Although many scientists have been doggedly pursuing the various attributes necessary for a habitable planet—Michael Hart, George Weatherill, Chris McKay, Norman Sleep, Kevin Zahnlee, David Schwartzman, Christopher Chyba, Carl Sagan, and David Des Marais come to mind—one name stands out in the scientific literature: James Kasting of Penn State University.
Kasting notes that whether habitable planets exist around other stars "depends on whether other planets exist, where they form, how big they are, and how they are spaced." Kasting stresses, as we do, the importance of plate tectonics in creating and maintaining habitable planets, and he suggests that the presence of plate tectonics on any planet can be attributed to the planet's composition and position in its solar system. But one of Kasting's most intriguing comments is related to our Moon. Kasting notes that the obliquity (the angle of the axis of spin of a planet) of three of the four "terrestrial" planets of our solar system—Mercury, Venus, and Mars—has varied chaotically.
Earth is the exception, but only because it has a large moon. ... If calculations about the obliquity changes in the absence of the moon are correct, Earth's obliquity would vary chaotically from 0 to 85 degrees on a time scale of tens of millions of years were it not for the presence of the Moon. . . . Earth's climatic stability is dependent to a large extent on the existence of the Moon. The Moon is now generally believed to have formed as a consequence of a glancing collision with a Mars-sized body during the later stages of the Earth's formation. If such moon-forming collisions are rare . . . habitable planets might be equally rare.
We have accumulated a laundry list of potentially low-probability events or conditions necessary for animal life: not only Earth's position in the "habitable zone" of its solar system (and of its galaxy), but many others as well, including a large moon, plate tectonics, Jupiter in the wings, a magnetic field, and the many events that led up to the evolution of the first animal. Let us explore what these conditions might mean for life beyond Earth.
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