The Solar System

CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 407

surface is not geologically active like Earth’s surface, some
moon rocks might have survived unaltered since early in
the history of the solar system. In fact, the oldest moon rocks
are 4.5 billion years old. Th at means the moon must be at least
4.5 billion years old.
Although no one has yet been to Mars, over a dozen mete-
orites found on Earth have been identifi ed by their chemical
composition as having come from Mars. Most of these have ages
of only a billion years or so, but one has an age of approximately
4.5 billion years. Mars must be at least that old.
Th e most important source for determining the age of the
solar system is meteorites. Radioactive dating of meteorites yields
a range of ages, but there is a fairly precise upper limit—many
meteorite samples have ages of 4.56 billion years old, and none
are older. Th at fi gure is widely accepted as the age of the solar
system and is often rounded to 4.6 billion years. Th e true ages of
Earth, the moon, and Mars are also assumed to be 4.6 billion
years, although no rocks from those bodies have yet been found
that have remained unaltered for that entire stretch of time.
One last celestial body deserves mention: the sun.
Astronomers estimate the age of the sun to be about 5 billion
years, but that is not a radioactive date because we have no
samples of radioactive material from the sun. Instead, an inde-
pendent estimate for the age of the sun can be made using helio-
seismological observations and mathematical models of the sun’s
interior (see Chapters 8 and 12). Th is yields a value of about
5 billion years, plus or minus 1.5 billion years, a number that is
in agreement with the age of the solar system derived from the
age of meteorites.
Apparently, all the bodies of the solar system formed at
about the same time, some 4.6 billion years ago. You can add this
as the fi nal item to your list of characteristic properties of the
solar system (■ Table 19-1).

CHAPTER 1CHAPTER 1

■ Table19-1 ❙ Characteristic Properties

of the Solar System

1. Disk shape of the solar system
   Orbits in nearly the same plane
   Common direction of rotation and revolution


2. Two planetary types
   Terrestrial—inner planets; high density
   Jovian—outer planets; low density


3. Planetary rings and large satellite systems
   Yes for Jovian Planets (Jupiter, Saturn, Uranus, and Neptune)
   No for Terrestrial Planets (Mercury, Venus, Earth, and Mars)


4. Space debris—asteroids, comets, and meteoroids
   Composition, Orbits
   Asteroids in inner solar system, composition like
   Terrestrial planets
   Comets in outer solar system, composition like
   Jovian planets


5. Common age of about 4.6 billion years measured or inferred for
   Earth, the moon, Mars, meteorites, and the sun

SCIENTIFIC ARGUMENT

In what ways does the solar system resemble a disk?

Notice that this argument is really a summary of pieces of evidence.

First, the general shape of the solar system is that of a disk. The orbit

of Mercury is inclined 7° to the plane of Earth’s orbit, and the rest of

the planets are in orbits inclined less than that. In other words, the

planets are confi ned to a thin disk with the sun at its center.

Second, the motions of the sun and planets also follow this disk

theme. The sun and most of the planets rotate in the same direc-

tion, counterclockwise as seen from the north, with their equators

near the plane of the solar system. Also, all of the planets revolve

around the sun in that same direction. The objects in our solar

system mostly move in the same direction, which further refl ects

a disk theme.

One of the basic characteristics of our solar system is its disk

shape, but another dramatic characteristic is the division of the

planets into two groups. Build an argument to detail that evidence.

What are the distinguishing differences between the Terrestrial

and Jovian planets?

The Story of Planet

Building

The challenge for modern planetary scientists is to compare

the characteristics of the solar system with predictions of the

solar nebula theory, so they can work out details of how the

planets formed.

The Chemical Composition of the

Solar Nebula

Everything astronomers know about the solar system and star

formation suggests that the solar nebula was a fragment of an

interstellar gas cloud. Such a cloud would have been mostly

hydrogen with some helium and small amounts of the heavier

elements.

Th at is precisely what you see in the composition of the sun

(look back at Table 7-2). Analysis of the solar spectrum shows

that the sun is mostly hydrogen, with a quarter of its mass being

helium and only about 2 percent being heavier elements. Of

course, nuclear reactions have fused some hydrogen into helium,

but this happens in the sun’s core and has not aff ected the com-

position of its surface and atmosphere, which are the parts you

can observe directly. Th at means the composition revealed in the

sun’s spectrum is essentially the composition of the gases from

which the sun formed.

Th is must have been the composition of the solar nebula,

and you can also see that composition refl ected in the chemical

compositions of the planets. Th e inner planets are composed of

rock and metal, and the outer planets are rich in low-density

19-3