Mass extinctions periodicity
When, in 1984, D. M. Raup and J. Sepkoski employed statistical and permutation tests to analyze the temporal distribution of periods of mass extinction over the past 250 million years, they found that especially intense mass extinction was repeated on the Earth with a periodicity of approximately 26 million years (Raup & Sepkoski 1984) (Fig.. XXII.5). The results that they obtained were highly statistically significant and later studies (Raup & Sepkoski 1988) tended to confirm the existence of a periodicity of 26 million years, or somewhat longer. However, some authors have thrown doubt on these figures, pointing out that the periodicity is an artifact caused by incorrect rounding off of the ages of some events (Jetsu & Pelt 2000; Jetsu 1997). Thus, at the present time, the existence of similar periodicity remains only a theoretically interesting possibility.
It is obvious that an event with such long periodicity cannot have its origin in processes occurring directly on the Earth, but rather in processes occurring in the cosmos. As the age of giant craters on the Earth indicates a periodicity of approximately 30 million years, it is generally assumed that mass extinctions could have been caused by periodic bombarding of the surface of the Earth by enormous cosmic bodies, the cores of comets or asteroids (Matsumoto & Kubotani 1996; Trefil & Raup 1987). The simplest model assumes that the Sun, similar to more than half the stars in the universe, could be part of a binary star, where the second part of the binary star, assigned in advance the name Nemesis (a substantially smaller red dwarf, orbiting at a distance of up to 3 light years) could approach the Sun, to be more exact to the Oort comet cloud, extending to a distance of almost one light year from the Sun, with a periodicity of 26 million years. According to some concepts, Nemesis could be a substantially less common brown dwarf, black hole or a planet on a very distant orbital path – however, in these cases, we would not have much of a chance of discovering it with contemporary technical means. It could affect the orbits of comets in the Oort comet cloud when it approaches the Sun and could send some of them towards the Sun and into the area in which the inner planets have their orbits. The impacts of their cores or the impacts of asteroids, which the comets forced out of their orbits, on the surface of the Earth could have caused the mass extinctions.
Astronomic research to date has not confirmed the existence of Nemesis. Consequently, at the present time, it is rather assumed that phenomena with a periodicity of the order of tens of million years could have their origin in the periodic passage of the solar system through the plane of the disk of the galaxy where a great deal of matter is present whose gravity could affect the orbits of comets in the Oort cloud (Sepkoski 1989; Rampino & Haggerty 1996). It is interesting that the periodicity of passage through the disk of the galaxy is currently estimated at 37 ± 4 million years (Stothers 1998), which is very similar to the periodicity of 37.5 million years, which was also observed simultaneously with the periodicity of 26 million years in the age of craters on the Earth (Yabushita 2002).