396
The human race lives on
a planet in a planetary
system that appears to
have formed in a nebula
around the protostar that
became the sun. This
artist’s impression shows
such a nebular disk
around a forming star.
(NASA/JPL-Caltech)Guidepost
You have studied the appearance, origin, structure, and evolution of stars, galaxies,
and the universe itself. So far, though, your studies have left out one important type of
object—planets. Now it is time to correct that omission. In this chapter, once you learn
the evidence for how the solar system formed, you can understand how the processes you
have been studying produced Earth, your home planet.
As you explore our solar system in space and time, you will fi nd answers to four essen-
tial questions:
What is the theory for the origin of the solar system?
What are the observed properties of the solar system that the theory of its origin
can explain?
How do planets form?
What do astronomers know about other planetary systems?
In the next three chapters, you will explore in more detail each of the planets, plus
asteroids, comets, and meteoroids. By studying the origin of the solar system before
studying the individual objects in it, you give yourself a better framework for under-
standing these fascinating worlds.