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Chapter 6
Introduction to the Solar System
Guidepost
You are a planetwalker. What does that mean? You live on an
astronomical body, Earth, and it is time to find out how Earth is
connected to the story of the stars. As you explore our solar
system, you will find answers to four essential questions:
• What theories account for the origin of the solar system?
• What properties must a successful theory explain?
• How do planets form?
• Is our solar system unique?
Guidepost (continued)
Exploring the origin of the solar system will give you new insight
into what you are, but it will also give you a chance to answer two
important questions about science:
• Why are scientists hesitant to accept catastrophic theories?
• Why are scientists skeptical of new ideas?
Once you understand the origin of the solar system, you will have a
framework for understanding the rest of the planets and the
mysterious smaller bodies that orbit the sun.
Outline
I. Theories of Solar System’s Origin
A. Early Hypotheses
B. The Solar Nebula Hypothesis
II. A Survey of the Solar System
A. A General View
B. Two Kinds of Planets
C. Space Debris: Comets, Asteroids, Meteorites…
D. The Age of the Solar System
III. Planets Orbiting Other Stars
A. Planet-Forming Disks around Other Suns
B. Extrasolar Planets
Misconceptions to be cleared out….
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The Solar System is very crowded.
The Solar System contains only the Sun, Moon, and planets.
Meteors are falling stars.
Comets sweep across the sky like meteors.
Comet tails stream behind the comet as the comet moves.
Comets are burning.
Planets are close together or are large compared with the distances between
them.
The planets are always arranged in a straight line away from the Sun.
Alignments of planets are dangerous and can affect the Earth.
Your personality and future are determined by the positions of the Sun, Moon,
and planets.
Planetary orbits are circular.
Mercury is hot everywhere on its surface.
Giant planets have solid surfaces.
Saturn is the only planet with rings.
Jupiter and Saturn are made of gas.
The Sun is made of molten lava and is solid in parts.
The asteroid belt is crowded and dangerous.
I. Theories of Earth’s Origin
Early Hypotheses (1)
1.
Catastrophic hypotheses,
e.g., passing star hypothesis:
Star passing the sun closely tore
material out of the sun, from which
planets could form
2. Evolutionary hypotheses,
e.g., Laplace’s nebular hypothesis:
Rings of material separate from the
spinning cloud, carrying away
angular momentum of the cloud 
cloud could contract further (forming
the sun)
Early Hypotheses (2): No Longer Considered
1.
Catastrophic hypotheses,
e.g., passing star hypothesis:
•
Stars very far apart – Very
infrequent collisions
• Gas pulled from Sun too hot
Catastrophic hypotheses predict: Only
few stars should have planets!
2. Evolutionary hypotheses,
e.g., Laplace’s nebular hypothesis:
• Sun would be rotating too fast,
and planets slow.
Evolutionary hypotheses predict:
Most stars should have planets!
The Solar Nebula Hypothesis: Video
• By 1940, astronomers were beginning to understand how
stars form and how they generated their energy.
• Solar Nebula Theory: planets form
in theofrotating
disks
of gas
Basis
modern
theory
of and
dust around young stars.
planet formation.
• If planet formation is a natural part of star formation, most stars
Planets form at the same
should have planets.
time from the same cloud as
the star.
Planet formation sites
observed today as dust disks
of T Tauri stars.
Sun and our Solar system formed ~
5 billion years ago.
Scientific Argument
• Why does the solar nebula theory imply
planets are common?
• Why would a catastrophic hypothesis for
the formation of the solar system suggest
that planets are not common?
Chapter Summary (1)
• The passing star hypothesis suggests that a passing star
pulled matter out of the Sun to form the planets.
• Laplace’s nebular hypothesis required a contracting nebula
to leave behind rings that formed each planet.
• The solar nebula theory is a more extensive version of the
nebula hypothesis, which is an evolutionary hypothesis.
II. A Survey of the Solar System
Survey of the Solar System
Relative Sizes of the Planets
Assume, we reduce all bodies in
the solar system so that the Earth
has diameter 0.3 mm.
Sun: ~ size of a small plum.
Mercury, Venus, Earth, Mars:
~ size of a grain of salt.
Jupiter: ~ size of an apple seed.
Saturn: ~ slightly smaller than
Jupiter’s “apple seed”.
Pluto: ~ Speck of pepper.
Planetary Orbits
Mercury
Venus
Earth
All planets in
almost circular
(elliptical) orbits
around the sun,
in approx. the
same plane
(ecliptic).
Sense of
revolution:
counter-clockwise
Sense of rotation:
counterclockwise (with
exception of
Venus, Uranus,
and Pluto)
(Distances and times reproduced to scale)
Orbits
generally
inclined by no
more than 3.4o
Exceptions:
Mercury (7o)
Pluto (17.2o)
Scientific Argument
• In what ways does the solar system
resemble a disk?
