Copyright © 2007 Dorling Kindersley
Our tiny corner of the Universe is dominated by a star we call the Sun. Trapped in the gravity of the Sun is a huge family of bodies – PLANETS, MOONS, asteroids, comets, and other smaller bodies – which hurtle with it through space. This family is our Solar System. But the effects of the Sun – its heat, gravity, light, and particles – extend far beyond Pluto, to about a quarter of the way to the next nearest star, Proxima Centauri.
The Solar System is around 5 billion years old. It formed out of a huge cloud of gas and dust called the solar nebula. Under gravity, the cloud collapsed and the material formed the Sun and a disc of matter in which the planets were born.
Nine planets orbit the Sun at different distances. The four inner planets are balls of rock and metal. The outer planets are giant balls of gas and liquid, except for Pluto, the most distant planet, which is made of ice and rock. The time it takes a planet to orbit the Sun is its orbital period (its year). Planets also rotate (spin round) as they travel. The time it takes a planet to rotate once is its rotation period (its day).
The planets vary widely in size. Earth is one of the smallest, just 12,756 km (7,926 miles) in diameter. More than 1,300 Earths could fit inside the largest planet, Jupiter. However, the Sun makes up 99.9 per cent of the mass of the Solar System. The planets are not upright in relation to their orbits around the Sun. The axis (the line around which it turns) of each planet is tilted at a different angle.
A moon is a body that orbits a planet. Altogether we know of more than 120 moons in the Solar System. Earth has one, the Moon, while Jupiter has 63. Most of these moons are small asteroids captured by Jupiter’s gravity, but its largest moon Ganymede, with a diameter of 5,268 km (3,266 miles), is bigger than Mercury.
No one is certain how the Moon formed, but many astronomers believe that it was born when a body the size of Mars collided with the young Earth over 4 billion years ago. In the collision, material from the two bodies was heated up, became molten, and was thrown out into space. In time, the material clumped together to form the Moon.