What are Planets Like?

            Perhaps, early in the morning or evening, you have seen what looked like a very bright star low in the sky. People call these bright objects the morning star and the evening star. But they are really planets. Telescopes show that the planets are spherical bodies that go around the sun. The earth is one of the medium-sized planets. Compared to the hot materials in the sun, the materials in the planets are very cool. The planets cannot give out their own light as the sun and other stars do. They shine at night because they reflect light from the sun. Some planets appear brighter than the stars. This is because the planets are much closer to us than the stars and because some planets reflect much light. But you can tell a planet from a star because planets shine steadily while stars twinkle.
            One way planets are different from stars is that they are made up of solids such as rocks and dust. Another way planets are different from stars is that they are many times smaller than most stars. Planets are also different from stars in that planets move around the sun.
            Before the telescope was invented, only five planets had been observed in the sky. These were Mercury, Venus, Mars, Jupiter, and Saturn. Since then, astronomers have discovered three other planets: Uranus, Neptune, and Pluto. So including the earth, there are nine planets that we know about. Mercury and Venus are nearer the sun than the earth is, while the other six planets are farther away. Each planet moves around the sun in a curved path, or orbit.
            All the planets except Mercury, Venus, and probably Pluto, have one or more moons, or satellites. Each satellite moves in its own orbit around a planet, much as the planet moves in its orbit around the sun. In recent years, many artificial satellites have been sent into orbit around the earth. Some have even been sent into orbit around the sun, like planets.
            Of course, much more is known about the earth than about any other planet. Until recently, the only way we could get information about the other planets was by studying the light and other radiant energy that came from them to the earth. By using what has been learned about the earth, astronomers can get some idea of what the other planets may be like.
            Scientists are almost sure that most of the planets cannot have living things like those on the earth. To stay alive, living things on the earth need air, water, and food. Food comes directly or indirectly from green plants, and green plants must have carbon dioxide and water. Living things also need oxygen. Furthermore, the temperature must not be too hot or too cold. Most of the planets do not seem to have the right conditions to support living things. Yet it is possible that one or two of them may have living things of a very simple kind.

How do Planets Move?

            Sir Isaac Newton explained why the earth and the other planets go around the sun in orbits, or revolve. He knew that objects on the earth fell to the ground when they were not held up by something. He thought that this same force might hold the moon and planets in their orbits. After much study and many calculations, he was able to show that this is true. He taught us that every body in the solar system, and everywhere else for that matter, pulls on every other body. This force is called gravitation.
            If gravitation were the only force acting on bodies, everything would be pulled together into one large lump, if we waited long enough. All of the planets and the sun and moon and stars would pull each other together in one place. So Newton knew that there must be something that prevented this. He discovered that any moving object will keep on moving at a constant speed in a straight line unless something happens to make it speed up, slow down, or change its direction. This property of matter is called inertia. This may seem strange at first, because we are used to seeing things that are in motion slow down and stop, or fall to the ground if they are thrown through the air. But this is because something is making them slow down and stop, or fall to the ground. An object that is pushed along the ground stops because friction makes it stop. And the gravitational pull of the earth makes an object that is thrown through the air fall to the ground.
            Out in space there is practically no friction. The planets are all moving very rapidly through space, and their motion is not slowed to any great extent by friction. But there is one force that acts on the planets and makes them keep changing their direction. This force is the gravitational pull of the sun. As a planet rushes along, this pull makes it swerve toward the sun. The planet does not move directly toward the sun because its inertia tends to keep it moving in a straight line. Instead, the inertia of the planet and the pull of the sun combine to make the planet keep going around and around the sun in an orbit. The orbits of the planets are not perfect circles. Instead, the orbit of each planet is an ellipse. An ellipse is like a circle that has been flattened somewhat so that it is longer in one direction than the other. The sun is not at the center of a planet's orbit, but closer to one end that the other. The more a planet's orbit is flattened, the farther the sun is from the center.
            At the same time the planets revolve around the sun in their orbits, they spin around, or rotate. The axis is an imaginary line through the center of a planet around which it rotates. The axis of the earth passes from the North Pole to the South Pole through the center of the earth. As the earth rotates, it carries places on its surface around from west to east. That is why the sun, moon, stars, and other bodies appear to move across the sky from east to west each day, but they also seem to move about among the stars. This is because they are moving around the sun in their orbits. As they move along in their orbits from day to day, we see them in front of different stars.