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Grade 9 Academic Science – Unit 3 Space Planets – Interesting Stuff Section 8.3 Pages 313-317 A planet is a large space object that revolves (i.e., to move in an orbit or circle around something) around a star. It also reflects (i.e., to throw back light, heat or sound) that star's light. Eight planets have been discovered in our solar system. Mercury, Venus, Earth, and Mars are the planets closest to the Sun. They are called the inner planets. The inner planets are made up mostly of rock. The outer planets are Jupiter, Saturn, Uranus and Neptune. Jupiter, Saturn, Uranus, and Neptune are large balls of gases with rings around them. No longer a planet, Pluto is the farthest planet from the Sun. It is coated with ice. All eight planets travel around the Sun in a different orbit (i.e., The path followed by an object in space as it goes around another object; to travel around another object in a single path). Mercury Mercury is the second smallest planet in our solar system. Only the planet Pluto is smaller. Mercury is about the same size as the Earth’s Moon. Mercury is very close to the Sun. Mercury travels around the Sun faster than any other planet. (Note: That is how Mercury got its name. It was named after Mercury, the swift messenger of the gods in ancient Roman mythology). Mercury can be seen from Earth just before sunrise or just after sunset. Why? Mercury always appears near the Sun when viewed from Earth. Mercury has a very thin atmosphere. Humans would not be able to live there. The surface of Mercury has holes in it where objects such as meteorites (i.e., a piece of stone or metal from space that falls to a planet’s surface) and asteroids (i.e., a rocky space object that can be a few feet wide to several hundred miles wide. Most asteroids in our solar system orbit in a belt between Mars and Jupiter) crashed into it. Pluto (Dwarf Planet) Pluto is usually the farthest large celestial body in our Solar System from the Sun. It has a very unusual orbit. Once every 248 Earth years, Pluto swings inside the orbit of Neptune. It stays there for twenty years. During those twenty years, Pluto is closer to the Sun than Neptune. While it is closer to the Sun, Pluto has an atmosphere. The methane and nitrogen frozen at the poles thaw out, rise and temporarily form an atmosphere. As it moves toward its farthest point from the Sun, Pluto's atmosphere freezes and falls back on the surface of the planet. Pluto has one known moon. Pluto's moon, Charon, is half the size of Pluto. Pluto is a dwarf planet. Some astronomers believe that Pluto may be a moon that escaped Neptune's gravitational pull (i.e., attraction that one object has for another object due to the invisible force of gravity).