The asteroid belt, an illustration showing the position of the main asteroid belt in the solar system. The six innermost planets are shown (from centre orbit); Mercury, Venus, Earth, Mars, Jupiter and Saturn. The main asteroid belt is in the space between the orbits of Mars and Jupiter. The existence of a planet between Mars and Jupiter was proposed by a group of six astronomers in 1800, the proposal based on Bode's Law of the relationship of planetary orbits. The first of the asteroids (or minor planets) was discovered a year later. Several thousand have had their orbits accurately computed, but it is thought that the solar system contains over 40,000 asteroids.