Planets beyond Neptune
Different hypotheses have been put forward: A hypothetical star, located at about 50,000 AU distance, has been called Nemesis. Nemesis might be either a red dwarf or a brown dwarf. In the original article, it was claimed Nemesis would orbit the sun at about 95,000 AU. The existence of Nemesis could explain events of mass extinction on earth which seem to occur at intervals of 26 million years. However, its existence is not necessary to explain the extinctions. There are actually fewer brown dwarfs in our cosmic neighborhood than previously thought. Rather than one star for every brown dwarf, there may be as many as six stars for every brown dwarf. The majority of solar-type stars are single.