The solar system orbits the galactic core of the Milky Way about once every 100 million years. Rather than staying withing the plane of the galaxy, however, our system tends to rise "above" and fall "below" it in a bobbing sort of motion. One way to picture this is to think of the up-and-down motion of the horses on an old-time carousel or merry-go-round.

Researchers are suggesting that such motion may take the solar system into the hydrogen clouds that exist in our galaxy's spiral arms. These clouds tend to block sunlight, thus having a cooling effect on the Earth. Additionally, the gravitational effects of these clouds may cause comets and asteroids to veer off their paths and slam into the the planet, as scientists think happened 63 million years ago at the end of the Cretaceous Period (late "dinosaur age").

A recent study by Nature magazine shows that the fossil record of the past half-billion years does indeed show sudden mass extinctions about every 62 to 64 million years. This is roughly the time required for the Sun and its planets to move from its "lowest" to its "highest" position in relation to the galactic plane, and back again.

Mikhail Medvedev of the University of Kansas in Lawrence suggests that "solar winds" generated by nearby stars in the spiral arm form a shield against potentially deadly cosmic rays. When the solar system is at its "northernmost" point above the plane of the galaxy, the Earth becomes vulnerable to these rays, which may have contributed to mass extinctions in the past.