The "Multi or Rare" Earth Debate: Are We the Sole Intelligent Life in the Milky Way?

The "Rare Earth" hypothesis is the idea that life is a staggeringly unlikely event, and that the reason we haven't seen hide nor hair (nor scale nor weird gel-layer) of aliens is that there aren't any. It's had some time in the spotlight, it makes us sound very important, and it's wrong.

The Rare Earth argument ignores a number of essential factors, the first being how staggeringly huge the numbers involved are. Even the Milky Way has 200 to 400 billion stars, and it's only one of a hundred billion galaxies in the observable universe, and there have been billions of years for things to happen. Countering "it's really unlikely" with "but there are lots of things!" might sound weak, but it's the Rare Earthers who are taking the burden of proof - claiming that nothing happens anywhere else ever. The more places there are, the worse their argument gets.

Geologist Peter Ward and astrobiologist Donald Brownlee, both of the University of Washington have outlined a short list of conditions needed: 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.

Claims that there aren't many suitable planets over all these stars are like hiding in a closet and claiming there's no such thing as coffee tables - we're now detecting planets at an ever-increasing rate, because now we have technology actually capable of detecting planets. Almost as soon as we try any new planet-detecting technique it detects a whole bunch of the things. We're even edging into the ability to find Earth-size planets, and what do you know? There they are! And some even have water!

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