Planetary Catastrophes

Even if intelligence is a likely consequence of evolution, f_i will probably be much lower than 1, based on recent insights into the stability of solar systems and planetary climates. Just because a planet starts out good for life doesn't mean it will stay that way.

Computer simulations by Fred Rasio and Eric Ford (Massachusetts Institute of Technology) among others show that Earthlike planets are probably unable to survive the gravitational tug-of-war in a system with two (or more) massive, Jupiterlike giants. They would be slung out of the system or sent careening into the central star.

Conversely, systems with no giant planets at all might also have dire consequences for life-bearing planets. Computer simulations by George Wetherill (Carnegie Institution of Washington) indicate that Jupiter acts as the solar system's gravitational vacuum cleaner, efficiently thinning out the population of hazardous comets that venture into Earth-crossing orbits. Without a Jupiter the current impact rate of comets might be about 1,000 times higher, says Wetherill, with truly catastrophic collisions (like the one that killed the dinosaurs 65 million years ago) happening about once every 100,000 years. This would surely frustrate any slow evolutionary progress from simple life forms to higher intelligences.

Also, dynamical studies by Jacques Laskar and Philip Robutel (Bureau des Longitudes, Paris) have shown that rocky, Earthlike planets show chaotic variations in orbital tilt that could lead to drastic climate changes. Fortunately, Earth's chaotic tendencies are damped by tidal interaction with the Moon. Without a relatively large satellite, Earth might have experienced variations in axial tilt similar to those of Mars, possibly as large as 20° to 60°. This would cause extreme variations in the patterns of the seasons. According to one analysis of planet formation, a world like Earth has only about a 1 in 12 chance of ending up with a nice, mild axial tilt that is safely stabilized by a large moon. (On the other hand a moonless Earth might have retained its original rapid spin, which would tend to stabilize its axis.)

It's anyone's guess how large axial swings would influence the evolution of life and the chance for the emergence of intelligence. Change and stress actually promote the emergence of new, versatile, adaptable species, biologists say. For instance, Paul F. Hoffman (Harvard University) and three colleagues proposed in 1998 that the series of intense global ice ages between 760 and 550 million years ago were the crisis that drove the remarkable "Precambrian explosion" of new life forms around or shortly after that time. The disastrous great extinctions later in Earth's geologic record were always followed by vigorous recoveries, eventually spawning more species than existed before. (Complete recovery from any great extinction, regardless of size, always seems to take a mere 10 million years.) Humanity's own emergence as a species during an unusual run of ice ages is sometimes cited as an example of stress-driven evolution leading to adaptability and intelligence. So a planet with a tippy axis might actually speed evolution along.

But planetary crises that are too extreme or frequent would kill off everything, or keep life beaten down to a low level. In any case, our existence here and now seems to be the accidental result of a number of astronomical coincidences that were unimagined in 1961.

Such coincidences are discussed in the book Rare Earth by Peter Ward and Donald Brownlee (Copernicus Books/ Springer, 2000). Ward and Brownlee argue that only very rarely will a good planet form and remain life-friendly for the billions of years that advanced creatures took to appear on Earth. Seth Shostak of the SETI Institute argued in a rebuttal essay that some of their points are overstated, that once life is established it is probably adaptable enough to thrive in un-Earthly conditions, and that it therefore need not require a planet with a narrowly Earthlike history.

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