Two Kinds of Planets
Planets of our solar system can be divided into two very different kinds:
Terrestrial (earthlike)
planets: Mercury, Venus,
Earth, Mars
Jovian (Jupiter-like) planets:
Jupiter, Saturn, Uranus, Neptune
Terrestrial Planets (Video Trailer)
Four inner
planets of the
solar system
Relatively small
in size and
mass (Earth is
the largest and
most massive)
Rocky surface
Surface of Venus can not be seen
directly from Earth because of its
dense cloud cover.
Craters on Planets’ Surfaces
Craters (like on
our Moon’s
surface) are
common
throughout the
Solar System.
Not seen on
Jovian planets
because they
don’t have a
solid surface.
The Jovian Planets (Video Trailer)
Much lower
average density
Mostly gas; no
solid surface
All have rings
(not only Saturn!)
Scientific Argument
• What are the distinguishing differences
between the Terrestrial and Jovian
planets?
Latest Pluto’s Flyby
New Horizons July 2015
Space Debris (Video Trailer)
In addition to planets, small bodies orbit the sun:
Asteroids, comets, meteoroids
Asteroid
Eros,
imaged by
the NEAR
spacecraft
Common Misconception
• Asteroids are the remains of a planet that
broke apart.
Comets (Video Trailer)
Icy nucleus, which
evaporates and gets
blown into space by solar
wind pressure.
Mostly objects in highly elliptical orbits,
occasionally coming close to the sun.
Meteoroids (Video Trailer)
Small (mm – mm sized)
dust grains throughout
the solar system
If they collide with Earth,
they evaporate in the
atmosphere.
 Visible as streaks of
light: meteors.
 (Sound of a meteor
shower)
(Video Trailer)
The Age of the Solar System
Sun and planets should
have about the same age.
Ages of rocks can be
measured through
radioactive dating:
Measure abundance of a
radioactively decaying
element to find the time
since formation of the
rock
Dating of rocks on Earth,
on the Moon, and
meteorites all give ages
of ~ 4.6 billion years.
Chapter Summary (2)
Review Questions
1. Which characteristic listed below describes the Jovian planets?
a. Small masses.
b. Low density.
c. Solid surfaces.
d. Slow rotational period.
2. All terrestrial planets have ________
a. Thick atmospheres.
b. Strong magnetic fields.
c. Craters.
d. Liquid water oceans.
3. What are the bodies between Mars and Jupiter (sometimes called the
‘failed planet’)?
a. Kuiper belt objects
b. Ort cloud objects.
c. The asteroids.
d. The meteorites.
Why Isn't Pluto a Planet Any
More?
India response to Pluto’s
Denomination!!!!
III. Planets Orbiting Other Stars
Video Trailer 3:
Hubble and Exoplanets
Hubble Cast 14
Evidence for Ongoing Planet Formation
Many young
stars in the Orion
Nebula are
surrounded by
dust disks:
Probably sites of
ongoing planet
formation right
now!
Dust Disks Around Forming Stars
Dust disks
around
some T
Tauri stars
can be
imaged
directly
(HST).
Extrasolar Planets
Modern theory of planet formation is evolutionary
 Many stars should have planets!
 planets
orbiting around other stars = “Extrasolar planets”
Extrasolar planets
can not be imaged
directly.
Detection using same
methods as in binary
star systems:
Look for “wobbling”
motion of the star
around the common
center of mass.
Indirect Detection of Extrasolar Planets
(Video Trailer: How to Detect)
/ Video 2
Observing periodic
Doppler shifts of
stars with no
visible companion:
Evidence for the
wobbling motion of
the star around the
common center of
mass of a planetary
system
Over 250
extrasolar planets
detected so far.
Direct Detection of Extrasolar Planets
Only in exceptional
cases can extrasolar
planets be observed
directly.
Preferentially in the
infrared:
Planets may still
be warm and emit
infrared light;
stars tend to be
less bright in the
infrared than in
the optical
Summary (3)
Is our Solar System Unique?
•
Hot disks of gas and dust have been detected in early stages of star
formation and are believed to be the kind of disk in which planets could
form.
•
Planets orbiting other stars, extrasolar planets, have been detected
–
–
by the way they tug their stars about, creating small Doppler shifts in the stars’
spectra.
as they cross in front of their star and dim the star’s light.
•
Some extrasolar planets are big enough to be Jovian worlds but are
quite close to their star and are called hot Jupiters.
•
A few of these planets have been detected when they crossed behind
their star and their infrared radiation was cut off.
•
Nearly all extrasolar planets found so far are massive, Jovian worlds.
Lower-mass terrestrial planets are harder to detect but are presumably
common.