293Chapter 27The Solar System and
the Planet Earth
The Solar System consists of the Sun and the various bodies, which
rotate about it. These include the eight planets (Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus and Neptune); the five dwarf planets
(Ceres, Pluto, Haumea, Makemake and Eris); the many moons of the
planets and dwarf planets; the Asteroid Belt between Mars and Jupiter
which also contains Ceres; the Kuiper Belt, which is similar to the
Asteroid Belt but lies just outside the orbit of Neptune; comets which can
be found in the inner Solar System; the Kuiper Belt; two other trans
Neptunian structures, the Scattered Disk and the Oort Cloud to be
described later and finally innumerable meteoroids. The distances
between these objects are so great that the Solar System consists of a
great deal of space, which contains gas, dust and plasma (or ionized
gases and electrons). The Earth is 150 million km from the Sun or one
astronomical unit (1 AU), which is the unit used to measure distances in
the Solar System. The planet farthest from the Sun is Neptune, which is
30.1 AU away. Neptune does not mark the end of the Solar System. The
torus-shaped Kuiper Belt extends from 30 AU out to 50 AU and contains
Pluto, Haumea and Makemake. The Scattered Disk contains objects
including the dwarf planet Eris with highly elliptical orbits that come
within 30 AU of the Sun and go as far out as 100 AU. The Oort Cloud
extends out to 50,000 AU or one light year and is made up of icy objects
and comets.
The planets, dwarf planets and their satellites orbit the Sun in
remarkably stable orbits. These nearly circular orbits display a great deal
of order and uniformity. The orbits of the planets and their satellites lie in
the same plane, which is also the plane in which the Sun rotates about its
own axis. The Sun’s axis of rotation is perpendicular to the plane of the