Is There Life Beyond Our Solar System?

A meditation inspired by a new book, Probability 1, Why There Must Be Intelligent Life in the Universe, by Amir D. Aczel and published by Harcourt Brace & Co, number QB 54 .A25 1998 in Hamilton Library
This is an old question and Amir D. Aczel updates an old answer in his new book: in a universe as big as ours seems to be, the probability of life elsewhere must be close to 1 (1 represents certainty). Aczel quotes Epicurus from about 300 years B.C.:

There are infinite worlds both like and unlike ours. For the atoms being infinite in number are borne far out into space....So there exists nowhere an obstacle to the infinite number of worlds. We must believe that in all worlds there are living creatures and plants and other things we see in this world.
Aczel also quotes by the Roman poet Lucretius (about 75 B.C.), who wrote
Granted, then, that empty space extends without limit in every direction and that seeds innumerable are rushing on countless courses through an unfathomable universe. It is in the highest degree unlikely that this earth and this sky is the only one to have been created.
But these answers were not shared by the philosophers whose ideas came to dominate Christian Europe: Aczel notes that Plato wrote about 400 B.C. in the "Timaeus" that "there is and ever will be one only-begotten and created heaven", and that Aristotle (about 350 B.C.) opposed a multiplicity of worlds. Aristotle's opinions dominated the Christian world until the modern era. Giordano Bruno, who was burned at the stake by the Catholic Church for heresy, suggested that there was an infinity of worlds in the 1500's (A.D.).

Aczel brings to this question some elementary probabilty (taught at UHM in Math 371 and Math 471). Among all galaxies in the universe what is the probability of another solar system with life? Aczel estimates that

P(a planet with life in a particular solar system)=5 * 10-14
He then views the universe of solar systems as being mutually independent coin tosses with respect to having life: P(Heads)=P(life)=5*10-14 and P(Tails)= P(no life)=1-5 * 10-14=0.99999999999995. From the standard probability of mutually independent coin tosses, the probability of never having a head among n independent coins is the same as the probability of all tails:
The probability of AT LEAST ONE other solar system having life among n stars is
1-P(all tails)=1-(.99999999999995)n
The current estimate for the number of stars in the universe (n) is about 3 * 1022, because there are about 3*1011 stars in each of 1011 galaxies. An elementary calculation with logarithms will show that P(all tails) is almost 0. This answer doesn't change much if we radically lower our estimates to 1010 stars per galaxy for 109 galaxies. The vastness of the universe overwhelms the low probability of life evolving from the primordial soup in any given solar system.

A Few Easy Criticisms That Probably Won't Change Aczel's Answer
Itis easy to criticize some of Aczel's assumptions---why should neighboring solar systems near the violent center of a galaxy be independent "coin tosses" in regard to the evolution of life within them? Also, just how likely is it that something like DNA can evolve in the primordial soup? But the sheer number of stars tends to swamp these concerns (but the criticisms provide fertile fields for more papers on this subject, refining the estimates). A much more difficult question is the probability that DNA can emerge from the primordial soup.

What Is The Probability Of Life Near Any Star?
On page 15 of Probability 1, Aczel reminds us of Frank Drake's 1961 equation for the number N of planets in our galaxy which could communicate with other solar systems:

The table below describes each factor in this expression. The equation comes from multiplication rule concerning conditional probabilities. For example,
  • P(A and B)= P(B given A) P(A) when P(A)>0
  • P(A and B and C)=P(C given A and B) P(B given A) P(A), when P(A and B)>0
  • P(A and B and C and D)=P(D given A and B and C) P(C given A and B) P(B given A) P(A), when P(A and B and C)>0
  • Etc.
  • Hence P(a sun has a planet with life)=P(there is life given a planet with a good environment)P(a planet has a good environment given there are planets about a sun)P(there are planets about a sun).
For Aczel, the probability that a star has life is just the subproduct fpnefl from Drake's equation. This product is not controversial; the only issues are good estimates for its separate factors.
Coefficient Definition Estimate
N* No. of stars in our galaxy 300 billion
fp P(stars has planets) 0.5 (note from TR: don't be surprised if further research pushes this close to 1)
ne P(planet has a good environment) 1/9 (not too hot; not too cold; etc! This is based on direct observation of about 9 stars and our own solar system.)
fl P(DNA will evolve on a planet with a good environment) Aczel suggests 1 in one trillion; there is no question that space has plenty of the proteins and elements needed to form DNA---the only issue is how can one create DNA from the random meeting of appropriate constituents). Back in 1961, Drake's colleagues thought .1 or .2 would be a reasonable guess for this factor.
fi P(life will evolve intelligence) In 1961, this was guessed to be about 0.1 to 0.5. Aczel is persuasive that evolving intelligence is very likely, given enough time without environmental disasters, but that disasters are fairly frequent and may prevent advanced evolution in many solar systems. About 65 million years ago, a large meteor struck the Yucatan in Mexico and killed off dinosaurs throughout the Earth. It is now estimated that a catastrophe of this magnitude (or greater) occurs on Earth about every 30 million years. Moveover, volcanoes themselves can cause disasters on a global scale (in 1814, a volcano in Indonesia put so much dust in the air that New England had no summer, crops failed and people starved). Other parts of our own galaxy are probably much more dangerous than our own neighborhood. The jury is clearly out on this factor.
fc P(intelligent life would be capable to communicate with other solar systems) The jury is really out on the size of this factor. The best argument for the product of this factor and the next one being nearly 0 is that any such communication should be obvious to everyone by now.
L P(long life for a civilization, long enough to communicate effectively with other solar systems) This could be rather low. Meteors striking a planet may end a civilization, or a civilization could end itself through nuclear war or global warming. There are also ice ages, volcanoes, continental drift, etc.

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