54 The Solar System-Mid

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. Understanding our solar system and how it formed might give us clues about other solar systems. Let's take an imaginary flight through the solar system. We'll start at the center: the Sun. As we move away from the Sun, we will pass many planets and other objects. One of those planets is our own Earth. All the objects in the solar system are circling the Sun. In this book, you will read about all parts of the solar system. Climb aboard! Let's begin our journey.

The Sun The Sun is one of many many, stars in the universe. Stars are huge, fiery balls of gas. The Sun is made mostly of a gas called hydrogen. The Sun is medium-sized compared to other stars. It looks bigger because it is so much closer. The Sun is about 150 million kilometers (93 million mi.) from Earth. Other stars are trillions of kilometers away. Atoms of hydrogen in the center of the Sun are forced together by heat and pressure. They join to form a gas called helium. This process gives off huge amounts of energy and causes the Sun to be hot and bright. Temperatures in the Sun vary. The hottest part of the Sun is its center. The surface of the Sun is much cooler.

The Sun has nearly all of the matter in the solar system. The Sun's huge amount of matter gives it very strong gravity. Gravity is a force of attraction between all objects in the universe. Gravity keeps the planets orbiting the Sun. It also keeps moons orbiting planets. 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 supply of hydrogen will almost run out The Sun will then begin to cool down But it will take 5 billion years for the Sun to use up that much hydrogen

The Planets Our trip through the solar system will take us by eight planets. The number used to be nine. But one planet, Pluto, is no longer called a planet. Scientists place the planets into two groups. Mercury, Venus, Earth, and Mars are called the terrestrial (tuh-RES-tree-ul) planets. They have solid, rocky surfaces. Terrestrial means "like Earth." Saturn, Jupiter, Uranus, and Neptune 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 Terrestrial Planets The first planet we see as we leave the Sun is Mercury. It is small and covered with craters. Mercury looks a lot like Earth's Moon. And like the Moon, it has no air. If you were standing on Mercury, the Sun would look 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 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