Several articles this year discuss the role that gas giants (like Jupiter and Saturn) play in the rise and evolution of life on Earth, and how this may apply to other solar systems.
On one hand, gas giants clear out a lot of solar system debris early in a solar system's history, sweeping up many asteroids. This is, of course, because of the massive gravitational pull a gas giant has. In this manner, Jupiter and Saturn, but especially Jupiter, act as body guards for the Earth. George Musser, in his article, "Jovian Protector" in the September 2008 issue of Scientific American, reports that two researchers, Jonathan Horner and Barrie Jones of the Open University in England, suggest that if Jupiter was 80% smaller the Earth would've had 400% more asteroid strikes. Making it much more difficult for intelligent life to have evolved, since intelligence evolves slowly as discussed in my 25 November 2007 post, "Sentient Life:"
It seems the more complex the sentient mind, the slower it evolves. Apparently this is because the genes in the complex mind code for proteins that have complex interaction with other molecules in the body: "change a gene too much and it will be unable to continue its existing functions" (Barone, Par. 2). Thus, the more a brain evolves, the slower its evolution becomes. Though some postulate that the recent information revolution, with its explosion of information and rapid technological change may add extra evolutionary pressure on our brains.
Another factor to consider is that despite Jupiter's great size, it is still small compared to the sun and to the diameter of its orbit. If it is a shield, it is a moving shield - most of the time it is not between us and an incoming asteroid (likewise, most of the time it will not be deflecting asteroids toward us).
Jupiter, Ice Ages, and the Dinosaurs
What is left unanswered is which wins out? Did Jupiter clear out more asteroids than it flung in toward us, or did it fling more toward us than it cleared out? If Jupiter was smaller, would Earth have actually been more protected, and, as the author of the Scientific American article wonders, would, then, the dinosaurs still be alive?
Some scientists feel that a change in global climate unrelated to the meteor strike was already working to kill off the dinosaurs - the meteor strike just greatly sped up the process. Would the dinosaurs have survived the climate changes? That is, would the dinosaurs, if they were not finished off by the meteor, had a chance to survive the ice ages?
Fewer meteor collisions could have given them just enough time that some could evolved enough to adapt to changing weather conditions.
Also, the death of large predators would also make life easier for the smaller evolving omnivore; after the weather improves the smaller creature can now be much more free to multiply and command the land. Because of the ice ages prehistoric humans no longer had to contend with saber tooth tigers, giant cave bears, and other huge, powerful predators.
And so, in this scenario of fewer meteor strikes, the dinosaurs may very well have developed an intelligent, sentient mind before the mammals (and, thus, before humans) having such a head start (no pun intended - well, not consciously).
Does Orbital Distance Matter?
However, as mentioned earlier, gas giants like Jupiter also sweep up many asteroids - asteroids that otherwise may have struck the Earth. Would having two or three Jupiter sized - or larger - gas giants in the outer solar system protect the Earth more? Or if Jupiter was larger would it have cleared away even more meteors, or would it have flung even more of them toward the Earth?
If Jupiter was in Mercury's orbit, would it fling less meteors toward the Earth and would that counter the fact that it is not able to sweep away as many meteors? Though it could end up sweeping away more of the meteors and comets that have elliptical orbits that bring them close to the Sun - as they come close to the Sun they would run the risk of being diverted in toward the Sun by Jupiter or being drawn into Jupiter itself.
If Jupiter was much further out, say in the orbit of Neptune, would it be too far away to sweep away meteors as well as be less likely to fling one toward the Earth? In a configuration where more meteors strike the inner terrestrial planet, could it having a very large moon help counter this increase? Are there even better solar system configurations, then, than ours for life to form and evolve in? If there are many configurations which can allow for life to form and evolve in, does this mean that life is not rare in the Universe?
These are questions that the exoplanetary science will help answer.
Comets are made of ice, rock, and organic compounds. They can be as large as several miles in diameter.
Asteroids are generally made of rock with some containing metal (usually nickel and iron). They can be as small as boulders or the size of mountains (hundreds of miles in diameter).
Barone, Jennifer. "Not So Fast, Einstein." Data. Discover. October 2007. 12. Print.
"Frequently Asked Questions." Near Earth Object Program. NASA/JPL. 23 Aug. 2008. Web. 23 Aug. 2008. <http://neo.jpl.nasa.gov/faq/>