No Nemesis

There seems to be an odd biological cycle on Earth. About every 27 million years, the planet suffers a mass extinction. Scientists have no real idea why that would be, but astronomers, noting the time scale, suggested in 1984 that the Sun might have a tiny, dim stellar partner that rattles the orbits of objects in the Oort Cloud such that roughly every 27 million years Earth is pounded by comets, creating the mass extinctions. Astronomers called the proposed star Nemesis.

A new study, interestingly, confirms the cycle does exist, but rules out any Nemesis-like star as the cause. Using computer models and simulations, the study argues that no star like the one proposed could remain in the necessary orbit over such large time spans because of the gravitational influences of other bodies as the Solar System swept through various regions of the galaxy on its orbit around the center of the Milky Way.

So, there is no Nemesis, but there is likely an extinction cycle, according to the fossil record. The next mass extinction is due in about 16 million years, which should give scientists plenty of time to solve the puzzle.