53 The Solar System-Low

THE SOLAR SYSTEM Written by David Dreier

Introduction The solar system is our home in space. It's special to us because it's where we live. Let's take an imaginary flight through the solar system. We'll start at the center, with the Sun. As we move away from the Sun, we will pass many planets. One of those planets is our own Earth. Earth and the other planets all circle the Sun. Climb aboard! Let's begin our journey.

The Sun The Sun is one of many stars in space. Stars are huge, fiery balls of gas. The Sun is made mostly of a gas called hydrogen. The Sun looks big and bright because it is fairly close to us. Other stars are much much farther away. That's why they're just little specks of light. Atoms of hydrogen in the center of the Sun are forced together. They turn into a gas called helium. This process gives off huge amounts of energy. It causes the Sun to be hot and bright.

The Sun's huge size gives it very strong gravity. Gravity is a force that pulls objects together. Gravity keeps the planets orbiting the Sun. Fast Facts About the Sun Diameter: 1.39 million kilometers (864,000 mi.) Temperature: Core about 15 million ºC (27 million ºF) Surface about 5,500ºC (10,000ºF) Corona more than 1 million ºC 1.8 million ºF Mass compared with Earth: 332,000 times as massive Wowser! Over time, the Sun's hydrogen will almost run out. The Sun will begin to cool down. But it will take 5 billion years for that to happen!

The Planets Our trip through the solar system will take us by eight planets. Scientists place the planets into two groups. One group has Mercury, Venus, Earth, and Mars. They all have a solid surface. The outer four planets are Saturn, Jupiter, Uranus, and Neptune. These planets are called gas giants. They do not have a solid surface. You wouldn't be able to breathe the air on these planets. Do You Know? Here's a memory aid to remember the order of the planets from the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune): My Very Educated Mother Just Showed Us Neptune.

The Inner Planets The first planet we see after leaving the Sun is Mercury. It is a small planet covered with craters, like our Moon. And like the Moon, it has no air. On Mercury, the Sun looks three times larger than it does on Earth. When the Sun is overhead on Mercury, the temperature rises to about 430°C (800°F). After sunset, it drops down to about -170°C (-280°F). Fast Facts About Mercury Diameter 4,879 kilometers (3,032 mi.) Average distance from the Sun: 58 million kilometers (36 million mi.) Period of rotation (one turn on axis): 59 Earth days Period of revolution one orbit around Sun 88 Earth days Number of moons: 0