Star of Vengence

Could the Sun be a double star system? Could it have a companion star that orbits around it but which is so far away that we cannot see it? There are good reasons for believing that this may be true. Many people have a strange prejudice about the stars. Just because we appear to orbit a single sun, they develop a kind of single‑sun chauvinism. They take it for granted that single stars like ours are ``normal'', and that double and triple star systems are oddities to be made the subject of fantastic paintings.

But this is not the case. In 1990 Antoine Duquennoy and Michel Mayor examined every Sun‑like star within 72 light years, and found that only one third of them were single stars. All the rest were doubles, triples or quadruples. The distances between the components of these multiple star systems vary tremendously. They may be so close that they are almost touching–with two stars nearer to each other than Mercury is to the Sun–or they may be so far apart that it takes tens of millions of years for one star to complete a single orbit round the other. This fact led Richard A. Muller to propose in 1984 that ours was just such a system. He suggested that the Sun might have a companion star with a highly eccentric orbit that approached the Sun closely every 26 million years. During such visits its gravity would disrupt Earth's orbit round the Sun, or subject our planet to a rain of cometary collisions. Either way, the result would be periodic mass extinctions. He therefore named this hypothetical star Nemesis, or the bringer of vengeance. And there is evidence that vast extinctions of animal life do occur roughly every 26 million years! The asteroid or comet that destroyed most of the dinosaurs 65 million years ago was one of these. (Luckily Nemesis isn't due to return to cause another such catastrophe for 15 million years.) And so one half of Muller's sinister theory stands up. The difficulty is in finding the star that causes all this trouble. If it exists it must shine very faintly no more than about 2.8 light years away, only two‑thirds of the distance to Proxima Centauri, generally thought to be the nearest star. And it must be very faint because if it were a bright star its identity would be obvious. But knowing the distance of Nemesis is not enough. Nobody knows in what direction to seek it, or how to distinguish it from the many thousands of background stars that are tens or hundreds of light years away. No one as far as I know is actively investigating the mystery at present, for an obvious reason. Fifteen million years is such a long time that it does not seem to matter very much what may happen at the end of it. Nevertheless, finding Nemesis would be a heroic work of astronomical detection. One clue would be that it would have a parallax of about one arc second, revealing its proximity. Alternatively, one can enjoy Isaac Asimov's splendid 1989 novel Nemesis, an adventure story based on this mystery.