When you consider that astronomers have hypothesised a companion star to our Sun that they've called "Nemesis", you may be thinking two things. One, how can the Sun have a companion? We'd be able to see it shining in the sky, wouldn't we? And, two, why would scientists theorise this in the first place? Let's address the second question...
Of course, the most famous mass extinction that everyone knows about is the demise of the dinosaurs, which occurred (so the theory goes) as the result of a large meteorite impacting our planet, some 65 million years ago. But, over the millennia, there have been many more such events.
In 1984, a couple of palaeontologists, David Raup and Jack Sepkoski, noticed that these mass extinctions seemed to have occurred at regular intervals - every 26 million years in fact. The only thing they could think of that caused this regularity had to be non-terrestrial. Over to the astronomers, then.
It was two separate teams of astronomers, in fact, that came up with the Nemesis theory independently. They reasoned that, as extinction events are usually caused by impacts of objects normally residing within the Oort Cloud, something must happen every 26 million years to disturb the Cloud, thus increasing the chance of terrestrial impacts.
So, they came up with the idea that our Sun actually has a small companion star, with an extremely elliptical orbit, that periodically takes it close to the Oort Cloud, thus causing the disturbance. It was christened "Nemesis", or, more dramatically, the "Death Star". One team suggested it was a red dwarf, the other maintained that it was a brown dwarf. In any case, it was reckoned to be, at the furthest point of its orbit, around 1.5 light years away.
The search for this possible companion star has been in progress since the 1980s, without success. It is best to look for red and brown dwarves in the infrared part of the spectrum and, currently, the best NASA project to do this is the WISE mission. Standing for Wide-field Infrared Survey Explorer, it is a space telescope launched in 2009.
So far, WISE has found no evidence to support the existence of a companion star to our Sun. In fact, to compound things even further, NASA has issued a statement pointing out that current scientific thought is now that extinction events do not happen at regular intervals at all, thus negating a need for the presence of such an object.