Editing Nemesis (hypothetical star) (section)

==Claimed periodicity of mass extinctions==
In 1984, [[paleontologists]] [[David M. Raup|David Raup]] and [[Jack Sepkoski]] published a paper claiming that they had identified a statistical periodicity in extinction rates over the last 250 million years using various forms of [[Time series|time series analysis]].<ref name=Raup1984/> They focused on the extinction intensity of [[fossil]] families of marine [[vertebrates]], [[invertebrates]], and [[protozoans]], identifying 12 [[extinction event]]s over the time period in question. The average time interval between extinction events was determined as 26 million years. At the time, two of the identified extinction events ([[Cretaceous–Paleogene extinction event|Cretaceous–Paleogene]] and [[Eocene–Oligocene extinction event|Eocene–Oligocene]]) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested a possible non-terrestrial connection. The challenge to propose a mechanism was quickly addressed by several teams of astronomers.<ref name=Whitmire1984/><ref name=Davis1984/>

In 2010, Melott & Bambach re-examined the fossil data, including the now-improved dating, and using a second independent database in addition to that Raup & Sepkoski had used. They found evidence for a signal showing an excess extinction rate with a 27-million-year periodicity, now going back 500 million years, and at a much higher statistical significance than in the older work.<ref name="melott"/>