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66 constituents of our atmosphere, although our ability to do this at interstel- lar distances will depend on the resolving power of our instruments. This type of research might even reveal the presence of a space-faring civilization. Finding two objects in an alien solar system that show signs of life in their atmospheres would suggest that one of them has been “terra- formed. That, in turn, would tell us that an intelligent species had expanded beyond its home planet. The Specialness of Earth Those who are optimistic about finding extraterrestrial life and intelli- gence tend to see our Earth as typical of a class of planets that may exist in many systems. Others question that assumption. For the first time in 500 years, Rood and Trefil proposed in 1981, we are coming to see the Earth as something special. It is in the system of a single G-class star; its orbit is in that zone where water will neither boil nor freeze for billions of years; it has a large moon which produces tides on Earth that favor tidal pools where life might have evolved; the tilt of the Earth’s axis is just enough to cause periodic changes in the climate. Geologist Peter Ward and astronomer Donald Brownlee revived this theme in their book Rare Earth, claiming that the conditions needed for the evolution and survival of higher life are so complex and precari- ous that they are unlikely to arise in many other places. The key factors include the probability of planetary systems of the right kind, the per- sistence of oceans and moderate temperatures, and a large moon. If the Rare Earth Hypothesis is correct, they declared, then SETI clearly is a futile effort. “There probably are other civilizations in the galaxy that have radio telescopes,” they acknowledged, “but the vast numbers of stars and the vast distances involved are barriers that may always keep SETI more an experiment than a large-scale scientific endeavor.” Planetary scientist David Grinspoon challenged the Rare Earth thesis, particularly its failure to fully recognize the role of life in creat- ing the Earth’s unusual character. However, he agreed that our Moon has influenced Earth in numerous ways, slowing its rotation, raising tides in its oceans, steadying its spin axis and climate. Without such a moon, our planet might have undergone radical changes in the tilt of its axis, provoking extreme excursions of climate (there is some evidence that Mars has gone through such traumas). A few claim that life might not exist at all if the Earth had no moon, particularly because of its effects on tides. On the other hand, some computer simulations suggest that small planets with big moons are likely to be quite common. As many as one in three young Earth-like planets may be struck hard enough to make big moons.” Probabilities: The Astronomical Factors