Doubting Didymus

Although I will point out that the reason certain people are widely considered to be right may just possibly be because they are, I have to disagree with sagan on this point. The crux of the debate boiled down to the probability of high intelligence evolving, which the probabilities derived from our sole sample (earths biosphere) seem to strongly oppose. Sagan did not adequately address this point, saying such frankly weird things as "an overall trend toward intelligence can be perceived in the fossil record", and this utterly unsupported statement: "other things being equal, it is better to be smart than to be stupid", something that may well be true, but in the competitive world of evolution, 'other things' are never equal.

There are some things that selection definitely loves. If we ever discover macroscopic extraterrestrial life, I will put good money on it having eyes of some kind and a head with a mouth (if it is a mobile organism). I also expect legs to a lesser degree of confidence. These things have all evolved independantly in many different lineages. Intelligence has only evolved once. Those are not good odds.

On the other hand, the degree of confidence we are talking about here on both sides of the debate is massively low. We have a stupendously biased sample that could have billions of uncontrolled variables in it that skew the inferences we take from it severely. For all we know, our planet might be unusually bad for intelligent life. For all we know, it could also be the only planet that ever did and ever will evolve any life at all. Our error margins here are so truly massive that it's on par with total ignorance. For this reason I agree with sagan and all the others in his closing petition statement:

"We are unanimous in our conviction that the only significant test of the existence of extraterrestrial intelligence is an experimental one. No a priori arguments on this subject can be compelling or should be used as a substitute for an observational program."

Rhaedas

I agree with the rest of it, but what of the whales? And there's been speculation that had the dinosaurs survived the various things that caused their extinction, they might have continued along a path similar to the mammalian one. Certainly many forms of life other than humans exhibit various levels of intelligence and even tool usage.

But it's true...we have only one test case, the earth, to go by thus far. Earth might be a slow planet, and others may spawn multiple intelligence forms. We may be it, period. Or something inbetween.

Doubting Didymus

Whales don't even come close. They're clever on the nonhuman animal scale, but the gulf between their intelligence and ours is pretty damn big.

Walnut-brained dinosaurs? What is that speculation based on exactly? and why dinosaurs anyway? There have been plenty of other much more intelligent candidates since the K-T extinction, all of whom have failed to produce anything at all. Why should dinosaurs have been any different?

And while it's true that many other species have various levels of intelligence, they've generally had plently of time to run with it, and none ever has.

Jesus Tap-Dancin' Christ

More or less true--BUT--given lifes propensioty to make billions of speicies, why should there not be intelligent life eventually on a planet that spawns it? So what if the odds are 1 in 20 million? That many species are born and die in the blink of a planets eye, right?

Doubting Didymus

The bottom line is; no-one has a frigging clue. Maybe that many species commonly show up on a planet, maybe nowhere near that many will arise. The odds might be 1 in 20 million, but for all we know, they might be a trillion times as low as that: nigh-impossible. We can tell almost nothing from the sample we have.

Will I Am

fp - fraction of stars with planets. An awful lot of stars are binary, and even if
there's a planet or two in the mix, they are going to experience some harsh
seasons, so might as well leave them out. I think 20% is optimistic.


Actually, Science says that 50% of systems are binary. 50%.

Doubting Didymus

Forgive my astronomical ignorance (i.e. ingorance about astronomy), but is there something about binary systems that influences the probability of life forming? Are such systems incapable of forming planets, or somesuch?

Hatchet

Its to do with a slightly outdated idea about the probability of planet formation.

Basically there are two competing models of solar system formation. "top down" is where gravitational instabilities cause the protoplanetary disk around a star to collapse quite "rapidly" to form a planet. "bottom up" is where small particles aggregate together and the planet slowly gets built out of aggregated chunks.

Computer modelling of both these scenarios suggests that the protoplanetary disk won't form planets in a binary star system. (basically the disk gets too "hot" to condense into any planets)

Thats the theory. Unfortunately for the theory planets have been found in binary systems, so the computer models are wrong. And the prelimary results of the latest generation of models shows that it might actually be more likely that planets form in a binary system because of the additional gravitational instabilities caused by having 2 stars.

As far as I know there is no reason that a binary system couldn't support life.


ttfn,

Hatchet

Rhaedas

It's all about the habitable zone, and what's in that zone.

The Habitable Zone

Having more than one large star can certainly influence the radiation a planet will get as it orbits, so you'd have to either have the companion star distant, cooler, or perhaps two very close stars, with the planets appropriately distanced away to receive the correct consistant luminosity from the combination. Impossible for life around a binary, no, but the added variables make things a lot more interesting.

The calculations on that site are I think a bit more "accurate" than Drake's equation. But again, they do make the big assumption, that life needs conditions like ours.

Also, a big factor the above site mentions, the length of time a planet will spend in a habitable zone...ie, the star(s) luminosity will change over time, and even if life gets a foothold early on, if the planet's condistions become lethal, the life can't evolve to a point where the OP is concerned. Likewise with life evolving late in a star's life...

Hatchet

True, for a binary system the habitable zone is smaller, but that will usually be more to do with restrictions on the range of stable orbits rather than radiation from the other star. A planet needs to be within about 1/5 the distance of the closest approach of the 2nd star for the orbit to be stable.

Consider a ternery system like Alpha Centauri, the closest approach of Alpha Centauri A and B is about 11AU. That gives us about 2 AU where we can have stable orbits. Alpha Centauri A is fairly similar to our sun, so a planet 1-2 AU away should be habitable.

The distance between Alpha Centauri A and B varies between 11 and 35 AU. Even at 11 AU "B" will have little impact on any planets orbiting "A". Consider that in our own solar system once you get more than a couple of AU away from the sun you don't get liquid water.

So I still believe that binaries are excluded from the Drake Equations because it was thought that planets wouldn't form in them rather than problems with habitablility of those systems.


ttfn,


Hatchet