 |
 |
such
as asteroids and comets.
Most of the asteroids
orbit in a region between
the orbits of Mars and
Jupiter, while the home
of comets lies far beyond
the orbit of Pluto, in
the Oort Cloud. The
four planets closest to
the Sun--Mercury, Venus,
Earth, and Mars--are
called the terrestrial
planets because they have
solid rocky surfaces. The
four large planets beyond
the orbit of Mars--
Jupiter, Saturn, Uranus,
and Neptune--are called
gas giants. Tiny,
distant, Pluto has a
solid but icier surface
than the terrestrial
planets.
Nearly
every planet--and some of
the moons--has an
atmosphere. Earth's
atmosphere is primarily
nitrogen and oxygen.
Venus has a thick
atmosphere of carbon
dioxide, with traces of
|
|