The solar system holds a special position in the Milky Way, close to (but not exactly at) the co-rotation distance - the one distance from the core where stars orbit the galaxy at the same rate as its spiral arm structure does. A star or planetary system located at the co-rotation distance and between two spiral arms would seemingly remain at that safe place. However, stars and planetary systems exactly at the co-rotation distance would experience a "mean motion resonance," repeated gravitational "kicks" exerted by the galactic arm structure. Such kicks would send the star and its possible planetary system flying out of the habitable zone.
Earth's solar system is located just inside the co-rotation distance. So it is safe from the mean motion resonance. Because the solar system revolves around the galactic center only slightly faster than the galactic arm structure, it crosses the spiral arms only one about every billion years. The last spiral arm crossing occurred 560 to 600 million years ago (just before the Cambrian explosion, when complex animals first came on the scene), so Earth currently resides in the safest possible position).
This protected location is truly exceptional. Not all spiral galaxies are like the Milky Way. In the vast majority, the co-rotation distance and the habitable zone fail to overlap. Not only is there a match for the Milky Way Galaxy, but also the best possible place for a newly forming planetary system to accumulate all the heavy elements and long-lived radioactive isotopes requires for advanced life happens to lie just inside the co-rotation distance." (pp. 68-70)
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