Even if another planet like earth exists and possesses all the hundreds of properties that are necessary to engender higher forms of life - a suitable star, an orbit not too close, not too far from the star, a large moon, continental plates, water, the proper period of rotation, and so on - this still may not be enough. It seems that it also has to reside in a system of planets that consists of a belt of asteroids and a large planet like Jupiter:
Here's the lede:
Solar systems with life-bearing planets may be rare if they are dependent on the presence of asteroid belts of just the right mass, according to a study by Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and astronomer Mario Livio of the Space Telescope Science Institute in Baltimore, Md.There's much more to this story at the link. The interesting thing about it is that it fits the thesis of two books that have been criticized for suggesting that life-bearing planets and thus life itself, may be very rare phenomena in the universe and indeed may exist only on earth. The two books are Rare Earth by Ward and Brownlee and Privileged Planet by Gonzalez and Richards.
They suggest that the size and location of an asteroid belt, shaped by the evolution of the Sun's protoplanetary disk and by the gravitational influence of a nearby giant Jupiter-like planet, may determine whether complex life will evolve on an Earth-like planet.
This might sound surprising because asteroids are considered a nuisance due to their potential to impact Earth and trigger mass extinctions. But an emerging view proposes that asteroid collisions with planets may provide a boost to the birth and evolution of complex life.
Asteroids may have delivered water and organic compounds to the early Earth. According to the theory of punctuated equilibrium, occasional asteroid impacts might accelerate the rate of biological evolution by disrupting a planet's environment to the point where species must try new adaptation strategies.
The astronomers based their conclusion on an analysis of theoretical models and archival observations of extrasolar Jupiter-sized planets and debris disks around young stars. "Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet," said Martin, the study's lead author. "Our study suggests that our solar system may be rather special."
This notion contradicts the claim of people like astronomer Carl Sagan who argued that there are likely billions or hundreds of billions of planets in the universe capable of supporting life and that life is doubtless common in the cosmos.
If life really is rare or even unique to earth then it upsets the so-called "principle of mediocrity" which states that there's nothing special about earth and nothing special about human beings. If earth is the only life-bearing planet in the universe and human beings are the highest form of life then there really is something special about humanity after all.
The more scientists learn, the more reason there is to think that the principle of mediocrity is going to someday quietly disappear from the discussion of life in the universe.