On 08/01/2011 04:19 PM, Warp wrote:
> Paul Fuller<pgf### [at] optusnetcomau>  wrote:
>> Makes you wonder what life on a Kuiper Belt Object might be like.
>
>    I don't think it would be physically possible for any kind of life to
> form that far from the Sun.

Certainly life, as we understand the concept, requires energy. Most life 
on *our* planet uses the Sun for that. But not all of it: it's well 
documented that there are organisms fed by chemicals originating from 
the Earth's core, for example. It's plausible that such a thing could 
happen on more distant planets. (Although I have no idea whether the 
chemistry of any /specific/ known planet makes this likely.)

>    It's hard to imagine how life could form without water.

Life, as we understand is, is a dense network of chemical reactions. 
That would logically appear to require some kind of solvent. There's no 
specific reason why it should be water; some scientists have suggested, 
say, ammonia.

More fundamental perhaps is carbon, which is capable of forming very 
complex compounds. (Silicon is also sometimes suggested.)

>    Of course for the water to be any good, it has to be in liquid form.
> If you are too far away from the Sun, all the water will be frozen solid.
> This isn't a very fertile ground for life to form. There are little chemical
> reactions going on, chemicals are not very free to move, and there are
> probably a huge bunch of other properties necessary for *any* kind of
> life to form which just aren't possible with deep-frozen ice.

You're aware that (on Earth) there are organisms that live in solid ice, 
right?

(Of course, further from the Sun, that probably wouldn't be viable.)

Live as it exists on Earth fundamentally requires liquid water, but 
there are reasons to suppose that this isn't necessarily a hard 
requirement for "life" to exist.

>    Now, perhaps if there was a liquid which remains an liquid form at those
> temperatures, it could ostensibly happen. However, such liquids are both
> extremely rare (iow. there wouldn't be enough of it in any given planet),
> and their chemical properties are probably inadequate for any kind of
> lifeforms.

What makes you think that chemicals which are "rare" on Earth would 
necessarily be rare elsewhere?

(E.g., O2 used to be "rare" on Earth - extremely rare, in fact.)

 From what I understand, other planets have been found to have vastly 
different chemistry to Earth.

> (Also, most liquids other than water get denser when they
> solidify, which is a big problem.)

Care to explain why?

>    (Conversely, a planet which is too *close* to the Sun cannot form life
> either, this time because there's no water because it's all vaporized away.
> It also makes forming a viable atmosphere quite hard, making it a very
> hostile environment, where strong radiation hits directly the surface
> of the planet, destroying any complex chemicals that might form by chance.)

I would have thought the biggest problem with living close to the Sun 
would be simply that any complex molecules would get smashed to pieces 
very rapidly.

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