The history of learning about planets in space

The Earth is one of nine planets orbiting the Sun that together make up the solar system. The planets, in order of their distance from the Sun, are:

Mercury 4900km (3000 miles) in diameter and 58 million km (36 million miles) from the Sun; no moons; rotates once in 59 Earth days; orbits Sun once in 88 Earth days.

Venus 12,100 km (7500 miles) in diameter and 108 million km (67 million miles)from the Sun; no moons; rotates once in 243 - 4 days; orbits Sun once in 225 days.

Earth 12,756km (7926 miles) in diameter at the Equator and 150 million km (93 million miles) from the Sun; one moon; rotates once in 24 hours; orbits Sun once in 3651; days.

Mars 6800km (4200 miles) in diameter at its equator and 228 million km (142 million miles) from the Sun; two moons; rotates once in 25 hours; orbits Sun once in 687 days.

Jupiter 143,000km (89,400 miles) in diameter at its equator and 778 million km (480 million miles) from the sun. 16 known moons. Rotates once in 10 years, and orbits the sun once in 11.86 years.

Saturn 120,000km (75,000 miles) in diameter at its equator and 1427 million km (886 million miles) from the sun, 17 known moons, and rotates once in roughly 10 years, and orbits the sun once in every 29.46 years.

Uranus 52,000km (32,300 miles) in diameter at its equator and 2870 million km (1783 million miles) from the Sun; five known moons; rotates once in about 16 - 28 hours; orbits Sun once in 84 years.

Neptune 48,000km (30,000 miles) in diameter at its equator and 4500 million Km (2800 million miles) from the Sun; tree known moons; rotates once in about 18 - 20 hours; orbits Sun once in 165 years.

Pluto Approximately 3000km (1900 miles) in diameter and averages 5970 million km (3700 million miles) from the Sun; one moon; rotates once in 6.4 days; orbits Sun once in 248 years.

Deimos, the second moon of Mars, is an irregularly shaped lump of rock no more than 15km (9 miles) across. Because of its size, its gravitational pull is so feeble that an astronaut could launch himself into space from it simply by jogging at about 11km/h (7mph) and jumping into the air.

Saturn's rings are extraordinarily thin in relation to their 270.000km (170,000 miles) diameter. Voyager observations show that the rings are no more than 100m (300ft) thick. On the same ratio of thickness to diameter, a gramophone record would be 5km (3 miles) across.

Unlike the other planets in the solar system, Venus spins not from west to east. but from east to west. It also takes longer to spin on its axis than it does to orbit the Sun, making a Venusian 'day' longer than a Venusian 'year' . The reason for the reverse rotation is unknown.

Until 1980, many scientists believed that the infant planet was set offon its backward spin when it collided with another embryonic planet or moon during the formation of the solar system about 4600 million years ago. But a mathematical analysis of Venus's movement has shown that theory to be incorrect and, so far. no satisfactory new theory has been devised.