The science behind the probability of extraterrestrials

[Dauphin]

Originally posted by Boris Godunov


The basis, IIRC, has to do with the Oort Cloud and the density of stars in the galaxy. As the density of stars becomes greater, the Oort Clouds that surround their solar systems become an increasing risk to neighbors, as comets will be drawn into their systems at a much higher rate.

That theory is supposed to send a few big comets or asteroids our way every time we pass throught the spiral arms of the galaxy. As a large portion of stars pass in and through the spiral arms I don't see the Solar system as being overly unique in that regard.

[Immortal Wombat]

Originally posted by Agathon
http://www.amazon.com/exec/obidos/tg...books&n=507846

This one is very good as well.

I find everyone's sweeping statements about where life may or may not be possible to be very amusing. As stated, we have sample size 1, and to assume that the highly specific conditions of earth are necessary is foolish. There could be a million life forms in a million different solar systems saying exactly the same thing about wildly different conditions. There might be life which requires high radiation, and repeated bombardment by asteroids. It is unlikely to be intelligent, but we have no way to know how unlikely it is to exist.

[Ecthy]

Originally posted by Lancer
Already exist do skull sucking locust aliens. Coming for us they are.

Ignore the yidtalk you should

[MichaeltheGreat]

Originally posted by Boris Godunov
The basis, IIRC, has to do with the Oort Cloud and the density of stars in the galaxy. As the density of stars becomes greater, the Oort Clouds that surround their solar systems become an increasing risk to neighbors, as comets will be drawn into their systems at a much higher rate.

AFFFFF-LAC!

1) We don't know squat yet about the number of objects, mass distribution, shape or extent of the Oort cloud.

2) We also don't know squat about the origin of the Oort cloud, and the extent to which it is dependent on having a string of gas giant planets. This also means that assuming Oort clouds form in other planetary systems with dissimilar configurations, we have no idea of its likely longevity and density over lifetime.

3) In the areas where density of stars is a problem, we don't have enough data to suggest that accretion disks will even form sufficiently to produce planets in a configuration that would create lots of Oort-cloud type objects, and increased radiation from extrastellar sources and termination shock effects would certainly have adverse effects on the size and stability of any Oort-cloud type objects, making objects large enough to cause damage even scarcer than would be the case otherwise.

4) For every disruption/ejection event, the possible orbital deflection is essentially random if the origin of the disruption/ejecting is extra-stellar system in origin, so the probability of collision with an earth mass planet close in to its star is extremely small.


5) Even in "dense" areas of the galaxy, stellar distances are such that disruption/ejection events would be extremely rare, except in cases where two stars proper motions are such that they interact. This is not uncommon on the scale of the entire galaxy, but extremely rare for any given star.

6) The areas where proper motion between stars makes disruption events likely are already pretty much no-brainers with respect to hosting life, namely the galactic core, globular clusters and active star forming regions with hot, young stars.

Every 300 million years or so? Perhaps not. Every 50,000 years though? Probably bad.

Then again, every 50,000 years means you'll pretty quickly sweep your orbit clear, and there aren't that many random colliders coming from other directions over a long period of time, so the likelihood is pretty high of getting a peaceful billion or two years in before the star goes red giant.

[MichaeltheGreat]

Originally posted by Immortal Wombat

This one is very good as well.

I find everyone's sweeping statements about where life may or may not be possible to be very amusing. As stated, we have sample size 1, and to assume that the highly specific conditions of earth are necessary is foolish. There could be a million life forms in a million different solar systems saying exactly the same thing about wildly different conditions. There might be life which requires high radiation, and repeated bombardment by asteroids. It is unlikely to be intelligent, but we have no way to know how unlikely it is to exist.

We have a sample size of one to observe the biology, but we have all the sample size we need to observe the chemistry and geology, physics, etc. necessary for any life, even if we can't imagine in any detail what that life would be like.

