Rare Earth - The verdict
twi...@worldnet.att.net
In parts 1, 2, & 3 of Rare Earth, I listed the factors which
tend to make Earth a very rare place.
The Rare Earth hypothesis is testable and is, in fact, being
tested now, although that testing is the purpose of the
experiments. The search for microbial life in the Solar
System is one test. SETI and the search for planets and the
planned search for the evidence of life on distant planets
via new instruments are more tests of the hypothesis.
Let's review quickly the arguments and evidence offered so
far. A star must exist in the habitable zone of the galaxy.
It must be heavy element rich. It cannot be too large nor
too small.
It must exist as a main sequence star for many billions of
years. For the first 2 billion years or so, Earth didn't
even have an oxygen atmosphere.
A G0, surface temp about 10,400 degrees F, seems to be about
the largest star that has a reasonable chance of lasting
long enough on the main sequence to allow complex animals
enough time to form. Our Sun is a G2 and almost perfectly
fit for the task.
An F3 (mass about 1.5 times that of Sol, our Sun) lasts on
the main sequence only about 2 billion years. Or about 1.4
billion years after a heavy bombardment if our systems
period of heavy bombardment is average. The habitable zone
is larger but moves outward much more rapidly than did our
Sun's habitable zone. Yet, our Sun's continuous habitable
zone is quite narrow.
A K5 has a luminosity of about 4% of our sun's. Thus the
continuous habitable zone about that star is small and very
close in. A real probability of tidal lock exists when the
star is this close in. An M class has even less luminosity
than a K class. So the K class seems to be about the
smallest that seems reasonable. A K5 is quite probably below
the luminosity required. (K7 and K8 on the Yerkes
classification typing aren't full subtypes according to the
Classification of Stars by the Jascheks. However, for those
interested, a K8 has a luminosity of about 3% of our Sun's.
A M7 would have a luminosity of about 0.0001 Sol's! That is
a very small habitable zone!)
(For those who have forgotten the spectral classes of stars,
just remember: Oh, Be A Fine Girl, Kiss Me. An O8 spectral
class has a luminosity of about 100,000 Sols and exists on
the main sequence for only about 4 million years. An M7 has
a luminosity of about 0.0001 Sols and exists on the main
sequence for about one million million years. IOW, we ain't
lost no M7s from the main sequence yet!)
So on the basis of spectral type alone, only about 1 or 2%
of stars seem to be suitable. When you take into account
the need to be in the habitable zone of the galaxy, you wind
up with a very, very, very small portion of 1% which seem
to be suitable for complex animals.
You need a large moon, such as Luna, you need a Jupiter
class planet outside the orbit of the habitable zone to
sweep the debris from the system.
Interestingly, the planet finders are finding Jupiter-sized
planets only around 5 - 6% of the stars examined. (This may
get larger because of the longer possible orbits some
planets might be in.) However, none of the Jupiter-sized
planets found so far would allow complex animal life in the
same system. Most are too close to the primary and the rest
are in highly elliptical orbits. Our Solar System may be
exceedingly rare to have virtually all of its planets in
neatly spaced and nearly circular orbits. So planets of the
Jupiter size are looking less common than believed and all
found so far, outside of our Solar System, do not permit
complex animal life to flourish in that stellar system.
The planet must maintain a nice temperature for most of the
time during those billions of years.
The heavy bombardment period must end within several hundred
million years.
There apparently must be plate tectonics.
The extinctions must not occur during periods when the
diversity and disparity of life cannot handle the
extinctions and have some life remain. Nor can there be too
many extinctions.
There must be plenty of water but not too much or too little
water. Shallow seas seem to be a requirement to remove
sufficent CO2 from the atmosphere.
The nebula that form the star and the planets must be heavy
element rich. (In a study of 174 stars, astronomer G.
Gonzalez discovered that the sun was one of the very richest
stars in metal content. Even in this category, we orbit a
rare star!)
Snowball Earth events and/or an IIE (not discussed in this
book review) may be a requirement.
With this in mind, the Drake Equation would be modified by
the Rare Earth Hypothesis to:
N* X fp X fpm X ne X ng X fi X fc X fl X fm X fj X fme = N
Where:
N* = stars in the Milky Way Galaxy
fp = fractions of stars with planets
fpm = fractions of metal-rich planets
ne = planets in a star's habitable zone
ng = stars in a galatic habitable zone
fi = fraction of habitable planets where life does arise
fc = fraction of planets where complex metazoans arise
fl = fraction of a lifetime of a planet that is marked by
the presence of complex metazoans
fm = fraction of planets with a critically low number of
mass extinction events
(Notice that the Snowball Earth factor and an IIE factor
have not been included.)
As any term in the above equation approaches zero, the
result also approaches zero. If several terms approach
zero, the result gets small even quicker.
Several of the terms seem to approach zero from the evidence
found so far.
The final paragraph of the chapter on Assessing the Odds and
the conclusion is:
"How much stock can we put in such a calculation? Clearly,
many of these terms are known in only the sketchiest detail.
Years from now, after astrobiology has matured, our
understanding of the various factors that have allowed
animal life to develop on this planet will be much greater
than it is now. Many new factors will be known, and the
list of variables involved will undoubtedly be amended. But
it is our contention that any strong signal can be perceived
even when only sparse data are available. To us, the signal
is so strong that even at this time, it appears that Earth
indeed may be extraordinarily rare."
Again, this is a very good book and well worth reading,
whether you agree with the conclusions or not. Indeed,
anyone discussing the possibility of complex animal life on
other planets who has not read this book is definitely not
informed about the subject.
"Facts do not cease to exist because they are ignored.
Aldous Huxley
Neal Plantz
fact, I couldn't have said it so well. I get a headache when I see
equations;.
But what twitch said, i have believed for a long time, namely, there are
so many variables in the universe and so little leeway in suitability
for
life, that life beyond the amoeba is probably a rare and wonderful
phenomenon. In fact, the amoeba's live is a rare and wonderful
phenomenon'
the monk
people place you as a dead man/woman or what ever your
little fat sissy but is.
God bless you Robert Bills, from the Monk...
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robert...@webtv.net
POLLOCK" SUB RACES NEED NOT SPEAK!!!!!!!!!!!!!!
I am"He who wil NOT be DEFEATED"