2. Chapter 15: The Solar System The Solar System The Planets

3. The solar system Ancient observers noticed that five bright objects seemed to wander among the stars at night. They called these objects planets, from the Greek word meaning “wandering star,” and named them Mercury, Venus, Mars, Jupiter, and Saturn.

4. The solar system Today we know that planets are not stars. Stars give off their own light.

5. The Sun is the largest object in our solar system.

6. Gravitational force Newton’s law of universal gravitation explains how the strength of gravity depends on the mass of the objects and the distance between them.

7. Gravitational force Gravitational force is the force of attraction between all objects. All objects that have mass attract each other.

8. Orbits An orbit is a regular, repeating path that an object in space follows around another object. An object in orbit is called a satellite.

9. Orbits In 1600, German mathematician Johannes Kepler determined that the orbits of the planets were not perfect circles but slightly elliptical.

10. Orbits Isaac Newton explained that an orbit results from the balance between inertia (the forward motion of an object in space), and gravitational force. Without the pull of gravity, a planet would travel off into space in a straight line.

11. The solar system The solar system includes the Sun, eight major planets, and their moons. A large number of smaller objects are also part of the solar system, including dwarf planets, asteroids, comets, and meteors.

12. 7.2 Overview of the planets The planets are commonly classified in two groups. The terrestrial planets include Mercury, Venus, Earth, and Mars. The gas planets include Jupiter, Saturn, Uranus, and Neptune. Pluto is not a terrestrial or gas planet, but now placed in a class of its own.

13. Comparing size and distance The Sun is by far the largest object in the solar system. One astronomical unit (AU) is equal to 150 million km, or the distance from Earth to the Sun.

15. Asteroids and comets An asteroid is an object that orbits the sun but is too small to be considered a planet. The largest asteroid, named Ceres, is 933 kilometers (580 miles) across.

16. Asteroids and comets We believe comets are made mostly of ice and dust. A comet’s tail can stretch for millions of kilometers into space and faces away from the sun as the comet continues its orbit.

17. Meteors and meteorites Occasionally, chunks of rock or dust break off from a comet or asteroid and form a meteor. As Earth orbits the sun, it passes through this debris, creating a meteor shower as the small bits of dust burn up in the atmosphere.

18. Meteors and meteorites If a meteor is large enough to survive the passage through Earth’s atmosphere and strike the ground, it becomes a meteorite.

19. Earth Earth is a small, rocky planet with an atmosphere that is made of mostly nitrogen (78 percent N 2 ) and oxygen (21 percent O 2 ).