[Immortal Wombat]

No. We have the sample size needed to observe the chemistry and geology, physics, etc. necessary for earthlike life. Possibly even aqueous-planet carbon-life. Given that we can't adequately explain how life began on earth, I don't think we can say with any confidence what limits exist on the conditions that make them "suitable".

[MichaeltheGreat]

We can't define the precise limits, no, but we can define what is clearly outside any possible limits, based on the full range of known chemistry and physics.

Case in point, UV radiation might be suitable as a thermal energy source for life of an unknown type living under a shielding material, but we can very well determine (and test and observe) the effects of a given frequency and quantity of UV energy on any number of specific chemical interactions, and we can determine the absorbtion of that UV energy by different materials.

We don't need to observe a range of macrobiology directly - we can deal with bonding energy of various compounds and catalysis and enzymatic effects to determine boundary conditions beyond which we know life can't exist as we currently define life. (let's assume we stay a bit past replicating molecules).

Do we assume DNA or RNA is a requirement? Obviously not. Do we assume an earth or mars like planet is a requirement? Obviously not.

But we can rule out the sun-facing surface of Mercury, for instance, and quite a few other places.

[Immortal Wombat]

Indeed. I suppose the continuing mission is to find out exactly which other places.

[MrFun]

Originally posted by MichaeltheGreat


Another point is that mass-extinction phenomena and collisions are not bad things. If you look at the earth's history, each mass extinction phenomena for which we have definite knowledge in the fossil record opened up many new ecosystems and opportunities. Wiping out all species and starting from scratch isn't ideal, but wiping out a lot of dumb species that have achieved dominance through something other than intelligence is useful in creating evolutionary opportunities for the survivors.

makes sense

[MrFun]

Originally posted by MichaeltheGreat


Sunlight would only partly be blocked out on a global scale - think of houseplants near, but not in, a small window.

You're right -- I fell into the trap of the all or nothing absolutism.

[MrFun]

Originally posted by Immortal Wombat

This one is very good as well.

I find everyone's sweeping statements about where life may or may not be possible to be very amusing. As stated, we have sample size 1, and to assume that the highly specific conditions of earth are necessary is foolish. There could be a million life forms in a million different solar systems saying exactly the same thing about wildly different conditions. There might be life which requires high radiation, and repeated bombardment by asteroids. It is unlikely to be intelligent, but we have no way to know how unlikely it is to exist.

It's also interesting to find microbial life here on Earth that can live in boiling vent steams in the ocean, or live in complete dark caves, subsisting off of rock for energy.

[MrFun]

Originally posted by MichaeltheGreat


But we can rule out the sun-facing surface of Mercury, for instance, and quite a few other places.

I certainly agree with you and IW that we need to open our minds to forms of life by thinking outside of the box -- to a reasonable extent.



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?

[MichaeltheGreat]

Originally posted by MrFun


I certainly agree with you and IW that we need to open our minds to forms of life by thinking outside of the box -- to a reasonable extent.



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?

Water is a fairly weak solvent (but a good solvent for many organic molecules) and an excellent mixing media for suspension and exchange of objects in the size range from large complex molecules to microorganisms, but I wouldn't presume it's an absolute requirement for all primitive lifeforms. However, since "terrestrial" planets are by definition water-abundant, it's a safe presumption that water-bearing environments will be orders of magnitude more viable and diverse ecosystems.

We have iron, methane and sulphur eaters on earth, but they all live in aqueous ecosystems. We have stuff that lives in every temperature and pressure range of liquid water other than superheated geothermal resources (stuff on the edge or boundary doesn't count, I'm talking about the superheated 350°-600° stuff down in the vents.), so it's pretty clear that liquid water under virtually any condition will be a viable environment for something.

I just don't think it's a given that it's the exclusive environment. Gently heated rocks with trapped methane bubbles and oxygen bearing compounds might potentially provide an ecosystem for something, as would a liquid methane sea.

[MrFun]

thanks for the prompt response

[MichaeltheGreat]

Slow day at work.