The nine planets of the Solar System are very different from one another, as the following descriptions and tables show. The period of rotation is the time it takes the planet to turn once itself; it is the length of the planet's day.
	The period of revolution is the planet's year the time it takes to go once around the Sun. By axis we mean the central line around which a planet rotates. Its tilt is the angle at which it lies to a line drawn at right angeles to the direction of the Sun.
	Mercury. It is the smallest planet, and also the nearest to the Sun. It has a surface covered with craters. It turns so slowly that Mercury day - from one sunrise to the next - lasts two third of a mercury year.
	Venus comes nearer to Earth then any other planet but little is known of its surface because it is always hidden beneyth clouds. A picture from a Russian space probe shows the surface to be stony. Venus spins clockwise, in the opposite direction to the outer planets.
	Mars is the best-known planet apart from Earth, as space probes have mapped the surface and landed there. Their pictures show craters and huge volcanos, the ground being stony and red in colour. There are white ice-caps at the poles.
	Jupiter is the largest of the Solar System. Running around it are wide bands of different colous, probably clouds, among which can always be seen the Great Red Spot, probably a continuous in the atmosphere. Surface temperature at cloud level -150 c.
	Saturn with its many rings is the most beautiful sight in the Solar System. The rings are composed of particles and stones, possibly ice-covered, floating in belts only a few kilometres thick. They may be the remains of a moon that broke up.
	Uranus was the first planet to be discovered with a telescope, although it can be made out without one. It is greenish in colour, and probably has a surface of ice and frozen gases beneath swirling clouds. In 1977 it was discowered to have a ring system.
	Neptune has a blue tinge when seen in telescope but no marking can be observed. It is probably like Uranus.
	Pluto is usually the most distant planet, but part of its orbit passes within that of Neptune. For this reason Neptune will in fact be the most distant planet until about 2000. Pluto is a small planet and little is known about it, though it is thought to have once been a moon of Neptune.