Oh man, as always I don't see a thread until it's knee-deep in personal attacks. Well I've actually done a bit of research in this area (previous to this thread, not because of it), the chances of any life developing on our own planet, much less intelligent life are smaller than you might realize. I won't get in to specifics, but it is truly staggering all the slim, by-chance, events that caused us to evolve as intelligent beings. However, the true scale of the interstellar realm is often underestimated. In our galaxy alone I would argue that there is life on other planets, without a doubt. As for intelligent life, a whole new group of factors come into play, but I would still guess that right now, there are other self-meditative organisms in our galaxy...and they are probably carbon based.

However, an important factor you've overlooked so far is time scale. Our sun will last another 7.5 billion years as a main sequence star. However, it will only support life on earth for another 1.5 billion years. In stellar evolution, this is quite short. On top of that, you must allow a star's terrestrial planets about 1 billion years to form, and at least another 4 billion for them to resemble anything that could potentially bear life. As star's age they grow larger and this heat their surroundings more, so if a star has a main sequence life of, say, 12 billion years, it would only be able to support life on any planet within reasonable distance for, perhaps a third of that. Factor in the time it takes for a planet to form and develop into a life-supporting entity, and this leaves a sun-like star only about 3 billions years, maximum to support life on any given planet...and that's assuming that the development of the planet and age of the star match up just right.

So short answer...yes, there is life on other planets, yes there is intelligent life, but more importantly there has been much much much more life that has grown, evolved, contemplated life on other planets, and died out long before the earth was even born....also we will probably never experience any meaningful communication with other intelligent life...distances are just too vast, even if we somehow came up with some kind of light-speed travel...which has been proved impossible.

So there's you astronomy lesson for the day.

you would say that a technologically advanced society would be unable to leave their own solar system to find a new planet to live on or develop artificial means to sustain life? life on this planet for another 1.5 billion years..... well lets assume we survive and manage to keep our planets ecosystem chugging along in that length of time we might not need the sun. we could upload our personality onto a chip and send them off into space. live out our own earth in a virtual world. far fetched now true...but 1.5 billion years is an awful loooong time.

I'm surprised this debate is so one sided. There are a lot of very good reasons to believe that life on Earth is unique in the universe and I happen to agree with them (see the Rare Earth Hypothesis). For one thing, the sheer improbability of life arising in the first place in my view cancels out the admittedly massive size of the universe. There are billions of galaxies and billions of stars but relatively few of them are in the feasible zone - far enough from the centre of the galaxy that radiation and astronomical interference makes life impossible, but not so far out that there is not enough matter to create the necessary fission reactions to create heavier elements. There are dozens of other factors which reduce the probability even further (see the Drake Equation), plus the relatively young age of the universe. I think to automatically assume that the universe is so big that there MUST be other life somewhere is dangerous and probably a little influenced by emotions. Who is to say that life arising is not just as rare or rarer than the circumstances that led to the Big Bang? And we know we've only had one of those so far...

'as far as we know'...what if 'our' singularity is not the first ?...'feasible'
are genes the only way of begetting ?...and as Sean says...if any alien bumps into us too soon..s/he/it, better be strong on patience

We've only had one of those... I use the term "we" as in our universe. If there has been a Big Bang in our universe since the Big Bang that created it, I think we'd know about it. Anything that happened before the Big Bang is pure speculation and not really relevant, just as for humans no-one knows what happens after we die or before we are conceived. (My personal best guess in every case is a big bucket of nothing!)

The point is, the Big Bang is an example of a very unlikely event that has only happened once in the history of the universe, despite the universe being very big and by our terms, very old. Life may or may not be another example, the difference being if that happened somewhere else in our universe, it wouldn't destroy us. But I don't know on what basis anyone can compare the probability of the two events (given that we don't know the cause of either), and one is just as likely to be more or less improbable than the other. The fact that they have both happened once is irrelevant, because they would both have had to happen once for us to be here to talk about them!

All this so called proof you bring up has one problem- it's proof as we know it. Science as we know it... Get it? Just because our brains can't comprehend something, does not mean its impossible. We are learning things about the universe everyday. What was not possible before, is now. So i find it arrogant that people still assume that ALL forms of life must be carbon based. All because its what WE KNOW.

