2. Astronomy

3. Chapter Eighteen: Earth Moon and Sun 17.1 About the Solar System 17.2 The Planets 17.3 Other Solar System Objects

4. 17.3 Other solar system objects Pluto is a dwarf planet. Most of the time Pluto is the farthest from the sun. Discovered in 1930, Pluto was named for the Roman god of the underworld.

6. 17.3 Other solar system objects Outside the orbit of Pluto is a region called the Kuiper Belt. The Kuiper Belt stretches to 1,000 AU and is believed to contain many asteroid-size and a few Pluto-size objects.

7. 17.3 Pluto and the Kuiper Belt To avoid confusion, astronomers no longer count Pluto as a planet. Instead, Pluto is grouped along with Sedna, Xena, and similar distant bodies in the Kuiper Belt Objects (or KBOs). Triton is Neptune’s largest moon. Some astronomers believe that Pluto may be an “escaped” moon of Neptune.

8. 17.3 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.

9. 17.3 Asteroids and comets We believe comets are made mostly of ice and dust. Comets revolve around the Sun in highly elliptical orbits.

10. 17.3 Asteroids and comets The inner core of the comet is the nucleus. As a comet gets closer to the Sun, it forms a tail.

11. 17.3 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.

12. 17.3 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.

13. Astronomy Connection Good-bye Pluto On August 24, 2006, Pluto was dropped as the ninth planet in the solar system. Astronomers had to vote on the definition of the term planet.

14. Activity The plants and animals that live on Earth are uniquely suited to Earth’s environment. In this activity, you will create an organism that could live on another planet. Extraterrestrial Design