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Asteroids Giveth and Asteroids Taketh Away

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If dinosaurs could have left written records, then we all probably would have read by now the horror stories describing the mass extinction of some 65 million years ago. Such first-hand documents would also have put an end to the debate on whether it was a large asteroid impact or increased volcano activity that brought about the demise of the dinosaurs. In any case, there is no doubt that a huge asteroid impact on Earth would be devastating to many life forms. However, we often forget that asteroid impacts may have played a crucial role in the emergence of life on Earth, and possibly even in the fact that humans are here to talk about them!

How are asteroids potentially important for life? First, recent studies by astrophysicist Rebecca Martin and myself have shown that the Earth most likely formed in the dry region of the disk of gas and dust around the young Sun. This explains why less than one percent of the Earth's mass is water. Consequently, the water (that we believe was essential for life to emerge) had to be brought to Earth, and the most likely delivery agents were asteroids (although comets, and the interaction between the magma and the atmosphere, may have also played a role). Second, heavy elements that are important for life -- such as iron -- may also have been delivered to the Earth's crust by asteroids (the iron in the initial molten Earth sank to the core). Third, some researchers have even suggested that primitive life forms, or the building blocks for life, may have arrived to Earth by way of asteroids. Fourth, the formation of the Earth's relatively large Moon is also thought to have ensued from a collision with a large body (although perhaps planet-sized and not an asteroid). The Moon stabilized the Earth's rotation axis against chaotic motion that would have resulted in weather extremes that could not have been "healthy" for life. Finally, one could wildly speculate that were it not for the extinction of the dinosaurs -- large, ferocious animals with extremely small brains -- mammals would not have become the dominant clade, and intelligent life might not have emerged. The key point is that even if only one of these potential roles for asteroids in the evolution of life on Earth is true, then the existence of an asteroid belt may be a necessary condition for intelligent life in extrasolar systems.

The key point is that even if only one of these potential roles for asteroids in the evolution of life on Earth is true, then the existence of an asteroid belt may be a necessary condition for intelligent life in extrasolar systems. - Mario Livio

There is, however, another issue that must be considered. Observations of extrasolar planets have shown that giant planets often migrate inwards through the protoplanetary disk. Such a migration can scatter the asteroid belt (if one had formed at all), with asteroids being either accreted by the central star, or ejected to large orbits. Consequently, systems in which significant migration has occurred are not expected to harbor a compact asteroid belt. At the other extreme, if the giant planet that was responsible for the formation of the asteroid belt in the first place (by gravitationally preventing a planet from forming) does not migrate at all, the asteroid belt may be too massive. Frequent impacts from such a belt would be harmful for life. In other words, for life to evolve, a just-right asteroid belt may be needed (Figure 1). In this respect, the solar system found itself in the "Goldilocks" paradigm, with Jupiter having migrated just enough to deplete the asteroid belt, but not destroy it.

Rebecca Martin and I have shown that among the giant extrasolar planets observed to date, about four percent appear to have the right conditions for a potentially favorable asteroid belt. My guess would be that terrestrial planets in the habitable zones (that allow for liquid water on the planet's surface) of those systems are good candidates for life.

Concerning the Earth, we should do everything we can to prevent future impacts, but at the same time, it is quite possible that we should be thankful for past ones.

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