Dark Star, or Nemesis, is a theoretical brown dwarf in the Solar System that was proposed to explain a perceived cycle of mass extinctions in Earth's history. Scientists speculated that such a star could affect the orbit of objects in the far outer solar system, sending them on a collision course with Earth. 
In the early 1980s, scientists noticed that extinctions on Earth seemed to fall in a cyclical pattern. Mass extinctions seem to occur more frequently every 27 million years. The long span of time caused them to turn to astronomical events for an explanation.
In 1984, Richard Muller of the University of California Berkley suggested that a red dwarf star 1.5 light-years away could be the cause of the mass extinctions. Later theories have suggested that Nemesis could be a brown or white dwarf, or a low-mass star only a few times as massive as Jupiter. All would cast dim light, making them difficult to spot. This theory was coined "Nemesis".
Scientists speculated that Nemesis may affect the Oort cloud, which is made up of icy rocks surrounding the sun beyond the range of Pluto. Many of these chunks travel around the sun in a long-term, elliptical orbit. As they draw closer to the star, their ice begins to melt and stream behind them, making them recognizable as comets. If Nemesis traveled through the Oort cloud every 27 million years, some argue, it could kick extra comets out of the sphere and send them hurling toward the inner solar system — and Earth. Impact rates would increase, and mass extinctions would be more common.
The Kuiper Belt, a disk of debris that lies inside of the solar system, also has a well-defined outer edge that could be sheared off by a companion star. Researchers have found other systems where a companion star seems to have affected the shape of the debris disks.
The dwarf planet Sedna lends further credence in the eyes of some to the existence of a companion star for the sun. With an orbit of up to 12,000 years, the planet presents a puzzle to many. Scientists have suggested that a massive object such as a dim star could be responsible for keeping Sedna so far from the sun.
Mass extinction cyclesEdit
Though some scientists find the Nemesis theory plausible, others do not. The initial cyclical nature of mass extinctions is still under discussion. Studies of craters seem to indicate that such a pattern does not exist. Other studies of fossil records suggest that mass extinctions are more likely to happen around those peaks, though extinction events occur during other epochs.
In order to avoid significantly affecting the orbit of the planets, as well as to avoid observation, Nemesis must remain at a distance from the sun. But astronomers argue that such an orbit would be inherently unstable. Traveling so far out, Nemesis would be affected by other stars moving through the galaxy. The resulting orbit would not provide a steady kick to the Oort cloud but would be constantly changing.
Archaeologist Jonathan Gray, author of Dead Man's Secrets, postulates that mass extinction cycles is more of a modern concept. Gray affirms that there is evidence for only one global extinction event, and beyond that, it is theoretical. Destructive forces come in various sizes. A repeated event on a global scale has yet to be realized.
- ↑ 1.0 1.1 1.2 Nemesis Star Theory: The Sun's 'Death Star' Companion, Space.com, August 26, 2013 06:43pm ET
- ↑ Coast to Coast AM, October 1, 2016 1:49:25, Jonathan Gray on destructive cycles
- Black Star
- Near Earth misses
- SPACE Big, Shiny Asteroid to Fly (Safely) Past Earth (April 10, 2017) 06:38am ET, by Hanneke Weitering, Staff Writer-Producer