From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: sci.space
Subject: Re: ETs and Radio
Date: 30 Jul 92 22:05:44 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <Bs64M2.AK7@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>In article <1992Jul29.161716.3491@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>>Now subtract out all Population II stars, no heavy elements like iron,
>
>Some of them might have enough for a small planetary system.

I mentioned iron for two reasons. First, it's necessary for oxygen
transport via the blood. Second, and this is opinion, it's needed
for the planet to have a significant magnetic field to redirect energetic
radiation from the planetary surface. Now iron is the result of supernova
explosions. Population II stars formed *before* supernovas became common
enough to supply heavy elements. 

>
>>and subtract out all multiple star systems, no stable planetary orbits,
>
>I'm told this doesn't look like as big a problem as was once thought.

??? Let me put it another way, stable orbits in the liquid water zone.

>>subtract out all systems that don't have a planet in the liquid water zone,
>
>This is where we get into real guesswork.  Note, though, that the liquid-
>water zone is wider than we once thought -- Mars would have liquid water
>if it was bigger.  (The idea that the "habitable zone" is very narrow and
>Earth has just happened to stay in it has been discredited.)

The potential liquid water zone of a G star like ours is roughly from
near Venus orbit to somewhat beyond Mars orbit given specially designed
planets in the various places. Now on the solar system scale, that's a
small range of distances.

>>subtract out all planets not at the correct stage of planetary evolution,
>>ours is
>>billions of years old while conditions for life are a much smaller fraction
>
>Really?  Please elaborate.  All you really need is liquid-water temperatures
>and adequate materials.  The major changes in our planetary conditions over
>the last few billion years have pretty well all been due to the presence of
>life.  (The idea that Earth had a lucky escape from being another Venus has
>also been discredited -- Earth's oceans would still be liquid even if it
>had Venus's load of atmospheric CO2 today.)

What I'm saying is that the accretion process, the remelt, the second
cooling, and finally the out gassing of water vapor and methane that
formed the primordial atmosphere took about 1/3 the life of the solar
system to occur. (At least according to popular theories of planetary
formation) The great extinction, due to oxygen liberation, took another
4 billion years, before oxygen based life became dominant. I think we
can safely assume that intelligent life would require the energetics
only possible to oxygen breathers.

>>... now subtract out all the systems where life hasn't evolved from
>>primitive forms to advanced forms, life has existed on this planet for
>>a long time, humans much less so, etc.
>
>But add in the potential for life to take faster routes than ours.  Some
>of the dinosaurs might well have evolved intelligence if their history
>hadn't been cut short a bit too early, and several other species on Earth
>are not impossibly far from intelligence.  This is getting pretty far into
>guesswork again, though.

At best that shaves only 60 million years off the timescale of billions.

>>who've thought about the question say that there are perhaps 50 systems
>>in the galaxy that may have life as we know it. 
>
>References, please.  There have certainly been *much* higher estimates too.

One of Sagan's books quoted this figure. There have been other estimates
in the thousands to millions for our galaxy, but the factors I listed
above weed that number down dramatically. One of the greatest unknowns
is whether life bearing planets naturally evolve intelligent creatures,
or whether that's a rare accident. We don't have any data to support
either position very well.

>Also, why insist on it being life as we know it?

Because we wouldn't recognize any other kind. Any lifeform not based on 
carbon chemistry would be very strange indeed.

>The only argument against extraterrestrial life/intelligence that strikes
>*me* as being particularly telling is the Fermi Paradox:  if they're out
>there, why didn't they colonize this planet long before we evolved?

Assume Einstein is right. Interstellar flight takes longer than the
time available since the Universe began to visit all the stars. Or,
we are the *first* technological civilization to evolve in the galaxy.
Or, technological civilizations self destruct.

Gary