Nemesis

In a report they released in 1984, paleontologists David Raup and J. John Sepkoski asserted that massive extinctions happened on a regular basis every 26 million years. Two different teams of astronomers put out the following theories: Nemesis, named after the Greek goddess of vengeance, orbits the Sun every 26 million years and is an unknown companion star. Nemesis affects the comets in the Oort cloud when it is at its closest to the Sun, which causes a comet shower to comet inward. Some of these objects strike Earth and cause extinctions, like the one that wiped off the dinosaurs 66 million years ago.

Nemesis would have to be an extremely dim object, like a red dwarf star or maybe a brown dwarf, as it hasn't been spotted. This concept has been contentious in almost every area. Not all paleontologists concur that extinctions occur periodically. It has been remarked by several astronomers that such a broad orbit would be easily perturbed by passing stars. The deal-breaker is that, much like with Nibiru and Tyche, Nemesis would have been discovered by later surveys that looked at the entire sky at infrared wavelengths, but they haven't done so.