Nemesis (star)

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Nemesis is a hypothetical hard-to-see red dwarf star or brown dwarf, orbiting the Sun at a distance of about 50,000 to 100,000 AU (about 0.8-1.5 light-years), somewhat beyond the Oort cloud.[1] This star was originally postulated to exist as part of a hypothesis to explain a perceived cycle of mass extinctions in the geological record, which seem to occur once every 27 million years or so. In addition, observations by astronomers of the sharp edges of Oort clouds, similar to that of the Solar System, around various binary (double) star systems, in contrast to the diffuse edges of the Oort clouds around single-star systems, has prompted some scientists to postulate that a dwarf star may be co-orbiting the Sun.[1] Counter-theories also exist that other forces (like the angular effect of the galactic gravity plane) may be the cause of the sharp-edged Oort cloud pattern around the Sun. To date the issue remains unsettled in the scientific community.

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Claimed periodicity of mass extinctions

In 1984, paleontologists 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 analysis.[2] They focused on the extinction intensity of fossil families of marine vertebrates, invertebrates, and protozoans, identifying 12 extinction events 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-Tertiary and Late Eocene) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested that there might be a non-terrestrial connection. The challenge to propose a mechanism was quickly addressed by several teams of astronomers.

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