Rhaedas

I was like Sagan, having grown up watching his Cosmos series. But I've been reading Rare Earth, by Peter Ward and Donald Brownlee, and they have some possible valid points on how formation of larger lifeforms may be more limited than we first believed. There are many things about Earth and its circumstances and surroundings that may or may not be important factors in animal life.

But without more evidence to support either side, it could go either way. They may be exactly right, but earth type planets with its characteristics might be commonplace as well, nullifying the problem. Then again, life, even large frms that can support intelligent thought, may be so persistant that it's a given once a few factors are in place.

For now, I believe that we'll find microbes elsewhere in our solar system, Mars or Europa being the most likely. I'm even crossing my fingers to find something very alien, but alive, on Titan once Cassini gets to Saturn. But I now have doubts on anything bigger...

It's a big universe though...

Friar Bellows

I think just as important as that question is: what is the likelihood of intelligent life developing something akin to science and technology? A related question: what is the likelihood of us and them being mutually intelligible, even to a minor degree? Is the idea of cognitive universality a myth? Interesting questions we don't know the answers to.

Pyrrho

That reminds me of something I heard about early humans killing off "competing" intelligent life. It has been speculated, for example, that Neanderthals were killed off by our ancestors. So, the claim that there should be more than one "intelligent" species on the planet if it were a great advantage isn't convincing, because: 1) there was more than one on earth, and 2) humans (or their ancestors) killed off the other "intelligent" life. When two life forms "compete" for the same position, often one will wipe out the other. Perhaps there is, evolutionarily speaking, "room" for only one "intelligent" species in one ecosystem.

As for the specific debate, I find Sagan more convincing, though he seems, for my taste, overly optimistic. Still, his approach is fundamentally sound:

After all, we are reasoning from one case, and that is inadequate to give us anything approaching a definitive answer to the question. Looking is the best way to find out.

AJ113

Thank you very much for the input so far. This is the first time I have spoken with most of you, I tend to live over on GRD. Most illuminating.

I'd like to shift the emphasis if I may:

Currently there are 110 (approx?) known solar systems, i.e. stars with at least one planet. There are those who would extrapolate from this data that there are millions of solar systems in the universe. Personally I think that there is an extremely high probability of this, too.

But here's my argument: If the probability of millions of solar systems existing is high, or even very high, are we correct in declaring their existence as fact?

My argument is that we cannot do so until we have observed or experienced the solar systems in some way. My opponents' argument is that it is reasonable to make a statement such as "There are many millions of solar systems."

When it was pointed out that this is not proven, my opponents answer was that because it was HE who had written the statement, it should be understood by the reader that the statement is only HIS opinion. Therefore he is justified in making this, and other, apparently factual statements.

Once again, I am concerned more with the argument rather than the subject matter.

You may have guessed by now that I disagree with him, so your comments are appreciated.

Rhaedas

Obviously for each star you cannot say for sure until it is observed whether or not it has planets. But given a sufficient sample, you could predict the probabiity of finding X stars within another sample area.

Now, most if not all of the planets found have been large ones, not earth sized, given the method of finding them the planet needs to have enough mass to move the star, or partially eclipse it. But the fact that there's many systems where matter collected in orbit gives good odds we'll find smaller rocky bodies as well in the future.

So I guess I'd have to say that the more systems we find, the higher the probability grows. You are correct in that we have to observe each system before we can say for sure there are planets there, but the mroe we find, the better the odds the next one will have them as well.

Actually, given the current theory of solar system formation, I'm not sure you can have a star form and not have SOME type of bodies orbiting...whether they are life-friendly or not is up to where and what they are made of, but in general, could a star form without having planets?

Jack the Bodiless

As for whether there was ever life on Mars...

Apparently, life on Earth appeared very soon after the end of the "heavy bombardment" period of Earth's formation, in which life would have been impossible. Suspiciously soon after: hence the speculation that life forms rather readily.

But Mars had a much longer period in which life could have emerged there: between the planet's formation and the loss of its dense atmosphere and liquid water.

So, if life is common, it probably arose on both Earth and Mars. Whereas, if life is rare, it probably arose only once, on Mars, and was carried to Earth in meteorites.

In either scenario, there should be evidence of past life on Mars.

Shadowy Man

Not to be overly pedantic, but there is only one solar system.

ps418

Even if that is true, human-like intelligence is still extremely rare, and has only occured in one genus. There is no evidence as far as I know that any species prior to Homo had any remotely human-like technological "intelligence." Even if there were only room for one "intelligent" species on earth at a time, there was plenty of time before Homo came along, yet no evidence that such existed.

Patrick