Our Solar System is part of a much larger system called the Milky Way. This is a

vast mix of dust, gas, stars, and other objects that is called a galaxy. Our galaxy

rotates about the center, and if you could see it from a long, long way off it would

look like a wispy pin-wheel.

Within our Milky Way galaxy are clouds of dust and gas where stars are born.

Our Solar System was created in just such a cloud. A part of this cloud began to

collapse under the pull of its own gravity. As it got smaller, it formed a big,

spinning disk of gas and tiny particles of dust. This disk was thickest at the

middle, and this part slowly collapsed to form the early Sun.

The remainder of the disk continued to rotate about the Sun in the center. The

tiny particles of dust were now close enough so that they would often collide, and

some of these would stick together. The bits of dust would slowly collect to form

grains. These would in turn join to form lumps the size of gravel, then pebbles.

As the objects grew in size, a few grew larger than the rest and began to pull on

the other rocks with their gravity. The rocks grew ever bigger, reaching the size

of mountains which crashed together with powerful force. In the end there were

only a few massive bodies left, which swept up the rest of the disk to form the

planets, moons, and asteroids.

While this was going on, the Sun began to glow from the huge energy released

by its own collapse. The temperature at the center of the Sun reached a million

degrees, and it turned into a furnace that could create more energy by smashing

tiny atoms together. This energy rose up to the surface and was emitted as light,

heat, and other forms.

This energy being emitted by the Sun swept away any surviving tiny particles

and gas from the inner Solar System, leaving behind only the larger objects.

About four thousand million years from now . . .

This is what you will find in the first few pages of

The Solar System

Introduction............................................................................................................ 1

Solar System......................................................................................................... 2

The Sun................................................................................................................. 5

Mercury................................................................................................................ 11

Venus.................................................................................................................. 15

Earth.................................................................................................................... 22

Moon................................................................................................................... 28

Mars.................................................................................................................... 35

Mars/Phobos....................................................................................................... 40

Mars/Deimos....................................................................................................... 43

Asteroid belt........................................................................................................ 46

Jupiter.................................................................................................................. 49

Jupiter/Amalthea................................................................................................. 54

Jupiter/Io.............................................................................................................. 55

Jupiter/Europa..................................................................................................... 56

Saturn.................................................................................................................. 59

Saturn/Mimas...................................................................................................... 63

Saturn/Enceladus................................................................................................ 65

Saturn/Titan......................................................................................................... 66

Uranus................................................................................................................. 67

Neptune............................................................................................................... 70

Pluto.................................................................................................................... 73

Pluto/Charon....................................................................................................... 76

Comets................................................................................................................ 77

Kuiper Belt........................................................................................................... 81

Oort Cloud........................................................................................................... 83

About gravity, mass, and weight.......................................................................... 85

How the Solar System was born......................................................................... 89

Glossary.............................................................................................................. 91

GNU Free Documentation License..................................................................... 93

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