Download Origin of the Solar System

25.5
 Any theory about the
origin of the solar
system must explain
 why all of the
planets’ orbits lie
more or less in a
plane
 Why all of the
planets orbit the sun
in the same direction.
 It must also explain
the differences in size
and composition
between the
terrestrial planets
and the gas giants.
The Nebular Theory
 The nebular theory states that the solar system
formed from a rotating cloud of dust and gas.
The Nebular Theory
The generally accepted explanation for
the formation of the solar system is the
nebular theory.
A large, thin cloud of dust and gas like
the one that eventually formed our solar
system is called a solar nebula.
According to the nebular theory, the solar
nebula formed from the material expelled
by previous stars.
The Nebular Theory
 The contraction of the solar nebula, a large cloud of dust and gas, led to a diskshaped protoplanetary disk, from which the sun and planets formed.
Solar nebula
The sun forms at
the center of a
protoplanetary
disk.
Planetesimals form.
Protoplanets form.
The Nebular Theory
It is likely that a shock wave
from the explosion of a nearby
star caused the solar nebula to
collapse.
As the cloud contracted, it
began to spin faster.
The gravitational attraction
between particles became
stronger as the solar nebula
shrank.
The Nebular Theory
Formation of the Protoplanetary Disk
As the solar nebula rotated faster, it began to flatten
out.
Over about 100,000 years, a large disk-shaped cloud of
dust and gas called a protoplanetary disk formed.
The disk was densest in the center and thinner toward
the edges.
The Nebular Theory
Nearly all of the mass of the solar nebula became
concentrated near the center.
The density at the center of the disk increased until the
temperature reached a few million degrees.
About 10 million years after the formation of the solar
nebula, nuclear reactions began and the sun was born.
The Nebular Theory
Planetesimals and Protoplanets
The nebular theory also explains how the planets could
have formed.
Weak surface forces held dust grains together, forming
loose balls of dust. As these balls of dust collided, they
grew larger and larger.
The Nebular Theory
Planetesimals were asteroid-like bodies that
eventually combined to form planets.
Planetesimals grew by accretion, the process of adding
mass by colliding with other planetesimals.
The Nebular Theory
At about a kilometer in diameter,
planetesimals began to exert a
significant gravitational attraction
on nearby objects.
The planetesimals attracted more
material, causing them to grow
even faster.
The result was the accretion of
planetesimals into a much smaller
number of moon-sized
protoplanets.
The Nebular Theory
 Protoplanets joined to form the current planets in a
series of immense collisions.
 The nebular theory provides an explanation for the
current motions of the sun, planets, and most moons
nearly all revolve in the direction that the protoplanetary
disk was spinning.
Composition and Size of the
Planets
 The terrestrial planets are relatively small and rocky. In
part, that is because the inner solar system was too hot
during their formation for ice-forming compounds to
condense.
Composition and Size of the
Planets
 The gas giants are large and have low densities because
the outer solar system was cool enough for ice-forming
compounds to condense.
Composition and Size of the
Planets
At low pressures, such as those found in space,
cooling materials generally condense directly from a
gas into a solid.
• When the solar system formed, temperatures near the
sun were very high. Rocky materials condensed, but
water and other ice-forming materials did not.
Composition and Size of the
Planets
 Ice-forming materials could condense in the outer
solar system because it was much colder.
 As the outer planets grew, their gravity increased, and
they were able to capture hydrogen and helium gas.
Composition and Size of the
Planets
Astronomers have been able to observe protoplanetary disks
around distant newborn stars.
Astronomers have also detected more than a hundred
planets in orbit around distant stars.
Evidence of planets forming around other stars, plus the
results of computer simulations, provides support for the
nebular theory.
Composition and Size of the
Planets
 The Hubble
Space Telescope
took this photo
of what are
thought to be
protoplanetary
disks around
several young
stars.