Is There Life Beyond Our Solar System?
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
Aczel quotes Epicurus from about 300 years B.C.:
|A meditation inspired by a new book, Probability 1,
Why There Must Be Intelligent Life in the Universe, by Amir D. Aczel and
Harcourt Brace & Co, number QB 54 .A25 1998 in Hamilton Library
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
plants and other things we see in this world.
Aczel also quotes by the Roman poet Lucretius (about 75 B.C.), who
Granted, then, that empty space extends without limit in every direction
seeds innumerable are rushing on countless courses through an
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
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
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(no life)=1-5 * 10-14=0.99999999999995. From the standard
probability of mutually
coin tosses, the probability of never having a head among n independent
the same as the probability of all tails:
The probability of AT LEAST ONE other solar system having life among 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
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
of life evolving from the primordial soup in any given solar system.
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
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.
|A Few Easy Criticisms That Probably
Won't Change Aczel's Answer
On page 15 of Probability 1, Aczel reminds us of Frank Drake's 1961 equation for the
of planets in our galaxy which could communicate with other solar
The table below describes each factor in this expression. The equation
multiplication rule concerning conditional
What Is The Probability Of Life Near Any Star?
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
- 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
- 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
- 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).
|Coefficient ||Definition ||Estimate
||No. of stars in our galaxy
||P(stars has planets)
||0.5 (note from TR: don't be surprised if further research pushes
this close to 1)
||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.)
||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
DNA from the random meeting of appropriate constituents). Back in 1961,
colleagues thought .1 or .2 would be a reasonable guess for this factor.
||P(life will evolve intelligence)
||In 1961, this was guessed to be about 0.1 to 0.5. Aczel is
evolving intelligence is very likely, given enough time without
disasters, but that disasters are fairly frequent and may prevent
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
Moveover, volcanoes themselves can cause disasters on a global scale (in
a volcano in Indonesia put so much dust in the air that New England had
crops failed and people starved). Other parts of our own galaxy are
more dangerous than our own neighborhood. The jury is clearly out on
||P(intelligent life would be capable to communicate with other solar
||The jury is really out on the size of this factor. The best
the product of this factor and the next one being nearly 0 is that any
should be obvious to everyone by now.
||P(long life for a civilization, long enough to communicate
effectively with other
||This could be rather low. Meteors striking a planet may end a
a civilization could end itself through nuclear war or global warming.
also ice ages, volcanoes, continental drift, etc.