Planets

Any of the nine largest objects that move around the sun
are considered to be planets. Using the sun as the center
and moving outward, the order of the planets are Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and
Pluto. The sun, the planets and their satellites (moons),
and smaller objects called asteroids, meteors, and comets
make up the solar system. 

The planets are dark, solid bodies, much smaller than the
sun and stars. All light and heat comes to them from the
sun. They can be seen only because they reflect the light
of the sun. In the night sky, planets and stars look alike
but there are two ways to tell them apart. First, the
planets shine with a steady light, but the stars seem to
twinkle. Second, the planets change their positions in
relation to the stars. 

All the planets move around the sun in the same direction.
In addition, each planet rotates as it revolves. The
planets' rotation periods (the time required to spin around
once) range from less than 10 hours for Jupiter to 243 days
for Venus. The earth rotates once every 24 hours, or one
day. 

The conditions on the planets vary widely. They depend on
three things: (1) the planet's distance from the sun, (2)
the planet's atmosphere, and (3) the planet's rotation.
 
The planets nearest the sun receive more heat than those
far away from it. The temperature on the closet planet,
Mercury, rises to about 625 degrees F. during the day. The
temperature on a planet is estimated from measurements of
infrared radiation and radio waves that the planet sends
out. These measurements are difficult to make for objects
with low temperatures. 

Astronomers determine the kinds of gases in a planet's
atmosphere by studying the light, radio waves, and other
radiation coming from the planet. The atmospheres of the
terrestrial planets consist mainly of carbon dioxide and
nitrogen. The atmospheres of the major planets consist
mostly of helium, hydrogen, methane, and ammonia. The earth
is the only planet with a large amount of oxygen in its
atmosphere.
 
The movement of the planets remained a puzzle until the
1600's. There were two theories which brought about a
dispute. One theory was suggested by Ptolemy, a Greek
astronomer (A.D. 150) who thought the sun and the planets
traveled around the earth once a day. His theory explained
what could be seen in the sky and guided people's thinking
about the universe for over a thousand years. The other
theory was suggested by Nicolaus Copernicus in 1543 who
said that the earth and the other planets traveled around
the sun. This theory made it easier to describe the motions
of the planets and astronomers soon began to use it.
Discoveries by other astronomers gradually convinced people
that the Copernican theory was correct. 

During the space age, much progress has been made, however,
there are still many questions about the planets that
remain unanswered. One of the biggest questions is whether
any form of life exists on any of the planets, in
particular on Mars.