Transcript Chapter 25.5 notes

25.5 The Origin of the Solar System
Any theory about the
origin of the solar
system must explain
why all of the planets’
orbits lie more or less
in a plane and 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.
25.5 The Origin of the Solar System
The Nebular Theory
What is the nebular theory?
The nebular theory states that the solar
system formed from a rotating cloud of dust
and gas.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
Composition and Size of the Planets
How does the nebular theory explain the
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
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.
25.5 The Origin of the Solar System
Assessment Questions
1. According to the nebular theory,
a. the sun will eventually die out and become a solar
nebula.
b. the solar system began as a solar nebula.
c. the solar system evolved from a protoplanetary disk to a
solar nebula.
d. solar nebulas were asteroid-like bodies that combined
to form planets.
25.5 The Origin of the Solar System
Assessment Questions
1. According to the nebular theory,
a. the sun will eventually die out and become a solar
nebula.
b. the solar system began as a solar nebula.
c. the solar system evolved from a protoplanetary disk to a
solar nebula.
d. solar nebulas were asteroid-like bodies that combined
to form planets.
ANS: B
25.5 The Origin of the Solar System
Assessment Questions
2. Which of the following is not evidence used to
support the nebular theory?
a. The planets’ orbits lie generally in a single plane.
b. The planets all revolve around the sun in the same
direction.
c. The terrestrial planets are relatively small and rocky.
d. All of the planets have the same composition.
25.5 The Origin of the Solar System
Assessment Questions
2. Which of the following is not evidence used to
support the nebular theory?
a. The planets’ orbits lie generally in a single plane.
b. The planets all revolve around the sun in the same
direction.
c. The terrestrial planets are relatively small and rocky.
d. All of the planets have the same composition.
ANS: D