CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 397
atoms unchanged since the universe began. Evidence indicates
that almost no atoms heavier than helium were made in the big
bang.
Within a few hundred million years after the big bang, mat-
ter began to collect to form galaxies containing billions of stars.
You have learned that nuclear reactions inside stars are where
low-mass atoms such as hydrogen are combined to make heavier
atoms (see Chapters 12 and 13). Generation after generation of
stars cooked the original particles, fusing them into atoms such
as carbon, nitrogen, and oxygen that are common in your body.
Even the calcium atoms in your bones were assembled inside
stars.
Massive stars produce iron in their cores, but much of that
iron is destroyed when the core collapses and the star explodes as
a supernova. Most of the iron on Earth and in your body was
produced instead by carbon fusion in type Ia supernova explo-
sions and by the decay of radioactive atoms in the expanding
matter ejected by type II supernovae. Atoms heavier than iron
such as gold, silver, and iodine are created by rapid nuclear reac-
tions that can occur only during supernova explosions. Iodine is
critical to the function of your thyroid gland, and you probably
have gold and silver jewelry or dental fi llings. Realize that these
types of atoms, which are part of your life on Earth, were made
during the violent deaths of massive stars long ago.
Our galaxy contains at least 100 billion stars, of which the
sun is one. Astronomers have a variety of evidence that the sun
formed from a cloud of gas and dust almost 5 billion years ago,
and the atoms in your body were part of that cloud. How the sun
took shape, how the cloud gave birth to the planets, how the
atoms in your body found their way onto Earth and into you,
make up the story of this chapter. As you explore the origin of
the solar system, keep in mind the great chain of origins that
created the atoms. As the geologist Preston Cloud remarked,
“Stars have died that we might live.”Early Hypotheses for the Origin of
Earth and the Solar System
Th e earliest descriptions of Earth’s origin are myths and folktales
that go back beyond the beginning of recorded history. In the
time of Galileo, the telescope gave philosophers observational
evidence on which to base rational explanations for natural phe-
nomena. While people like Copernicus, Kepler, and Galileo tried
to fi nd logical explanations for the motions of Earth and the
other planets, other philosophers began thinking about the ori-
gin of Earth and its sibling planets.
Th e fi rst physical theory for the solar system’s origin was pro-
posed by the French philosopher and mathematician René
Descartes (1596–1650). Because he lived and wrote before the
time of Newton, Descartes did not recognize that gravitation is
the dominant force in the universe. Rather, he believed that force
was communicated by contact between bodies and that the entireWhat place is this?
Where are we now?
— CARL SANDBURG, “GRASS”M
icroscopic creatures live in the roots of your eye-
lashes. Don’t worry. Everyone has them, and they are
harmless.* Th ey hatch, fi ght for survival, mate, lay eggs,
and die in the tiny spaces around the roots of your eyelashes
without doing any harm. Some live in renowned places—the eye-
lashes of a glamorous movie star, for example—but the tiny beasts
are not self-aware; they never stop to say, “Where are we?”
You can study the solar system for many reasons. You can
study Earth and its sibling planets because, as you are about to
discover, there are almost certainly more planets in the universe
than stars. Above all, you can study the solar system because it is
your home in the universe. Because humans are an intelligent
species, we have the ability and the responsibility to wonder
where we are and what we are. Our kind has inhabited this solar
system for several million years, but only within the last few
hundred years have we begun to understand what the solar sys-
tem is.
The Great Chain of
OriginsYou are linked through a great chain of origins that leads back-
ward through time to the instant when the universe began,
13.7 billion years ago. Th e gradual discovery of the links in that
chain is one of the most exciting adventures of the human intel-
lect. In earlier chapters, you studied some of that story: the origin
of the universe in the big bang, the formation of galaxies, the
origin of stars, and the growth of the chemical elements. Now
you will explore further to consider the origin of planets.
The History of the Atoms in Your Body
Th e universe began in the big bang (see Chapter 18). By the time
the universe was a few minutes old, the protons, neutrons, and
electrons in your body had come into existence. You are made of
very old matter.
Although those particles formed quickly, they were not
linked together to form many of the atoms that are common
today. Most of the matter in the early universe was hydrogen,
and about 25 percent was helium. Although your body does not
contain helium, it does contain many of those ancient hydrogen
19-1
*Demodex folliculorum has been found in 97 percent of individuals and is a
characteristic of healthy skin.