olivergringold wrote:In summary, regarding our status in the universe, we are not alone.
That's a pretty strong statement. Do you have any empirical evidence that we are not alone?
Based on vibrations in distant starts we know that within our general vicinity there are several stars with large planets in their orbit. While the vibrations caused by planets of our size are too small to detect, it's not too great a logical leap to imagine that existence of other planets in general would indicate the existence of other planets similar to our own.
Actually, it would be quite a leap. Actually, several large leaps. Yes, we can detect Jupiter-size planets in other solar systems, but we have not detected any Earth-sized planets in other solar systems. That doesn't mean they might not be out there, but from current observations, we have no evidence of them.
Also, just because there might be a planet the size of ours, it doesn't mean it will have the same characteristics as ours. Size is only one factor. There are several other factors in the Drake equation that also needs to be considered.
Where:
N* is the number of stars in the Milky Way. This number is not well-estimated, because the Milky Way's mass is not well estimated. Moreover, there is little information about the number of very small stars. N* is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars.
ne is the average number of planets in a star's habitable zone. This zone is fairly narrow, because constrained by the requirement that the average planetary temperature be consistent with water remaining liquid throughout the time required for complex life to evolve. Thus ne = 1 is a likely upper bound.
fg is the fraction of stars in the galactic habitable zone. 0.1 at most.
fp is the fraction of stars in the Milky Way with planets.
fpm is the fraction of planets that are rocky ("metallic") rather than gaseous.
fi is the fraction of habitable planets where microbial life arises. W&B believe this fraction is unlikely to be small.
fc is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence W&B argue that this fraction may be very small. Moreover, the Cambrian Explosion, when complex life really got off the ground, may have been triggered by extraordinary climatic and geological events.
fl is the fraction of the total lifespan of a planet during which complex life is present. This fraction cannot be high because complex life takes so long to evolve. Complex life cannot endure indefinitely, because the energy put out by the sort of star that allows complex life to emerge gradually rises, and the central star eventually becomes a red giant, engulfing all planets in the planetary habitable zone. Also, given enough time, a catastrophic extinction of all complex life becomes ever more likely.
fm is the fraction of habitable planets with a large moon. If the giant impact theory of the Moon's origin is correct, this fraction is small.
fj is the fraction of planetary systems with large Jovian planets. This fraction could be large.
fme is the fraction of planets with a sufficiently low number of extinction events. W&B argue that the low number of such events the Earth has experienced since the Cambrian explosion may be unusual, in which case this fraction would be small. Such a low number again requires a very stable planetary system, with outer planets having nearly circular orbits, no gravitational perturbations from passing stars, and no nearby supernovas, quasars, or gamma ray bursts.
http://en.wikipedia.org/wiki/Rare_Earth_hypothesis
