QUOTE (anarkhy @ Jul 4 2008, 01:10 PM)

Just playing with numbers... in the reality i think would be a very small number, not millions but more likely a dozen habitable planets.
Take mars as the example, its a very viable planet, why there is no evolved life there (not bacterias) when the earth is so full? This tell us the conditions to support life are extremely rare.
So you're basing your judgement off of one solar system, out of trillions?? Keep in mind that the only solar system we have ever sufficiently explored had life. We've never sufficiently explored any other solar system. Sure, we've found a few odd ball solar systems with hot Jupiters in close orbits. However, as Frank Drake pointed out in the Rare Earth Debate on space.com,
"Only about 5 percent of the stars that have been studied sufficiently have hot Jupiters or Jupiters in elliptical orbits. The other 95 percent of the stars studied do not have hot Jupiters, and just what they have is still an open question. The latest discoveries, which depend on observations over a decade or more, are finding solar system analogs. This suggests that 95 percent of the stars- - for which the answers are not yet in -- could be similar to our own system. This is reason for optimism among those who expect solar system analogs to be abundant."
We don't even yet possess the necessary equipment to hunt for small, rocky, Earth-like worlds. We only have the capability at the present time of detecting massive Jupiter-like worlds by such methods as astrometry (used to look for the wobble caused by displacement by an orbiting object of great mass and gravity), transit photometry (periodic dimming of the star while eclipsed by massive Jupiter-like planets) and more complicated methods such as Doppler Spectroscopy (or radial velocity method; detects periodic velocity shifts of the stellar spectrum indicative of a large orbiting body). Within the next decade or two, NASA will be launching the
Terrestrial Planet Finder. Only then will we discover just how common Earth-like planets really are, but scientists today are overwhelmingly optimistic.
So, out of the solar systems we've explored, there is a 100% success rate (but of course, we have only one example, so of course the success rate is 100% -- the point is, to proclaim that life is rare using our solar system as a model simply makes no logical sense). I don't know where you're getting your information from. It wouldn't happen to be the Rare Earth proponents would it (i.e. Ward and Brownlee)? Mars is not as viable a planet as you are assuming. It is much smaller than Earth (about half the size with 1/3 the gravity), and the lack of a molten core gives it a very weak, almost non-existant, magnetic field. Without a magnetic field the solar wind from the Sun is simply stripping away Mars' atmosphere, and the low gravity isn't helping matters either. Hydrogen is a vital element for life as we know it (H2O). On Mars, the gravity is such that lighter elements such as hydrogen would be the first to be lost to space. Mars was likely warm and wet in the early years after the solar system began to settle down. It had a molten-metal core at one point due to the heat caused by friction generated by constant asteroidal impacts. However, the lack of plate tectonics led to a cooling phase within Mars, and without the charges generated by currents caused by a molten core, the magnetic field dissipated. The warm and wet environment was likely sustained by a greenhouse effect caused by a large quantity of CO2 in the atmosphere that has since left its mark as the carbonic rocks on the surface today. Because Mars is so small, plate tectonics were not a possibility, thus the CO2 dense atmosphere was not recycled back from whence it came by volcanic activity (although volcanic activity existed long ago, this was prior to the cooling of the core). Don't even get me started on Venus...
Mars is not a good example of a "viable planet that went wrong," in fact, it was doomed from the start.