What are Planets Like?
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
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.
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.
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.
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.
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?
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.
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.