“Ultimately, in the future of exoplanets, we would like an image of an Earth twin as beautiful as the Apollo images of Earth” .
Our planet is the very model of a habitable planet. Accompanied by its silvery Moon, the Earth forms our only current example of habitability. For this reason, these currently unique objects require a separate section, to consider why this might be so and how such a welcome situation came about. But the random processes involved during formation and the level of complexity exposed by subsequent geological and biological evolution give one pause in seeking clones among the exoplanets.
John Updike remarked that “this planet is exceptional; clearly Venus or Jupiter wouldn’t be agreeable to us” . How did it manage to achieve this status? Can one deduce any general principles from a unique planet accompanied by a unique satellite? A good example has been the difficulty in recognizing on the Earth that the formation of craters by the impact of asteroids, comets and meteorites is an important planetary process. Erosion has removed most of the evidence. It is only within living memory that professional geologists have come to accept that the craters were not caused by internal eruptions. Our experience with the geology and geochemistry of the Moon, with its subtle but crucial distinctions from our hard-won experience on the Earth, should also remind us of the hazards of trying to extrapolate from unique terrestrial conditions.