55 The Solar System-High

The Solar System Written by David Dreier

Introduction Earth is our home, and the solar system is our local neighborhood in the universe. Our solar system is just one of many, near the outer edge of just one of billions of galaxies— huge rotating masses of stars— in the universe. But it's special to us because it's where we live. Understanding our own solar system and how it formed might even give us clues about solar systems elsewhere in the universe. Let's take an imaginary flight from the center of our solar system to its outer edges. We start at the Sun, the center of the solar system. As we move away from the Sun and travel to the very edges of the solar system, we will pass many objects of varying sizes and shapes. All of them, just like Earth, are circling the Sun. Climb aboard our fantasy spaceship! Let's begin our journey.

The Sun The Sun is one of countless stars in the universe. Stars are huge, fiery balls of gas. The Sun is medium-sized compared to other stars— it only looks larger because it is much, much closer to us. The Sun is about 150 million kilometers (93 million mi.) from Earth, while other stars are trillions of kilometers away. The Sun— like other bright stars— is made mostly of hydrogen gas. Hydrogen is the lightest element in the universe. Atoms of hydrogen in the Sun's center are forced together by extreme heat and pressure. In this joining process, called fusion, four hydrogen atoms produce one atom of helium. Fusion releases huge amounts of energy that cause the Sun to be extremely hot and bright. Temperatures in the Sun vary greatly. The hottest part of the Sun is its core. The surface of the Sun is much cooler. As hydrogen atoms form helium, the supply of hydrogen decreases.

The Sun is by far the most massive object in our solar system. It contains more than 99 percent of all the mass— quantity of matter— in the solar system. The Sun's tremendous mass gives it extremely strong gravity. Gravity is a force of attraction between all objects in the universe. It is gravity that keeps planets orbiting the Sun, and 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 in 2006, the most distant planet, Pluto, lost its standing as a planet. Scientists place the planets into two groups. One group is the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. These planets are called the terrestrial (tuh-RES-tree-ul) planets because they have solid, rocky surfaces. The word terrestrial means "like Earth." The outer four planets— Saturn, Jupiter, Uranus, and Neptune— are called gas giants. These planets are big balls of gas and do not have a solid surface. 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 encounter as we leave the Sun is Mercury, a small, barren planet that is covered with craters. Craters are scars on the surface of a planet that form when meteorites or other objects strike it. Mercury looks a lot like Earth's Moon. And like the Moon it has no atmosphere. If you were standing on the surface of 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 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