Ch. 28 Sec. 1
Formation of the Solar System
Section 28.1
Formation of the Solar System
The solar system formed from the collapse
of an interstellar cloud.
Review Vocabulary
focus: one of two fixed points used to define
an ellipse
I. Formation Theory
NASA Cassini
A. Direct observation & data from probes
B. Explanation of observations
1. Shape of the solar system
2. Differences among the planets
3. Nature of asteroids, meteorites, and comets
II. Collapsing Interstellar Cloud
A. Nebula
1. Exist in space between the
stars
2. Hydrogen and helium gas
3. Small amounts of other
elements and dust
B
B. Radiate & reflect light
1. Dust blocks light (dark)
Eagle Nebula
2. Starlight
illuminates dust
3. Stars heat cloud (glow)
Eagle Nebula
C. Density of interstellar gas is low
1. Gravity slowly draws matter
together
2. Pressure balances gravity
D. Collapse
1. Begins slowly
2. Gradually accelerates
3. Cloud becomes much denser at its
center
4. If rotating, the cloud spins faster as
it contracts, due to centripetal
force
E. Collapse accelerates
As a collapsing interstellar cloud spins, the rotation
slows the collapse in the equatorial plane, and the
cloud becomes flattened.
Eventually, the cloud becomes a rotating disk with a
dense concentration of matter at the center.
The interstellar cloud that formed
our solar system collapsed into a
rotating disk of dust and gas.
When concentrated matter in the
center acquired enough mass, the
Sun formed in the center and the
remaining matter
gradually condensed,
forming the planets.
F. Matter condenses
1. Temperature varied greatly with location
a. Inner planet – rocky
b. Outer planets – gas & ice
III. Planetesimals & Planets
 Nebula begins to collapse
 Rotation begins
 Dust combines to form planetesimals
 More dust collects
 Collision of planetesimals forms planets
A. Gas giants form
1. Jupiter
a. Merging of icy planetesimals that
contained mostly lighter elements
b. Gravity attracted additional gas &
plantesimals
c. Disk in equatorial plane formed rings
and satellites
2. Saturn and the other gas giants formed
similarly
B. Terrestrial planet formation
1. Composed primarily of elements
that resist vaporization
2. No satellite formation
C. Debris
1. Icy objects known as comets
2. Rocky planetesimals known as
asteroids
Section 28.1
IV. Modeling the Solar System
A. Geocentric theory :
Earth-centered
1. Earth spins on its
axis
2. Explains
appearance of
celestial motions
a. Sunrise in east,
etc.
3. Could not readily
explain retrograde
motion
The apparent
backward
movement of a
planet is called
retrograde
motion. The
changing angles
of view from
Earth create the
apparent
retrograde
motion of Mars.
Fig. 28.4 page 799
B. Heliocentric model
1. Nicolaus Copernicus
– 1543
2. Explained retrograde
rotation
3. Inner planets move
faster than outer
planets due to
gravity
Which observation provided evidence for the
heliocentric model of the solar system?
a. the nightly motion of the
stars
b. the rising and setting of the
Sun
c. the retrograde motion of
planets
d. the occurrence of meteor 0%
showers
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C. Kepler’s first law of
Planetary Motion
1. From 1576–1601,
before the telescope
was used in
astronomy, Tycho
Brahe, a Danish
astronomer, made
accurate observations
to within a half arc
minute of the planets’
positions.
2. Kepler used Brahe’s observations
3. First Law – planet’s orbit is an ellipse
a. Foci
b. Major axis (semimajor axis)
c. Astronomical unit (AU) – average
distance from Earth to Sun
Each planet has its
own elliptical orbit,
but the Sun is
always at one
focus. For each
planet, the average
distance between
the Sun and the
planet is its
semimajor axis.
4. Earth’s semimajor axis is a unit used
to measure distances within the
solar system.
Earth’s average distance from the Sun is
1.496 × 108 km, or 1 astronomical unit
(AU).
5. The shape of a planet’s elliptical orbit is
defined by eccentricity, which is the
ratio of the distance between the foci
to the length of the major axis.
D. Kepler’s second
law states that
planets move
faster when close
to the Sun and
slower when
farther away. This
means that a
planet sweeps out
equal areas in
equal amounts of
time.
E. In Kepler’s third law, he determined the
mathematical relationship between
the size of a planet’s ellipse and its
orbital period. This relationship is
written as follows:
2
P =
3
a
P is time measured in Earth years, and a is
length of the semimajor axis measured in
astronomical units.
F. Galileo
1. Used telescope
a. Discovered 4 of Jupiter’s moons
b. Lunar mountains & craters
c. Sunspots
d. Phases of Venus
2. Favored sun-centered theory
The English scientist Isaac Newton described
falling as a downward acceleration produced by
gravity, an attractive force
between two objects. He
determined that both the
masses of and the
distance between two
bodies determined the
force between them.
Newton’s law of universal gravitation is stated
mathematically as follows:
F is the force measured in newtons,
G is the universal gravitation constant
(6.6726 × 10–11 m3/ kg•s2), m1 and m2 are the
masses of the bodies in kilograms, and r is the
distance between the two bodies in meters.
Which scientist first observed the
moons of Jupiter with a telescope?
a. Nicolaus Copernicus
b. Tycho Brahe
c. Isaac Newton
d. Galileo Galilei
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V. Gravity
A. Gravity and orbits
Newton observed the
Moon’s motion and
realized that its direction
changes because of the
gravitational attraction
of Earth. In a sense, the
Moon is constantly
falling toward Earth.
B. Center of mass
Newton determined that each planet
orbits a point between it and the Sun
called the center of mass.
Just as the balance point on a seesaw
is closer to the heavier box, the
center of mass between
two orbiting bodies is
closer to the more
massive body.
Fig. 28.9 page 803
VI. Present-Day Viewpoints
Recent discoveries have led many astronomers to
rethink traditional views of the solar system.
Some already define it in terms of three zones:
Zone 1, Mercury, Venus, Earth, and Mars;
Zone 2, Jupiter, Saturn, Uranus, and Neptune;
and Zone 3, everything else, including Pluto.
Examine the illustration. What
relationship exists between the areas
of the segments of the planet’s orbit?
Answer: Kepler’s second law
states that a planet sweeps out
equal amounts of area in equal
amounts of time. Therefore,
each segment of the planet’s
orbital ellipse has the same
area.
The solar system formed
from the collapse of an interstellar
cloud.
 A collapsed interstellar cloud formed
the Sun and planets from a rotating
disk.
 The inner planets formed closer to the
Sun than the outer planets, leaving
debris to produce asteroids and
comets.
 Copernicus created the
heliocentric model and Kepler
defined its shape and mechanics.
 Newton explained the forces
governing the solar system bodies
and provided proof for Kepler’s
laws.
 Present-day astronomers divide
the solar system into three zones.