Some astronomers maintain that the Sun might have a small companion star in orbit around it that comes close enough on a regular basis to disrupt the orbits of comets in the Oort Cloud. Some of those comets, in turn, blaze into the inner Solar System and slam into Earth, accounting for the rough regularity of mass extinctions observed in the fossil record. That mysterious companion star, which has never been seen, is called "Nemesis."
A new study of M-type dwarf stars, the most common type of star in the universe, didn't find Nemesis. but it did find 18 such stars that could possibly come fairly close to the Sun over the next billion years. None of them pose a real threat to the Solar System, however.
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