction portrays. Each planet is vastly different in just our solar system. The vast differences in environments across the galaxy will yield vastly different paths of evolution. We are only looking for “life” that fits our narrow definition based on Earth’s species - we have likely already missed signs of alien life.

I'm actually kind of excited for what we might find in the oceans of Europa. You never know!