But why should comets become more likely to hit us every million years? Here we launch ourselves into deep speculation. It has been suggested that the sun has a sister star, and the two orbit each other with a periodicity of about 26 million years. This hypothetical binary partner, which has never been seen but which has nevertheless been given the dramatic name Nemesis, passes, once per orbital rotation, through the so-called Oort Cloud, the belt of perhaps a trillion comets which orbits the sun beyond the planets. If there was a Nemesis that passed close to, or through, the Oort Cloud, it is plausible that it would disturb the comets, and this might increase the likelihood of one of them hitting earth. If this all happened - and the chain of reasoning is admittedly tenuous - it could account for the 26-million-year periodicity of mass extinctions that some people think the fossil record shows. It is a pleasing thought that mathematical unweaving of the noisy spectrum of animal extinctions might be the only means we have of detecting an otherwise unknown star.

Piet Hut, another Dutch astronomer fifty years younger than Jan Oort, decided to take seriously the possibility that comet showers are periodic. If they are periodic, the theory that they are caused by the random passing-by of alien stars cannot be right. If showers are periodic, they must be explained by a different theory. Piet Hut and his friend Rich Muller found an alternative theory to explain the periodicity in case it turns out to be real. The alternative theory is called Nemesis. Nemesis is an imagined star belonging to our own solar system. It is gravitationally bound to the Sun and forms with the Sun a binary star. We know rather accurately how far away it is, if it exists. It is 2.5 light-years away. But we do not know even roughly in which direction it lies, and we do not know whether it is bright enough to be seen with a telescope. Several astronomers are searching for it but nobody has found it. It may turn out, after all, to be a chimera, a mythical creature made out of dreams and travelers' tales.

Piet Hut's theory assumes that Nemesis has an elongated orbit, as many double stars do. Most of the time it will be at a large distance from the Sun and will have no effect on the comets in the Oort Cloud. But once every 26 million years it will come rather quickly through the part of its orbit which is close to the Sun. As it passes by the Sun it will disturb the comet orbits and cause a comet shower, just as an alien star would do. Since the passages of Nemesis occur at regular intervals, the comet showers will be periodic.