As our solar system orbits the center of the Milky Way galaxy, it bobs up and down like a pony on a merry-go-round. It’s not all a joyride, though. Each passage through the plane of the galaxy’s disk might trigger destruction here on Earth. Some researchers have found evidence that nasty things happen to our planet every 30 million years or so. That includes bombardment by comets, and huge volcanic eruptions, which trigger mass extinctions. 30 million years is also the gap between the solar system’s passages through the densest part of the galaxy’s disk. To some, that suggests there could be a relationship between these passages and the cycles of destruction. Perhaps the gravity of stars and clouds of gas and dust disturbs the orbits of comets far from the Sun. Some of the comets might fall toward the inner solar system, where they could hit Earth. Some recent studies say the comets might also be given a shove by dark matter, which produces no energy but exerts a gravitational pull on
