GunnerJ

Aren't the boiling and melting points of water also infuenced by air pressure? So wouldn't a planet with higher atmospheric pressure (e.g., a larger planet with a thicker atmosphere) have a different habitable range?

Photon

Not only air pressure, but also tidal forces that can generate heat in the core of a body can also cause huge variations in the habitable range. Also, you have to consider doping effects (anytime material other than water is mixed with the water) can also affect the range of temperatures over which water can be liquid. I'm sure there are more factors.

HerodionRomulus

If not whales, what about dolphins? I thought Flipper was supposed to be reallllly smart?
Heck he got a tv series.
No wait---that's an argument against intelligence.

never been there

As the site I linked to stated, the discussion is the main benefit of the Drake equation. Interesting posts, everyone.

I DO support continuing some level of SETI activity. It's not a top priority, but we can't NOT do it; the significance of even a possible find is too huge.

In his A Brief History of Time, Hawking makes what I think is a very convincing argument about the odds. The Drake equation implies a continuing universe - as if we're in the middle of an infinite stretch of time. However, if you compare the estimated age of the earth with the estimated age of the universe (give or take an eternity), and apply the same odds, there's a very good chance that we are the first communicating life in the universe. Just have to wait another couple billion years for some purple slime somewhere to get multicellular and invent radio. If you suppose that elements heavier than iron are necessary for life (I think they help, but aren't necessary) then it's quite amazing that we're here.

P.S. thanks to JTDC and contracycle - I had to work through it a few times before I was reasonably sure I wasn't counting fL twice unreasonably - good to hear the arguments put in other words.

Cheiron

So you're telling me that potentially Humanity as a species could end up being one of the "ancient" species of the universe that SciFi is so apt to throw around? Man i hope these millenia are the terrible 2's or we'll never get past puberty as a species.

gcameron

re the evolution of intelligent life...

1) It is possible that whichever species first evolves human-level intelligence will alter its environment so significantly that it is hard to extrapolate what will happen next. (Incidentally, whichever species first evolves human-level intelligence will also be the first to have the ability to ask, "why are we the first/only ones?" and so will invariably confront that dilemma.) What will the influence of humans (and their descendants, whether they be more humans, genetically-modified humans, super-AIs, or whatever) have been on the Earth's ecology when, say, 100 million years have passed? A miniscule blip ending in an orgasm of nuclear/biological holocaust, or a seminal change to the whole ecosystem, or destruction of the environment altogether, or something in between? Hard to say...

2) Is high level intelligence harder to evolve than, say, wings (which have evolved -- 4 times? (bugs/bats/birds/pterosaurs) -- in Earth's history?), or other structures that have been seen in convergent evolution? Is the "payoff" (in terms of improved reproductive/survival rate) more subtle, less available to many species? Is the "groundwork" harder to lay than it is for other structures?

What made primates the candidate of choice? Articulable digits? Necessity of navigating the arboreal canopy? Social infrastructure? etc.

Are any other species/families/genuses on Earth potentially "poised" to benefit from increases in intelligence? If we could fast-forward another 50 million years or so (and subtract humanity's own influence from the equation), what might we see? Would the primates take it again, or some other group, or nobody?

3) Hasn't it been argued that only certain chemistries seem likely to be able to provide a stable template for life/replication/evolution/etc.? I think Sagan talked about this in "Cosmos." Of course we are fondest of carbon because that is what we are made of, but can our understanding of the elements allow us to rule out most other chemistries as being viable for life?

SLD

I think there are two caveats to add to this point. One, life can only evolve around heavy metal stars (no, not that kind of heavy metal, the atomic kind ). However these are only in the outer reaches of the galaxies, not in the central bulge where most of the stars are. So you can cross off a large percentage of stars right off the bat.

Second, is that life cannot evolve inside the galactic arms, but only in between. Why? Too many supernovas.

These two factors alone should rule out 90% of the stars in our galaxy from even being considered in the drake equation. It would be better to use a figure of 10 billion stars in the galaxy and go from there. Still, using fairly pessimistic numbers, I came up with 81 civilizations - quite a bit more than I expected. Of course, I am of the opinion that if life can evolve it will 90% of the time and if intelligent life will evolve, it will learn to communicate 90% of the time. The real problem may be though in the evolution of multicellular life forms first. I would venture to guess that a lot of planets exist out there with nothing but bacterial pond scum like our planet was for the first few billion years.

SLD

Jabu Khan

Think of the different extremes in which life exists on this planet.
I think all thats really necessary is for a process of chemical volatility to continue uniterrupted for billions of years. I would be amazed if there isn't at least the precurssors of life on Mars.
It takes billions of years without loss of atmosphere or a collision strong enough to fry everything or an overpredation of interdependent firsts for more than that to occur though.
Once the ball gets rolling though it would take an extreme catastrophe to wipe out everything.