In my logic class tonght I presented the following "rare earth argument," after Ward and Brownlee:
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.)
4) Therefore, intelligent life probably exists only on Earth and nowhere else in the universe
Probably we are alone in the universe.
As students hear this argument many are interested. Some want it to be irrational. One objection to it is:
Maybe complex life can exist on a non-earthlike planet. If so, then rare earth factors are irrelevant.
I think this objection is a "just so story," an ad hoc fallacy that probably derives from reading too much science fiction. In the current search for extraterrestrial life all seekers are looking for earthlike planets. Astrobiologists are looking for a planet that is "warm and wet," like earth. Does the planetary candidate show, especially, evidence of liquid water? For such to be the case a number of rare earth factors must come together.
Mars, e.g., is interesting since it may have once had liquid water. Which means non-complex life may once have existed there.
See, e.g., the essay "Clay Not Evidence of Lakes on Mars," in Astrobiology Magazine. Could Mars have had a "wetter and warmer" past? If so, then maybe it contained life. If not.... then we'll be looking elsewhere.
Ward and Brownlee's "rare earth factors" include:
- Right distance from a star;
- habitat for complex life;
- liquid water near surface;
- far 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 plate tectonics;
- a Jupiter-like neighbor to clear out comets and asteroids;
- plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field;
- 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, ellipitcal 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.