The sun and the other heavenly bodies that move around it are called the solar
There is a
great distance between the orbits of Mars and Jupiter. You might expect to find
another planet between these two planets. However, instead of one planet,
astronomers have found more than 1500 small bodies that vary in diameter from 1
mile to over 400 miles. They are called planetoids, which means little
planets. They are also called asteroids. Each one revolves around the sun
in its own orbit. Some astronomers think that planetoids may be pieces of a big
planet which once revolved around the sun between Mars and Jupiter.
planetoids, the solar system contains many comets. Some comets can be
seen only through telescopes. From the earth, a comet looks like a hazy, round
spot of light. The main part, or head, of a comet may be from 10,000 to 150,000
miles in diameter. A comet usually has a tail. The longest tail ever observed on
a comet was 200 million miles long. Astronomers believe that comets are made of
gases and little bits of solids. A comet travels in a long, narrow orbit around
the sun. One end of the orbit may be very close to the sun, while the other end
may reach far beyond Pluto. As a comet travels in its orbit, its tail is always
pointed away from the sun. This is because light and other radiations that are
traveling away from the sun exert a force on the molecules of gas in the comet
and cause the molecules to move in the same direction.
have such long orbits that they take hundreds of years to travel around the sun.
Others, with shorter orbits, appear every few years. One of the most famous,
Halley's comet, appears every 75 years. A comet can be seen only when it comes
near both the earth and the sun. Comets shine partly because they reflect
sunlight and partly because the sun makes their gases glow.
are seen much more often than comets. We usually call them shooting stars or
falling stars, but they are not stars at all. Meteors are really small bodies of
metal or stone, moving at great speed. They are usually no bigger than the head
of a pin, although occasionally they may weigh many tons. A meteor cannot be
seen until it falls into the earth's atmosphere. Then friction between it and
the air makes it hot enough to give off light. Several billion meteors probably
strike the earth's atmosphere every 24 hours. Most of them are burned up, and
only the dust that is left falls to earth. Scientists estimate that this dust
adds thousands of tons to the mass of the earth every day.
Some of the
larger meteors fall to the earth before they are completely burned up. Then they
are called meteorites. Some meteorites are stones, but others are largely
metals such as iron and nickel. Very large meteorites may go deep into the
earth. Meteor Crater in Arizona is 650 feet deep and over 4000 feet wide.
Scientists are quite sure that it was formed a few thousand years ago by a huge
meteorite that hit the earth and then exploded.
How Was the Solar System Formed?
Many hypotheses have been suggested. Many scientists accept this hypothesis:
years ago there was a huge cloud of gas whirling in space. It whirled like an
enormous wheel. The center of the whirling cloud slowly became smaller and
hotter. This became the sun. Around the sun spun huge rings of gases. Slowly the
gas in each ring came together into a very hot ball of gas. Gradually these
gases cooled. Many of them became liquids. In time, many of the liquids became
solids. The balls of gas became the planets and moons.
Sun - Discusses
what the sun is like.
Planets - Discusses
what the planets are like, how planets move, Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and Pluto.
Moon - Discusses what the
moon is like and why the moon seems to change its shape.
Tides - Discusses how the
moon and sun cause tides.