The Sun is one of at least a hundred thousand million stars in our Galaxy, and many of these are of solar type. Therefore, most astronomers have always believed that it is unlikely to be unique in having a system of orbiting planets. If our Solar System had been formed by the near-collision between the Sun and a passing star, it would certainly have been a rarity, because close encounters seldom occur, but when this theory was abandoned there was no reason to believe that the Sun was a special case. Table 20.1 is a selected list of stars with known planets.

Proof was difficult to obtain. A planet will be very close to its parent star; it shines only by reflected light; it is much smaller than a normal star, and it will be drowned in its parent's glare, so that it will strain the power of even our largest telescopes. Only a few of these extra-solar planets (‘exoplanets’) have so far been actually seen, but fortunately there are other methods of detection. Of these, the most prolific are:

Astrometric. Proper motions of reasonably close stars are easy to detect with present-day equipment, and are measurable out to hundreds of light-years. If a star is attended by a planet of sufficient mass it will not move regularly, but will ‘weave’ its way along.