The Washington Post has a story about the discovery of a planet orbiting a star some 20 light years from earth that seems to be about the same size as the earth and about the right distance from its star to qualify as a "Goldilocks" planet - neither too far from the star nor too close to prevent simple life forms from existing on it. From these meager facts about the planet, named Gliese 581G, some media commentators have concluded that the planet is just right for life.
This is certainly a premature claim and goes beyond what the scientists themselves are saying.
The planet hasn't actually been seen, rather its presence is inferred by detecting slight wobbles in the star it orbits. From these it is deduced that there must be a planet of a certain mass and distance from the star. Scientists deduced a few other things as well. For instance they believe the planet always has one side facing the star and one side facing away, a condition called tidal lock, which means that whatever life might exist there must be restricted to a narrow band where the lit portion fades into darkness and the temperature is not too high nor too cool.
But there's much more to being a life-sustaining planet than just having the right temperature, and it's not known whether the new planet possesses any of these other necessities. For example, in order to sustain life of any kind, much less complex life, a planet needs to have water, a stable atmosphere, plate tectonics, a large moon, and a magnetic field. It also must be shielded from radiation and incoming meteorites. It also needs to have a rich supply of available oxygen, carbon and other elements, and the star itself must have an orbit that does not take it into the galaxy's spiral arms where life would be prevented or extinguished by violent collisions and radiation.
That planets of roughly the mass of the earth exist in the habitable zone of stars is not surprising. That any would be found that would possess all the conditions necessary to allow for life to emerge and survive would be. As Ward and Brownlee conclude in their book Rare Earth the conditions for life are so many and so rare that the earth may well be the only planet in the entire universe which possesses them.
