I agree with Survivorman. People our handicapped by our definition of 'habitable'. The organism could be a creature that live at the bottom of oceans, in the deepest trenches of their world, or a animal that lives inside volcanos. A world that wouldn't be habitable for us might be extremely habitable for some species. They might not be based off of carbon and oxygen. Perhaps they're based from nitrogon and argon?

I think whether or not we're visited by aliens depends on a few things. A: If they have some form of FTL travel, cryo sleep, or incredibly long lives. B: If they are genocidal and view every other alien race as a threat. C: If they want to observe us until we are ready for some form of contact and D: To see whether or not our world is something they would want. (Obviously, all of the others are contingent of A being true.)

How do we know this? The opponents claim the opposite it true (that the chances are closer to zero than one can comprehend).

Gosh i like this number. It's sooooo evident, it makes scientific notation blow up in smoke.

I'm offering one more opposite opinion to the 2 minimal formula unproven and most probably as near as  anyone ever reached for a conclusive hypothesis.

We, today, cannot imagine what the truth is for a number of reasons; our conscience and what senses allow us to perceive or express it. If there is (at least) a second life-form (be it different or smart enough to return the favors from Earth) -- it either WAS out there or WILL be there.

That -- nobody can deny. By replies in a thread or by publishing books.

I dont understand statistically there has to be other intelligent life out there. How could you possible argue that there isn't?

I agree with Survivorman thus far, or atleast his approach.

An alien species would be adapted to its own environment. They may not even have a concept of war, or peace, or trade, or religion, or any one of these or another. They could have have been born unto an extremely docile environment, so years of evolution and growth would/could give them no concept of the "survival of the fittest" or perhaps war. The reverse could also be true as well as many other variations, and concepts that we may have no idea about.

Did you walk up to every planet and ask, "do you have life?"

So far, we have no statistics.

. . . except maybe from SETI. And out of the heaven knows how many stars they've looked at, we're - well we've got a big, fat zero.

Problem with zero is no matter how many times you multiply it, you still get zero.

It's the Fermi paradox. Or, to put it bluntly: If they're really so probable, why haven't we found them?

Science loves to have evidence. Evidence is good, evidence is great. But we've got no evidence.

There are more stars in the Universe than there are grains of sand on all of Earth's beaches. I dont think you understand the shear amount of planets out there. And there is evidence that planets will form with water inside of the habitable zone quite often.

Im sorry but Earth is not going to be special, were not a fluke. if life formed here it will form there and eventually intelligent life.

That's interesting, but rather meaningless.

sheer.

Problem is, it doesn't matter - as we don't have any idea what fraction of planets have life. It could be an exceedingly small amount. It could even in fact be just us - the issue is not how many planets there are, but the fact that we know nothing about the probablities of life, or even if life was an accidental event at all.

Water is necessary for life, but not sufficient. What you just said is meaningless in this context.

It doesnt matter that we have not detected other life out there. We know about planet formation. We know what elements are available from a star by viewing its light. The planets will have atmospheres that are comprised of the readily avialable elements. All the organic compounds are available where some stars form. So other solar systems will contain similar planets as ours. Experiments show that basic organic compounds will form in variouse types of atmosopheres under the right conditions.

SO ITS ALL ABOUT THE NUMBERS.

There are so many planets out there that earth like planets WILL FORM.

Sure it does. If somebody presents a hypothesis, and the evidence contradicts it, then we need a new hypothesis.

We have guesses about planet formation based on various things we see in telescopes.

We know what elements are in the star at the time. The composition of the planet is AFAIK still not directly observable.

Well, I guess that means Earth's atmosphere is made of mostly Hydrogen and Helium.

Strange. Never heard of organic compounds being discovered in stars.

Do I need to explain the difference between "sufficient" and "necessary" conditions?

Yelling won't make your logic any better.

