ABSTRACT
Our Sun is one of at least a hundred thousand million stars in our Galaxy alone, and many of these stars are of solar type. 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, or even unique; but this theory has long since been discarded, and there is nothing unusual about the Sun. If a low-mass companion of a star has a mass greater than around 13 times that of Jupiter, the most massive planet in our Solar System, it is probably a brown dwarf. In November 1999 a team of astronomers using the Keck I telescope in Hawaii announced the discovery of six more extra-solar planets orbiting solar-type stars. The European Space Agency is already planning what is known as the Darwin project. This involves sending several small infra-red telescopes into the outer part of the Solar System.
