Ancient astronomers noticed points of light
that seemed to move among the stars. They
called these points of light planets, meaning
wanderers, and they named them for the
Jupiter was named for the king of the gods;
Mars, the god of war; Mercury, the
messenger of the gods; Venus, the goddess
of love and beauty and Saturn, the father of
Jupiter and god of agriculture.
The solar system consists of our sun, and
the eight planets that encircle it.
When I was in school, we were taught that
there were nine, because Pluto was
included. But recently, Pluto was dropped
from the list when similar worlds were
discovered deeper in the Kuiper Belt, a
disc-like zone beyond the orbit of Neptune
populated with thousands of miniature icy
worlds, which formed early in the history of
our solar system.
Icy Pluto was reclassified as a dwarf planet,
a new class of worlds which may offer some
of the best evidence about the origins of our
There are four other known dwarf planets,
tens of thousands of asteroids, as well as
comets and icy bodies.
The sun makes up 99.8% of the mass of the
solar system. If the sun were as tall as the
typical front door, earth would be about the
size of a nickel. It is roughly 109 times the
diameter of the Earth. About a million Earths
could fit in it.
The terrestrial planets—Mercury, Venus,
Earth and Mars—are the four planets closest
to the sun. They have compact, rocky
The Jovian planets, or gas giants, are the
four largest planets—Jupiter, Saturn, Uranus
and Neptune. They are huge compared with
the earth and have a gaseous nature like
Until 1977, Saturn was considered the only
planet with rings. We now know that Jupiter,
Uranus and Neptune also have ring
systems, though Saturn's is by far the
Particles in these ring systems range from
dust- to boulder- to house-size. They may be
either rocky or icy.
Almost all of the planets, and some of their
moons, have an atmosphere. Venus, Earth
and Mars have significant atmospheres,
while Mercury has almost none.
Some of the larger moons—Ganymede (a
moon of Jupiter) and Titan (a moon of Saturn)
—are larger than Mercury.
Most of the solar system seems to be an
empty void. But even it includes various
forms of energy as well as interplanetary
dust, which is microscopic solid particles,
and interplanetary gas, which is mostly gas,
protons and electrons.
Interplanetary gas is also called the solar
wind. It has an effect on comet tails and on
the motion of spacecraft.