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Asteroid belts may help life to evolve into more complex forms: study

Asteroids pose a significant threat to the Earth and its inhabitants. Astronomers scan the skies for new ones, track the ones that we know about and come up with ideas about how to deal with the dangerous ones. They're the star players in many apocalyptic and post-apocalyptic books and movies. However, according to a new study, asteroids may have helped life on this planet evolve from simpler forms to the complex forms we see today.

A NASA Sagan Fellow from the University of Colorado in Boulder, named Rebecca Martin, and an astronomer named Mario Livio, who works at the Space Telescope Science Institute in Baltimore, have published a paper about this in Monthly Notices of the Royal Astronomical Society: Letters. The idea behind their study is that most species that evolve on this planet go through long periods of evolutionary stasis, where they remain essentially the same, with only minor changes to the species. The only time that any major evolutionary changes happen is when there is some event that quickly (at least relative to geologic time scales) and radically changes their environment, forcing the species' to adapt or die off. In evolutionary biology, this is called 'punctuated equilibrium'.

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According to Martin and Livio, the position of the asteroid belt in our solar system is specifically based on two factors: Its location just outside our Sun's 'habitable zone', or beyond what Martin and Livio refer to as the 'snow line', where all water would be frozen, and the location of where Jupiter formed and then migrated to, 'just' outside the asteroid belt. Jupiter's immense gravity would have carved the asteroid belt's shape, while simultaneously preventing any larger bodies from forming from the material in the belt.

"To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt," said Livio, according to JPL News. "If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve."

As Jupiter migrated towards the sun, it stopped at just the right point that the asteroid belt was preserved, but also where its gravity imbued enough energy into the objects in the belt that it made their collisions more violent. This would have knocked some of the asteroids out of the belt, which would then have struck the inner planets. There have been several studies already that have suggested that both the water and the building blocks of life on Earth are here due to asteroid collisions just after the planet's formation. Martin and Livio propose that those asteroids also forced life on Earth to evolve, and our solar system may be very special in that regard.

To examine this further, Martin and Livio constructed computer models of solar systems, using the idea that asteroid belts always form just beyond the habitable zone. They placed protoplanetary disks around young stars of various masses and then calculated the 'snow line' of those stars — just far enough away where water remains frozen. To confirm their findings, they examined all of the infrared surveys from the Spitzer Space Telescope of 90 star systems that were known to have 'warm dust'. Matching the star types with the star types from their models, they found that the temperature of this dust was close to what they had found in their simulations.

"The warm dust falls right onto our calculated snow lines, so the observations are consistent with our predictions," said Martin.

They then used that information as a guide to survey 520 stars that are known to have giant planets orbiting them. Of those 520 giant planets, only 19 orbit outside the 'snow line', so it is likely that the remainder migrated too close to their star, thus disrupting any asteroid belt that might have formed in the system.

"Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet," said Martin.

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From their findings, it appears as though less than four per cent of the star systems they surveyed have a compact, slightly-dispersed asteroid belt.

"Based on our scenario, we should concentrate our efforts to look for complex life in systems that have a giant planet outside of the snow line," Livio said.

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