The Solar System

Summary

▶ Descartes proposed that the solar system formed from a contracting
vortex of matter—an evolutionary hypothesis (p. 398). Buffon later
suggested that a passing comet pulled matter out of the sun to form
the planets—a catastrophic hypothesis (p. 398). Later astronomers
replaced the comet with a star to produce the passing star hypothesis
(p. 398).
▶ Laplace’s nebular hypothesis (p. 398) required a contracting nebula
to leave behind rings that formed each planet, but it could not explain
the sun’s low angular momentum, a puzzle known as the angular
momentum problem (p. 399).
▶ Hypotheses for the origin of the solar system have been either
catastrophic or evolutionary. Catastrophic hypotheses depend on a rare
event such as the collision of the sun with another star. Evolutionary
hypotheses propose that the planets formed by gradual, natural
processes. The evidence now strongly favors the solar nebula theory
(p. 399), an evolutionary scenario.

▶ (^) Modern astronomy reveals that all the matter in the universe,
including our solar system, was originally formed as hydrogen and
helium in the big bang. Atoms heavier than helium were cooked up
in nuclear reactions in later generations of stars. The sun and planets
evidently formed from a cloud of gas and dust in the interstellar
medium.
▶ (^) The solar nebula theory proposes that the planets formed in a disk of
gas and dust around the protostar that became the sun. Observations
show that these disks are common.
▶ (^) The solar system is disk shaped, with all the planets orbiting nearly in the
same plane. The orbital revolution of all the planets, the rotation of most
of the planets on their axes, and the revolution of most of their moons
are all in the same direction, counterclockwise as seen from the north.
▶ (^) The planets are divided into two groups. The inner four planets are
Terrestrial planets (p. 402)—small, rocky, dense Earth-like worlds.
The next four outward are Jupiter-like Jovian planets (p. 402) that
are large and low density.
▶ (^) All four of the Jovian worlds have ring systems and large families of
moons. The Terrestrial planets have no rings and few moons.
▶ (^) Most of the asteroids (p. 401), small, irregular, rocky bodies, are
located between the orbits of Mars and Jupiter.
▶ (^) The Kuiper belt (p. 404) is composed of small, icy bodies that orbit
the sun beyond the orbit of Neptune.
▶ (^) Comets (p. 404) are icy bodies that pass through the inner solar
system along long elliptical orbits. As the ices vaporize and release
dust, the comet develops a tail that points approximately away from the
sun.
▶ (^) Meteoroids (p. 405) that fall into Earth’s atmosphere are vaporized
by friction and are visible as streaks of light called meteors (p. 404).
Larger and stronger meteoroids may survive to reach the ground, where
they are called meteorites (p. 405).
▶ (^) The age of a rocky body can be found by radioactive dating, based on
the decay half-life (p. 405) of radioactive atoms. The oldest rocks
from Earth, the moon, and Mars have ages over 4 billion years. The old-
est objects in our solar system are some meteorites that have ages of
4.6 billion years. This is taken to be the age of the solar system.
▶ (^) Condensation (p. 409) in the solar nebula converted some of the gas
into solid bits of matter, which grow by accretion (p. 409) to form
billions of planetesimals (p. 409).
▶ (^) Planets begin growing by accretion of solid material into protoplanets
(p. 409). Once a protoplanet approaches about 15 Earth masses, it can
begin growing by gravitational collapse (p. 410) as it pulls in gas
from the solar nebula.

CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 421

What Are We? Planetwalkers

The matter you are made of came from the

big bang, and it has been cooked into a wide

variety of atoms inside stars. Now you can

see how those atoms came to be part of

Earth. Your atoms were in the cloud of gas

that formed the solar system 4.6 billion years

ago, and nearly all of that matter contracted

to form the sun, but a small amount left

behind in a disk formed planets. In the

process, your atoms became part of Earth.

You are a planet-walker, and you have

evolved to live on the surface of Earth. Are

there other beings like you in the universe?

Now you know that planets are common,

and you can reasonably suppose that there

are more planets in the universe than there

are stars. However complicated the forma-

tion of the solar system was, it is a common

process, so there may indeed be more

planet-walkers living on other worlds.

But what are those distant planets like?

Before you can go very far in your search

for life beyond Earth, you need to explore

the range of planetary types. It is time to

pack your spacesuit and voyage out among

the planets of our solar system, visit them

one by one, and search for the natural

principles that relate planets to each

other. That journey begins in the next

chapter.