Tomorrow in one of my MCCC logic classes I'm going to present the "rare earth" argument concerning extraterrestrial life. This serves two purposes for me: 1) it's an example of the use of logic to formulate an argument for a belief; and 2) it's intrinsically interesting to students, many of whom are taken by the discussion. For some this is even a religious thing. Some students get pretty defensive when presented with the idea that, most likely, intelligent life does not exist outside earth. Why freak out about this possibility? Why care? But some do.
|Holland, Michigan light house|
Years ago I read Ward and Brownlee's Rare Earth: Why Complex Life is Uncommon in the Universe. Here's the argument:
1) Complex life is very fragile, and can only exist in a habitable zone.
2) A habitable zone suitable for complex life has many rare earth
factors that have come together and appear fine-tuned for complex life.
3) The odds of such factors is astronomical. (That is, of them coming
together randomly. If there are other such planets, it would be astounding on
the random approach, and would argue for the existence of God.)
Therefore, intelligent life probably exists only on Earth and nowhere else in
Note: this is a probableistic arguments which renders the massive size of our universe irrelevant.
See also the rare earth work done by Harvard's Howard Smith. See, e.g., Smith's "On Living Alone in the Universe: New Indications of our Probable Solution." The bullets are:
Smith places rare earth factors into four essential ones: "(1) Stability: a suitable planet’s orbit must be stable, that is, sufficiently circular or otherwise unchanging so that it remains suitable not only for one revolution around its star, but for the billions of years it takes for intelligence to evolve. The star it orbits must be stable in size and radiative output for this long too. (2) Habitability - Water: a suitable planet must reside in the "habitable zone" of
- Very few planets in our neighborhood are suitable and stable enough to nurture life to evolve and become intelligent.
- We are alone in the universe for all practical purposes. "We are
not likely to make contact with an alien intelligence, or even to know if one exists, for at least
100 human generations and perhaps for very much longer."
- The “Misanthropic Principle” is the
observation that, in a universe whose physical parameters are amazingly well suited for
intelligent life (the “Anthropic Principle”), the environments and situations necessary for
intelligence to develop are extraordinarily rare."
the star, that is, at a distance from the star where the temperatures allow water to be liquid – or else have some other mechanism to maintain liquid water. (3) Planetary Mass: The planet must be massive enough that its gravity can hold an atmosphere, but not so massive that plate tectonics are inhibited, since geological processing and its important consequences will be reduced. (4) Composition: The planet must be rich in all the key elements needed to build complex molecules (carbon for example), but also including heavy elements like iron, silicon, etc., that are perhaps not needed for making life itself but that are essential for an environment that can host intelligent life."
Smith's essay develops the four areas.
Ward and Brownlee's rare earth factors are:
distance from a star;
- habitat for complex life;
- liquid water near surface;
enough to avoid tidal lock;
- right mass of star with long enough lifetime and
not too much ultraviolet;
- stable planetary orbits;
- right planet mass to
maintain atmosphere and ocean with a solid molten core and enough heat for
- a Jupiter-like neighbor to clear out comets and asteroids;
- plate tectonics to build up land mass, enhance bio-diversity, and enable a
- not too much, nor too little ocean;
- a large moon at the right
distance to stabilize tilt; a small Mars-like neighbor as possible source to
seed Earth-like planet; maintenance of adequate temperature, composition and
pressure for plants and animals;
- a galaxy with enough heavy elements, not too
small, ellipyical or irregular;
- right position the galaxy;
- few giant impacts
like had 65 million years ago;
- enough carbon for life, but not enough for
runaway greenhouse effect;
- evolution of oxygen and photosynthesis;
- and, of
course, biological evolution.
It's 10 PM in Michigan. I think I'll step outside and look up at the stars, and think of the fact that we're all alone in the universe.