The 1980s discovery: Nemesis and its link to the Earth's widespread extinction.

"Nemesis" is an example of a red dwarf 'hypothetical star,' i.e. its existence has been postulated, but not yet confirmed through observation.

Nemesis was first introduced in 1984 in a paper presented by Dave Raup and Jack Sepkoski.

A link to a copy of the paper: Periodicity of extinctions in the geologic past by DAVID M. RAUP AND JACK SEPKOSKI, JR.

These researchers sought to explain a perceived 26-28 million year extinction periodicity in the geological record. Every 26-28 million years, Earth seems to experience a widespread extinction[1], according to their study.

They concluded that a shower of cometary debris whipped into the inner solar system every 26 million years under the influence of Nemesis, a companion star to the Sun.

This periodicity suggests a certain pattern for the extinctions.

According to their hypothesis, whenever Nemesis reaches the point of least distance between two binary stars (also known as periastron), the companion star's gravity perturbs myriad Oort Cloud comets. (The Oort Cloud is the spherical shell of long period comets extending between 2,000-50,000 AU.) These dislodged comets would then assault the inner solar system like an armada. Some of these interloping bodies then strike our planet, causing severe disruptions to the ecosystem which then precipitates mass extinctions.

In order for Nemesis' orbital period to be 26 million years, its mean distance from the Sun would have to be about 1.5 light years: beyond the outermost Oort Cloud comets.

But now there are two issues:

1. At such a distance, the gravitational bond between Nemesis and the Sun is quite tenuous. Perturbations [2] from other stars over the course of hundreds of millions of years would most likely have dislodged Nemesis from the solar system. Or, at the very least, it would have altered its orbit profoundly.

2. No astronomer has yet found Nemesis. It is assumed to be a red dwarf or a brown dwarf [3] and therefore quite faint. However, Nemesis would still be about three light years closer to the Sun than any other star. (The closest star, Proxima Centauri is 4.2 light years away.) At this distance, it should have been found by now, especially with the Wide-field Infrared Survey Explorer capable of detecting brown dwarfs within a 10 light year radius. So far, Nemesis has not been detected.

Does Nemesis exist?

Probably not, but we still can't claim whether it does or does not exist.

"When is the next 27 million year extinction scheduled to happen?"

The last was the Middle Miocene extinction about 14.5 million years ago, giving us about 12.5 million years until the next, provided the periodicity persists.

[1] A widespread extinction: (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth.

[2] In astronomy, perturbation is the complex motion of a massive body subject to forces other than the gravitational attraction of a single other massive body.

[3] A red dwarf is the least massive of the fusion stars, the stars that are powered by core thermonuclear fusion reactions. Brown dwarf stars are not massive enough to have ignited these fusion reactions.