Solar system orbits from Huygens's 'Cosmotheoros' (1698). From the Sun (centre) outwards, the planets are: Mercury, Venus, Earth, Mars, Jupiter and Saturn. The orbits of the planets and the orbits of the moons of the Earth, Jupiter and Saturn are also shown. This work by Dutch astronomer Christiaan Huygens (1629-1695) described the planets of the solar system and his theories on the existence of extraterrestrial life on those planets.