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On television, I heard a scientist who believes that our Sun has a companion (brown dwarf) Sun, that is responsible for comets destroying life here on Earth every 26 million years. He proposed the theory that when this companion Sun travels through the Ort Cloud, every 26 million years, it brings billions of Comets with it into our Solar System and inevitably one crashes into the Earth. Any thoughts on this?

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    $\begingroup$ In first place, why this star would be invisible to ALL our telescopes? ;) $\endgroup$ – Py-ser Oct 30 '14 at 4:43
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Ahh, I see you've met our friend, the hypothetical star known as Nemesis. I don't know which scientist(s) you're referring to, but the idea goes back quite a while. The original theory was created by Raup and Sepkoski way back in 1984. Their paper analyzed mass extinctions in the past and concluded that there was a pattern. Note, though, that the paper does not specifically say that a star orbiting the Sun is the cause; it merely favors general impacts of "extraterrestrial" origin.

However, two more teams analyzed the data and posited that there was a celestial body responsible. Whitmire and Jackson and Davis, Hut, and Muller (Regrettably, you must pay to see the papers, which I have not done) suggested that the Sun had a companion in the far reaches of the solar system, and it was perturbing comets and other bodies, which came towards Earth.

Honestly, I (and quite a few others) don't think there's much truth to the idea. Why? Because we haven't found any evidence of its existence. As Wikipedia explains,

Searches for Nemesis in the infrared are important because cooler stars shine in infrared light. The University of California's Leuschner Observatory failed to discover Nemesis by 1986. The Infrared Astronomical Satellite (IRAS) failed to discover Nemesis in the 1980s. The 2MASS astronomical survey, which ran from 1997 to 2001, failed to detect a star, or brown dwarf, in the Solar System.

and, later on,

In particular, if Nemesis is a red dwarf star or a brown dwarf, the WISE mission (an infrared sky survey that covered most of our solar neighborhood in movement-verifying parallax measurements) was expected to be able to find it. WISE can detect 150 kelvin brown dwarfs out to 10 light-years. But the closer a brown dwarf is the easier it is to detect. Preliminary results of the WISE survey were released on April 14, 2011. On March 14, 2012, the entire catalog of the WISE mission was released. In 2014 WISE data ruled out a Saturn or larger-sized body in the Oort cloud out to ten thousand AU.

In fact,

According to NASA, "recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals, and thus, the Nemesis hypothesis is no longer needed." And, indeed, a recent sky survey by NASA's WISE mission found no star or brown dwarf orbiting the Sun.

NASA's conclusions are laid out in this page, which talks about how WISE found no brown dwarf in the Oort Cloud . . . but found other brown dwarfs further out.

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