Nemesis Origin of the theory The "Nemesis Theory" was an outgrowth of the discovery of Alvarez et al., that the impact of a large (10 km diameter) comet or asteroid was responsible for the great mass extinction that took place 65 million years ago. Studies of the fossil record by Dave Raup and Jack Sepkoski shows that this was not an isolated event, but one of several mass extinctions that appear to occur on a regular 26 million …showed first 75 words of 4215 total…

…showed last 75 words of 4215 total…half the story of why Nemesis is not believed. The other half is that we predicted we would find it within a few years, and we haven't. So most people think our search found no such star. In fact, the search stalled soon after it started. There is no reason to believe that Nemesis is not the solution to the mystery of the periodic extinctions, and there is no alternative theory that has survived scrutiny.