The Oort Cloud and the Nemesis Hypothesis
One of the theories mentioned often in the study of “would-be catastrophes” is about a hypothetical star accompanying the Sun, orbiting the Solar system in a long, 26-million-year cycle, dragging a shower of meteorites “stripped” from the Oort Cloud and, attracted by the force of gravity, casting them towards the sun, and by proximity ultimately to Earth. There’s no need to go very far, Wikipedia has a full article on the so called Nemesis hypothesis.
A Brown Dwarf Accompanying the Sun
According to Wikipedia, the Nemesis hypothesis appeared in a research article by R. A. Muller (physicist, University of California, Berkeley), Piet Hut (physicist, Institute for Advanced Study of Princeton) and Mark Davis (Princeton) published in 1984 in Nature magazine. Nemesis would be a brown dwarf, with an orbit of tens, or hundreds or even thousand times farther than Pluto’s. It is calculated that its orbit passes every 26 million years through the so called Oort Cloud, which is another interesting place outside our solar system, this transit destabilizes the cloud triggering a shower of meteorites towards the Sun. This would explain the apparent recurrence of the mass extinctions and associated impacts (confirmed in the geologic record and the traces of Iridium, an extraterrestrial metal, in the geologic strata) that the history of our planet has suffered.
The Oort Cloud
The Oort Cloud is a spherical cluster of comets and asteroids not directly visible from earth, in the limit of the Solar System, almost one light-year away, and a quarter of the distance to Alpha Centauri, the nearest star to our Solar System. According to some estimates, the cloud could host between one and a hundred billion comets. It is believed that most comets approaching the earth come from this Oort Cloud, although not something happening frequently.
The mass extinctions that took place on Earth may have been caused, according to some theories, by this huge cyclic meteorite shower, and it is assumed that Nemesis, when transiting the Oort Cloud, disturbs the meteor cluster sending millions into the heart of the Solar System. By some estimates, we are presently in the middle of a Nemesis cycle, counting from its closest approach to the Sun, meaning that the last closest approach to our star occurred 12 million years ago, with another transit of Nemesis across the Oort Cloud is not very far in the future (at least in an astronomical time scale, I would think).
If this theory is validated as true and correct, we’d better run for cover because, if a meteorite shower comes our way in the future like the one life endured thousands of years ago, it’s going to be a hell of a show (and no umbrella will keep us safe).