>> TH
>> If life is a reaction to the sun/heat cycle, then
>> you can't have life without the sun/heat cycle.
>> And you can carry that further (same size
>> planet to hold in a similar atmosphere - watery
>> such that liquid water is there - temp zone
>> between 0-100C, and on and on.
>
>Ah...  Well, the more similar attributes that you abitrarily add between
>here and another life-giving planet, the more likely that life will be the
>same form, and on and on.  But there's no real reason that the same size
>is
>necessary nor that the atmosphere is similar - after all, life began with
>a
>different atmosphere.

TH
Yes but it is that different reducing atmosphere that
is a requirement for all life. And that specific environment can only be held
by the gravity of a planet
like earth. Bigger - (gas giant) and holds too much gas
smaller (Moon or Mars) and it's gravity  is too weak to hold in the necessary
atmosphere.
Then too we go a step further - rocky planets are near the Sun (mercury - to
Mars) while gas giants farther away.
So to have a rocky planet - size of earth, it has to be in the same life zone.
Also if all this is a sun too hot - it'll burn up in a billion years before
inteligent (?) life has time to form.
So it has to be a sun sized star. So for life to exist it has to have a similar
sized sun, a similar sized rocky planet in the life zone orbit, etc. etc.
And it is even likely that it took a planetoid collision that formed the Moon -
to set up the orbit spin that sets up the heat cycle that starts life.
Now that is very very very very very specific conditions for life (and there
are many more conditions) 

(snipped)

>> TH
>> Because chemistry obeys laws. You can't change
>> the chemical rules. They lead to the same result,
>> at the same temp (liquid water range) and under
>> the same conditions (liquid water - certain
>> atmosphere etc.) with slight variation
>
>True about basic chemicals.  But I'm suggesting that you can't know that
>Earth's (raw materials and) chemistry is the only one capable of producing
>life.  That is, a different elemental mix might produce a different form
>of
>life.  And for that matter, a similar mix might produce something pretty
>profoundly different from DNA-based life (I'm not thinking that we can even
>guess at what that might be - after all, did we successfully guess at a
>structure for DNA?).

Th
True - no one can pin down the future. But for a 
carbon based life form (and remember life here is
based on the most common elements available  - so
any other kinds of life (if there is such) must be built on
uncommon and hard to find elements - thus this too suggests that it would be
rare indeed if possible at all.
Also see new  post on 'Why Early"

(snipped)

>Interestingly, the last few decades has taken the opposite direction.
>Extremophiles are now found deep in the rocky mantle, high in the atmosphere
>and around oceanic vents, where we used to assume life was impossible. 
>If
>life has evolved elsewhere, I think we'll find it can thrive in even more
>extremely diverse and "inhospitable" (from our viewpoint) habitats than
>ones
>just here on this planet.  If I were a betting man and thought that any
>of
>this can be proven in my lifetime, I'd even propose a wager that whatever
>life we find will not have DNA, nor anything we'd consider a variant, as
>its
>base.  Regards,  Brett.

TH
I think the extremophiles are here because
they've had almost 4 billion years to adapt.
I think the common ancestor probably were
all in a similar environment such that no
extreme variants would have been stable
enough to survive at the start.


I assure you, no one loves to speculate more
than I on this. I've written a few sci-fi
pieces (plug here - see my website for the
short sci-fi novel "Library Planet" for ex.
musea.digitalchainsaw.com) But the more I
have learned , the more convinced I am that
for the most part life is very specific to
a certain place, condition, time, gravity,
and all the rest.

But again no one knows the future except
Cassandra and who's listening to her?
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