The science behind the probability of extraterrestrials

[MrFun]

Careful -- you risk proving to communists that capitalists really are lazy, while exploiting others to do the work.

[MichaeltheGreat]

Actually, we're so inherently efficient that we have leisure time.

And it was a slow day on the markets, too.

[Immortal Wombat]

Originally posted by MrFun
So given this, I have a question -- is it fair to assume that no matter what form life takes in any type of terrestrial planet, that water will be a fundamental, prerequisite for that planet's lifeforms?

Yes, what MtG said. It is likely to be important if it is present, but it is probably not absolutely required.

More exotic alternatives dispense with solvents completely- such as life within a gas giant.

[MrFun]

IF there would be any form of life within a gas giant.

[MichaeltheGreat]

There wouldn't.

[MrFun]

Thus, MTG hath proclaimed an infalliable statement -- let it be known to the world, and let it be recorded for posterity.

[MichaeltheGreat]

Given what Galileo and Cassini have shown so far about the physical conditions in the atmospheres, the physical chemistry problems are a bit tough for even replicating molecules to do much of anything.

[DaShi]

There are organisms that survive from no energy from the sun whatsoever. They're called computer geeks.

[Odin]

The people who think intelligent life is common are deluding themselves. If we played the tape twice (to take a phrase from S. J. Gould) the chance that sentience would evolve again is slim. The belief that sentience is common in the universe, to me, is an rediculous extension of the arrogant victorian idea that sentience, and by extension humankind, was predestined to evolve and that we were somehow the purpose of evolution. The people who believe scentience is common tend to be physicists and astronomers who naively think that sentience will automatically emerge once you have life, obviously showing thier ignorance of biology.

We are the end result of a long line of improbable events and luck. There was a good chance that the Snowball Earth of the late Precambrian could have snuffed out animal life early on. If the Permian extiction would been just a little bit worse it would of wiped out all multicellular life. It was just dumb luck that the same reorganization of the jaw joint that happened at the transition between mammal-like reptiles and true mammals that left the lower jaw as a single sturdy bone also resulted in a change in the braincase that allowed an increase in brain size without destabilizing the jaw joint (which is why the smartest dinosaur, Troodon, was an idiot compared to the early therian mammals that it ate). If the modern ice ages had not happended, Homo would had never emerged from Australopithicus. During that worst part of the second-to last glacial advance 150,000 years ago the population of early H. sapiens was reduced to 8,000. if Africa would of became just as bit drier we would not be here.

[Immortal Wombat]

That's nads, Odin. The belief that sentience is common in the universe doesn't imply purpose, it just implies that it is a successful adaptive strategy. Given enough time, it might be the most successful strategy, because it completely removes the species into environments it has more control over.

Every species is the end result of a long line of improbable events. Ok, so sentience is not inevitable, but I think intelligence is a viable, good, strategy which evolution will take wherever there is life. On earth that led to sentience pretty quickly, and to say that that event was the unlikeliest of all is far more victorian arrogance than the suggestion that sentience is everywhere and that humanity is not special.

If we played the tape twice (moving one snail two feet to the left to satisfy Laplace) from the Cambrian, of course we probably wouldn't have humans, nor even vertebrates as they are now. But I think we would have terrestrial, air breathing animals with an organ of some kind which processes sensory data and controls behaviour. It isn't a stretch from there to intelligence, providing that the ecological conditions favour it.

[Dr Strangelove]

I think that sentient life would have to be carbon based - no other element can form macromolecules. They'd have to be oxygen breathers - the other forms of respiration used by lower lifeforms don't yield enough energy to sustain a large body. They'd have to be animalian - photosynthesis also isn't efficient enough to sustain a brain. Intelligent beings would probably have hands similar to ours so that they could use tools and they'd have vocal cords and respiratory systems similar to ours so that they could use speech. In short intelligent beings pretty much have to be similar to us. Presumably the planet they develop on would have to be similar to Earth. A planet too small won't hold sufficient atmosphere, a planet too big might hold too much atmosphere, trapping toxic gases and the field of gravity might be too great to allow motile beings to evolve. Exactly how much leeway in size there might be I can't say. The planet's oxygen content would be similar to ours. Too little oxygen would preclude the evolution of animals our size, too much oxygen allows spontaneous combustion. There would have to an inert gas in the atmosphere also. Nitrogen is a good candidate for a number of reasons. The planet would also have to have alot of water in order to have sufficient water in the atmosphere to produce rainfall to water the land masses. I don't think that intelligent life could evolve on an ocean planet as a water environment doesn't encourage the development of the abillity to use tools or fire. It would have to have a circulating liquid core in order to generate a magentic shield against cosmic radiation. It's surface temperature would need to be close to ours. Remember that a planet this size with having an atmosphere is going to have a fairly broad range of climate zones. A truly "ice planet", "desert planet", or "jungle planet" with an atmosphere capable of sustaining life can't exist. If it meets all the other requirements to sustain life - size, atmosphere, liquid core, etc. - then it will have a diversity of climates.

So how common would such a planet be?

[pchang]

The science behind the probability of extraterrestrials consists of the following:

1) Make up some likely sounding premises
2) Follow them to their extreme conclusions

How much do you believe in your set of likely sounding premises?

[MichaeltheGreat]

When you've got billions of galaxies, billions of candidate stars in each, and billions of years, you have a lot of opportunity, so it really doesn't matter a rat's arse how "rare" any particular condition is for development of intelligent life.

The more absurd premise is that it's a unique event.

[pchang]

MtG's likely sounding premise is:

The universe is really, really, really big!!!!!!!!!!!!

[Immortal Wombat]

Originally posted by Dr Strangelove
So how common would such a planet be?

A large amount of earth's atmosphere's apparent suitableness for life was driven to balance by life itself. The photosynthetic microbes which pumped all the nasty toxic oxygen into the atmosphere probably caused a mass extinction. It was those harsh and eminently unsuitable conditions which led to the first eukaryotes' success. It is possible that a wide range of atmospheres will allow the development of the kind of life which leads to "perfect" oxygen/nitrogen/CO2 atmospheres.

It is fortuitous that in Europa we have a reasonably good test case for many of these variables- size, atmosphere, proximity to the sun.