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OVERVIEW

For generations, people have observed the sky in awe, wondering about the bright planets moving across the background
of stars, but they could do no more than wonder. You are among the first generations on Earth to see close-up
photographs of the planets, comets, and asteroids, and to see Earth as it appears from space. Spacecrafts have now
made thousands of photographs of the other planets and their moons, measured properties of the planets, and, in some
cases, studied their surfaces with landers. Astronauts have left Earth and visited the Moon, bringing back rock samples,
data, and photographs of Earth as seen from the Moon (Figure 15.1). All of these photographs and findings have given
us a new perspective of Earth, the planets, and the moons, comets, and asteroids that make up the solar system.
Viewed from the Moon, Earth is a spectacular blue globe with expanses of land and water covered by huge, changing
patterns of white clouds. Viewed from a spacecraft, other planets present a very different picture, each unique in its
own way. Mercury has a surface that is covered with craters, looking very much like the surface of Earth’s Moon. Venus
is covered with clouds of sulfuric acid over an atmosphere of mostly carbon dioxide, which is under great pressures with
surface temperatures hot enough to melt lead. The surface of Mars has great systems of canyons, inactive volcanoes, dry
riverbeds and tributaries, and ice beneath the surface. The giant planets Jupiter and Saturn have orange, red, and white
bands of organic and sulfur compounds and storms with gigantic lightning discharges much larger than anything ever seen
on Earth. One moon of Jupiter has active volcanoes spewing out liquid sulfur and gaseous sulfur dioxide. The outer giant
planets Uranus and Neptune have moons and particles in rings that appear to be covered with powdery, black carbon.
These and many more findings, some surprises and others expected, have stimulated the imagination as well as
added to our comprehension of the frontiers of space. The new information about the Sun’s impressive system of planets,
moons, comets, and asteroids has also added to speculations and theories about the planets and how they evolved over
time in space. This information, along with the theories and speculations, will be presented in this chapter to give you a
picture of the solar system.

15.1 PLANETS, MOONS, planet. Today, there are eight planets, five dwarf planets (Pluto,
Eris, Ceres, Makemake, and Haumea), and many, many small
AND OTHER BODIES
solar system bodies. These definitions may change again in the
The International Astronomical Union (IAU) is the governing future as more is learned about our solar system.
authority over names of celestial bodies. At the August 24, 2006, In this chapter, we will visit each of the planets and other
meeting of the IAU, the definitions of planets, dwarf planets, bodies of the solar system (Figure 15.2).
and small solar system bodies were clarified and approved. To The Sun has 700 times the mass of all the planets, moons,
be a classical planet, an object must be orbiting the Sun, nearly and minor members of the solar system together. It is the force
spherical, and large enough to clear all matter from its orbital of gravitational attraction between the comparatively massive
zone. A dwarf planet is defined as an object that is orbiting Sun and the rest of our solar system that holds it all together. The
the Sun, is nearly spherical, but has not cleared matter from its distance from Earth to the Sun is known as one astronomical
8
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orbital zone and is not a satellite. All other objects orbiting the unit (AU). One AU is about 1.5 × 10 km (about 9.3 × 10 mi).
Sun are referred to collectively as small solar system bodies. The astronomical unit is used to describe distances in the solar
Some astronomers had dismissed Pluto as a true planet system; for example, Earth is 1 AU from the Sun.
for years because it has properties that do not fit with the other Table 15.1 compares the basic properties of the eight plan-
planets. The old definition of a planet was anything spherical ets. From this table, you can see that the planets can be classified
that orbits the Sun, which resulted in nine planets. But then in into two major groups based on size, density, and nature of the
2003, a new astronomical body was discovered. This body was atmosphere. The interior planets Mercury, Venus, and Mars have
named Eris after the Greek goddess of discord. Eris is larger densities and compositions similar to those of Earth, so these
than Pluto and round and circles the Sun. Is it a planet? If so, planets, along with Earth, are known as the terrestrial planets.
the asteroid Ceres would also be a planet, as would the 50 or Outside the orbit of Mars are four giant planets, which are
so large, icy bodies believed to be orbiting the Sun far beyond similar in density and chemical composition. The terrestrial
Pluto. The idea of 50 or 60 planets in the solar system spurred planets are mostly composed of rocky materials and metallic
astronomers to clarify the definition of a planet. Because Pluto nickel and iron. The giant planets Jupiter, Saturn, Uranus, and
does not clear its orbital zone, it was downgraded to a dwarf Neptune, on the other hand, are massive giants mostly composed

378 CHAPTER 15 The Solar System 15-2