The Universe is filled with binary star systems in which planets rotate around more than one star at the same time. But the Solar System just has one star, the Sun, right? Well, perhaps not if the ‘Nemesis theory’ is correct.
The theory goes something like this: over the last 250 million years, Earth has seen mass extinctions every 26 million years or so. The idea is that these extinctions were caused by the catastrophic impact of comets sent hurtling into the inner Solar System by a small second star called Nemesis – the Sun’s twin star. We’ve never spotted Nemesis because it’s very dim and small compared to the Sun, and it’s moving very slowly from our perspective.
Large parts of the theory are open to dispute. The scientific consensus is that Earth’s mass extinctions don’t in fact occur on a predictable cycle, and there’s no evidence for periodic impacts in Earth’s crater record. Oh, and there’s also the fact that we’ve been searching the sky for decades with all manner of instruments and have never seen even a whiff of such a star.
But the theory lives on, most recently in observations of Sedna, a dwarf planet that’s locked into an extremely distant orbit of the Sun. According to our understanding of the Solar System, Sedna shouldn’t be where it is. We’ve got something wrong, we just don’t know what.