Noodlebug, a few of your arguments are a bit inaccurate. Unless a planet is withing the very central bulge of a galaxy, the radiation and interstellar dust would be negligible. Also to say that the small improbability of life cancels out the large scale of the universe is just ludicrous. In our galaxy alone there are several trillion stars, and our galaxy is relatively small. As to the heavier elements, most were produced by the earliest stars already reside in star-forming nebula. You are correct that space is mostly empty, and the distances between...well anything is truly staggering, but considering we've already charted over 2000 galaxies besides our own, and hundreds of thousands more star clusters, and considering that a new generation of stars with terrestrial planets are produced every few hundred million years, I think it is safe to assume that the factors contributing to life on earth are not unique.

They're not just my arguments, I didn't come up with them, there are very qualified and knowledgeable scientists who have researched into and written about this subject at length. All galaxies are denser towards the centre, that is where most of the stars are, any form of life based on any element would be impossible in the face of radiation which caused heavier elements to decay. In addition, if you are in a densely populated area of the galaxy, all the neighbouring stars' gravitational fields would play havoc. A stable system like ours, where stars pass by within range to affect objects rarely (I think there has been one example since life began, which has been proposed as a possible explanation for one of the periodic extinctions) would be statistically impossible. Just like on the planetary scale, there is an interstellar "goldilocks zone" where the formation of Sun-like systems and Earth-like rocky planets which could feasibly support life is possible. But being capable of supporting life and life actually being created is not the same thing, as I said there are dozens of other factors to take into consideration (and possibly others we haven't thought of!).

if you have even a basic knowledge of chemistry it is very clear that all life will be carbon based.
if you do not believe that the universe was created by god (or indeed sneezed out of nose of a supernatural being) you have to believe that there is live out there.

So I'll agree that the universe in its infinite size inevitable contains another intelligent species, however, if you are going off probability, they will be located so far away from us that we will never encounter them. So they may as well not exist. No one will ever develop faster-than-light travel. Period. We will be stuck in our own solar system and maybe at Alpha Proxima/Centauri forever, unless someone has the stones to go on a 10,000 year multi-generational voyage or something ludicrous like that.

Life exists under three conditions. Of course, this does not apply to human life on Earth only. There is no reason to believe that these rules could not apply, somehow, on Pluto. There is also no proof that God did not create these rules, as faith, by definition, cannot be proven.

1. Replication : things must be able to make copies of
themselves.
2. Mutation : these copies must contain small differences (or
mistakes).
3. Heritability : these differences must be the same in their
copies.

As usual, I'm not entering into any sort of discussion with you plebians. I'm just telling you all how it is.

Be fruitful and multiply....

It is clearly quite impossible that there should be absolutely no life on other planets. However, the chemical properties of carbon would make it nearly impossible for life to develop based on another element. In addition, I would say that while bacteria could have developed many places in the universe, anything more complex would be quite impossible given the massive number of neccessary prerequisites.

> no argument possible...maths and probability lay it out.
> Carboniphilia ...one element out of many
See the Drake Equation: http://en.wikipedia.org/wiki/Drake_equation

The uncertainties in each of the inputs is so large that we don't know whether there is any intelligent life out there, or whether there's loads of it all over the place. We don't have enough data to say one way or another how likely it is, because it's so hard to extrapolate from one data point :-(

Kestas is right of course, any comparison of the probabilities must be a guess. But I personally think that if life itself (ie bacteria etc) is not unusual, then given the size of the universe, and the probable large number of rocky Earth-like planets in the Goldilocks zone of planetary systems where water is liquid, with a large moon to cause tides, with a relatively stable temperature window, and big gas giants around to suck off all the dangerous space debris before it hits us, oh and don't forget that important strong magnetic core with a field that deflects the radiation of the sun in exactly the right way... however improbable these circumstances might all seem, there are so many stars in the universe (even discounting most of them for being too radiated/active/element-light) that if bacterial life is common, it frequently must evolve into complex and intelligent life. Of course we have no evidence of a universe teeming with intelligent life which could mean there isn't any, could mean it's not that common, or could mean we are in a quiet neighbourhood.

I just instinctively feel that it is that initial step - the step from inorganic molecules to organic life - through a process we have not been able to explain or replicate despite many efforts - that must have been astronomically improbable. Hence my comparison with the Big Bang - an event we know is possible but so improbable that it has only happened once in the existance of this universe. Therefore we know such improbable events do exist and can happen on just one occasion.

If our scientists ever manage to create life from inorganic materials, then it will start looking a whole lot more probable that the event may be replicated in nature in far off worlds. But for now that is the indicator to me that the birth of life itself was such a miraculous and unlikely thing that all the improbable events that have happened subsequently to bring us to this point seem virtually inevitable by comparison.

You were really going to let me have the last word..? :P