There are two kinds of large cosmic objects that strike the Earth - asteroids and comets. Asteroids were discovered at the beginning of the 19th century, when astronomers were intrigued by the apparent gap in the spacing of the planets between Mars and Jupiter. The gap shows up clearly in a simple mathematical expression which relates the distances of the planets from the Sun. As is often the case in science, the law does not carry the right, name - it was first set out by the German astronomer Johann Titius in 1766, but was popularised by his compatriot Johann Bode in the 1770s.

In the form presented by Bode, the key sequence of numbers is 0, 3, 6, 12, 24 and so on, with each number after the '3' twice the preceding number. Then, you add 4 to each number. If the distance of Earth (the third planet) from the Sun is set at 10 units, then the distances of Mercury, Venus and Mars are all at the equivalent distances given by the series: 4, 7, 10, 16. There is a gap at 28, with Jupiter at 52 and Saturn at 100 units from the Sun, as 'predicted' by the law.

The discovery of Uranus in 1781, close to 196 units from the Sun, was regarded as a confirmation of the accuracy of Bode's law, and in the early 19th century the asteroids were discovered in a belt between Mars and Jupiter, corresponding to the 'missing' planet.

Although Neptune and Pluto, discovered later, do not have orbits which match the predictions of Bode's 'law', there has recently been a revival of interest in the possibility that some simple rule of thumb of this kind must apply to the spacing of the orbits of planets in any planetary system, and to the spacing of moons in systems like those around Jupiter and Saturn.

The discovery of three planets orbiting a pulsar known as PSR B1257+12 has provided a boost for these ideas by revealing a system with properties that almost exactly match those of the innermost Solar System, made up of Mercury, Venus and the Earth. The similarities are so striking that it seems there may be a law of nature which ensures that planets always form in certain orbits and have certain sizes.

PSR B1257+12 is a rapidly spinning neutron star, containing slightly more matter than our Sun packed into a sphere only about 10 km across. As the star spins, it flicks a beam of radio noise around, like the beam of a lighthouse, producing regularly spaced pulses of radio noise detectable on Earth. It can only have been produced in a supernova explosion, long ago, which would have disrupted any planetary system the star possessed at the time. So the present planets associated with the pulsar are thought to have formed from the debris of a companion star disrupted by the pulsar. The three planets cannot be seen directly, but are revealed by the way in which they change the period of the pulsar's pulses as they orbit around it. There is enough information revealed in the changing pulses to show that the three planets have masses roughly equal to 2.8 times that of the Earth, 3.4 times that of the Earth, and 1.5 per cent of that of the Earth. And they are spaced, respectively, at distances from the pulsar equivalent to 47 per cent of the distance from the Earth to the Sun, 36 per cent of the Sun-Earth distance, and 19 per cent of the Sun- Earth distance.

Tsevi Mazeh and Itzhak Goldman, of Tel Aviv Uni- versity, have pointed out that the ratio of these distances, 1:0.77:0.4, is extremely close to the ratio of the distances of the Earth, Venus and Mercury from the Sun, which is 1:0.72:0.39. And the masses of the three inner planets in the Solar system are 1 Earth mass, 82 per cent of this mass (Venus) and 5..5 per cent (Mercury). In each case,. two outer planets with roughly the same mass have an inner companion with a much smaller mass.

So Bode's law also works for the planets of pulsar PSR B 125 7 + 12, and the indications are that there is a universal mechanism for the formation of planets around stars. If it works for systems as diverse as a pulsar and our Sun, the chances are that it works for all stars, and that'Solar' Systems very like our own may be the rule, rather than the exception, among the stars of the Milky Way.