Here is another one which relates it to the passing of our solar system throught the galactic plane: "Sun's Movement Through Milky Way Regularly Sends Comets Hu

Here is another one which relates it to the passing of our solar system throught the galactic plane: "Sun's Movement Through Milky Way Regularly Sends Comets Hurtling, Coinciding With Mass Life Extinctions", 2008 https://www.sciencedaily.com/releases/2008/05/080502092145.htm https://www.sciencedaily.com/releases/2008/05/080502092145.h... ``The Cardiff team found that we pass through the galactic plane every 35 to 40 million years, increasing the chances of a comet collision tenfold. Evidence from craters on Earth also suggests we suffer more collisions approximately 36 million years. Professor William Napier, of the Cardiff Centre for Astrobiology, said: "It's a beautiful match between what we see on the ground and what is expected from the galactic record." The periods of comet bombardment also coincide with mass extinctions, such as that of the dinosaurs 65 million years ago. Our present position in the galaxy suggests we are now very close to another such period.´´ ---- Edit: There are also papers which relates it to passing through spiral arms (different cycle).