That wasn't your claim. Your claim was:

I like to think about it like this; 1st, we as in humanity, are enough evidence for alien life. in a universe as large as ours, its impossible for an event such as life to only happen once. 2nd, even with numbers as pointed out by praetor(where exactly did you get that number anyway? seems a little exaggerrated to me), there is more than enough room for more than 1 instance of life. for example, a galaxy as large as ours will form many trillions of planets in its lifetime, that alone is an incredible amount, in addition to the many trilions of galaxies in our 1 arm of the universe(with many other braches to count), leads to a statistical impossibility that life will only form once. 3rd, for those who have faith in the bible, you have already discovered aliens. God, the Devil, Angels, and Demons are not from or of the earth. Nor are they some direct product or relation of humanity. so in that sense, they are very alien. 4th, also for believers, gods said he created angels ccording to the bible. if memory serves then he did not say what specific form they shall take or that they shall only reside in heaven. so once again, on those grounds, the faithful have already discovered alien life.

Whether or not the hypothetical alien species is anything like us at all, it still must obey the laws of physics.  Using only one criterion (that there should be no deadly radiation), we can eliminate over 99 % of the stars in the universe.  Each successive criterion (and there are many: the recipe for life is much more complex than "just add water") takes away over 90 % of the remaining stars.  Further, why do we keep assuming that all stars have planets?  To be sure, we have planets, and we know of SOME other stars that have them, but as far as we know, the VAST majority of stars have no planets.  Further, we have this odd idea that all habitable planets are inhabited.  As I said before, the simplest life has a little over two hundred proteins in it, each of which has approximately the complexity of a Rubik's Cube.  A Rubik's Cube could be solved by a blind man in approximately 1.6 trillion years.  (If you're wondering, I got those statistics from The Blind Watchmaker, by Richard Dawkins.)  The life-span of your average star is about ten billion years.  So we are asking the wind and the rain to solve two hundred Rubik's Cubes in less than ten billion years.  This is difficult, to say the least.  And further, we must cut down the time allotted to the wind and rain by about four billion years, as that is how long it takes for intelligent life to evolve from the primitve bacteria.

As a side note, I noticed someone above saying that "[these creatures] might not be based off of carbon and oxygen. Perhaps they're based from nitrogon and argon?"  Who taught you chemistry?  Please let me know, so I can slap them.  There are many good reasons why all life must be based on carbon or some other element in column IVa of the periodic table, but even putting all of them aside, argon is a noble gas; it can't react with ANYTHING!  Meaning that NO other chemicals are derivative from it!

Are we?

Is it?

It is. The estimates vary wildly depending on who you talk to. But there are some that peg is as pretty much impossible, especially if you are willing to deviate from the path of pure naturalism.

Or, for that matter, from or of our universe, according to most forms of Christinity. They are "aliens" in the sense that they are not from Earth, but not "aliens" in the sense of they are from another planet.

Without the survival of the fittest, evolution cannot occur: it is the driving force behind evolution.  Therefore, an intelligent being not produced by either survival of the fittest or the from-scratch fabrication of an Intelligence is a contradiction in terms.  As for war, war is the natural state of all creatures, and to be intelligent and have no conception of it is, again, quite impossible.  Whenever you have multiple individuals in one place, they are inevitably drawn into conflict, "bellum omnes contra omnia" as Thomas Hobbes put it.  It is only after intelligence arises that government and law can begin to make peaceful coexistence possible.  And even then, war is the natural state of the intelligent species.  Indeed, we have not made peace today: there has never been a time in all of human history without conflict!

And really, this approach, while valid, is altogether meaningless.  If we assume nothing about these hypothetical aliens we are encountering, we cannot usefully conjecture anything about what will happen if/when we do encounter them.  So unless we assume that these creatures are at least somewhat like us, this thread is pointless.

And, as a good many of you want to know where I got the figure from, I got the number 1/1000000000000000000000000000000000000000 from The Privileged Planet, by Guillermo Gonzales, a professor of Astronomy at the University of California at Berkely...