Formation of the Solar
System
Questions for this chapter…
How old is the Earth? How do we know???
Questions for this chapter…
•
How old are the
Sun and other
planets?
•
Were they
created during
the “Big Bang”?
•
How do we
know?
Questions for this chapter…
What is the value to exploring our solar
system, and spending tax dollars for these
missions?
How do we start?
Observe patterns, characteristics,
motions
How do we start?
Analyze and Research data
How do we start?
Develop models and test them with
hypotheses.
NASA supercomputer
models of our
solar system
(2010)
What does the solar system
look like?
Clear patterns of motion,
composition, and distribution.
The solar system exhibits clear patterns of composition and motion
and distribution.
These patterns are far more important and interesting than
numbers, names, and other trivia.
Patterns of Motion
Patterns of Composition
Patterns of Distribution
But don’t get too confident…
Planets are very
tiny compared to
distances
between them.
Sun
• Over 99.9% of solar system’s mass
• Made mostly of H/He gas (plasma)
• Converts 4 million tons of mass into energy each second
Mercury
• Made of metal and rock; large iron core
• Desolate, cratered; long, tall, steep cliffs
• Very hot and very cold: 425°C (day), –170°C (night)
Mercury
•Mariner 10 (1974)
•Messenger spacecraft (2004-2013…)
Venus
• Nearly identical in size to Earth; surface hidden by clouds
• Hellish conditions due to an extreme greenhouse effect
• Even hotter than Mercury: 470°C, day and night
• Mariner (1960s)
• Pioneer & Venera (1970s)
• Magellan (1989)
Earth
Earth and
Moon to scale
• An oasis of life
• The only surface liquid water in the solar system
• A surprisingly large moon
Mars
• Looks almost Earth-like, but don’t go without a spacesuit!
• Giant volcanoes, a huge canyon, polar caps, and more
• Water flowed in the distant past; could there have been
life?
Mars
•
•
•
•
•
•
Mariner (1960s)
Viking (1970s)
Pathfinder (1990s)
Odyssey
Spirit & Opportunity
Phoenix
Mars
• MRO (Mars Reconnaissance
Orbiter)
• Curiosity (Mars Science
Laboratory)
• MAVEN (Mars Atmosphere
and Volatile EvolutioN
Mars
• Mars Express (ESA)
• Mars Orbiter Mission
(Mangalyaan) - India
Mars
Mars failures…
• Mars Phobos-Grunt
(Russia/China)
• Mars Polar Lander
• Mars Climate Orbiter
• Nozomi (Japan)
Jupiter

Much farther
from Sun

Mostly H/He; no
solid surface

300+ times
mass of Earth

Many moons,
rings
Jupiter




Pioneer (1970s)
Voyager (1970s)
Galileo (198990s)
Juno (2011-16)
Jupiter’s moons can be as interesting as planets
themselves, especially Jupiter’s four Galilean
moons IO, EUROPA, GANYMEDE, & CALLISTO
Saturn



Giant and gaseous like Jupiter
Spectacular rings
Many moons, including cloudy Titan
Pioneer (1970s)
Voyager (1980s)
Cassini (2004- )

Cassini probe arrived in July 2004 (launched in 1997).
• Rings are
NOT solid
• Countless
small
chunks of
ice and rock
Artist’s conception
• Each
orbiting like
a tiny moon.
Uranus

Smaller than
Jupiter/Saturn;
much larger than
Earth

H/He gas and
hydrogen
compounds (H2O,
NH3, CH4)

Extreme axis tilt
Neptune

Similar to
Uranus (except
for axis tilt)

Many moons
(including Triton
with geysers)
Pluto and Eris



Much smaller than other planets
Icy, comet-like composition
Pluto’s moon Charon is similar in size to Pluto
Pluto and Other Dwarf Planets
Much smaller than other planets
 Icy, comet-like composition
 Charon is similar in size to Pluto

What features and patterns of
our solar system provide clues
to how it formed?
Motion of Large Bodies

All large bodies in the
solar system orbit in
the same direction and
in nearly the same
plane.

Most also rotate in that
direction.
Two Major Planet Types

Terrestrial planets are
rocky, relatively small,
and close to the Sun.

Jovian planets are
gaseous, larger, and
farther from the Sun.
Swarms of Smaller Bodies

Rocky asteroids
populate the inner solar
system.

Pluto-like bodies
beyond Neptune

Icy comets populate
the outer solar system
Notable Exceptions

Several exceptions
to normal patterns
must be explained.
Venus’ rotation
 Earth’s Moon
 Uranus’ tilt

What theory best explains
the features of our solar
system?
Um… What really is a
scientific theory?
What is a scientific theory?
• An explanation that is right all of the time
• An explanation that everyone agrees with
• An explanation that most people agree with
• An explanation that has been proven by a
scientist
What is a scientific theory?
A scientific theory is a well-substantiated
explanation of some aspect of the natural
world, based on knowledge that has been
repeatedly confirmed through observation and
experimentation.
Scientific theory. (2013, October 22). In Wikipedia, The Free Encyclopedia.
Retrieved 16:46, October 23, 2013, from
http://en.wikipedia.org/w/index.php?title=Scientific_theory&oldid=578316195
What is a scientific theory?
A scientific theory is not the end result of the
scientific method; theories can be proven or
rejected, just like hypotheses.
Zimmerman, K. (2012) “What is a Scientific Theory?” LiveScience.
http://www.livescience.com/21491-what-is-a-scientific-theory-definition-oftheory.html
What is a scientific theory?
A scientific theory summarizes a hypothesis or
group of hypotheses that have been supported
with repeated testing.
“If enough evidence accumulates to support a
hypothesis, it … becomes accepted as a valid
explanation of a phenomenon” – a scientific
theory.
What is a scientific theory?
• An explanation that is right all of the time
• An explanation that everyone agrees with
• An explanation that most people agree with
• An explanation that has been proven by a
scientist
Can scientific theories be
proven absolutely?
A scientific theory can be falsified (proven
wrong) with just ONE test.
But… a scientific theory can never be proven
absolutely true with just one test!
Can scientific theories
change?
A scientific theory MUST change when new
evidence is discovered.
So… what qualifies as “evidence”?
What theory best
explains the
features of our solar
system?
Assemble evidence from many sources
Integrate many observations and results of
many prior experiments
Test the general theory against all known facts
But…
What theory best
explains the
features of our solar
system?
But…
Always be aware, you can’t PROVE the theory
correct!
You can, however, disprove it or cause it to be
re-evaluated with new evidence!
What theory best
explains the
features of our solar
system?
We have only ONE example to study
Could we over-generalize based on a unique
case?
Are there mutliple ways to “build” a solar
system?
Space isn’t empty…
We know space is filled with gas and dust – the
raw materials from which planetary systems
form!
…and its composition changes
Spectra of exploding and old stars shows
heavier elements being ejected, too
The Nebular Theory
According to the
nebular theory, our
solar system formed
from a giant cloud of
interstellar gas.
(nebula = cloud)
Why would it
“contract”?
•
Mass of material
could be ~100 x
Mass of the Sun
•
Universal Law of
Gravity
Why would it
“flatten”?
•
Rotation speed of
the cloud increased
as cloud contracted.
•
Conservation of
Angular Momentum
Why would it
“flatten”?
•
Collisions within the
rotating disk even
out motions
•
Conservation of
Angular Momentum
Rotation of a
contracting cloud
speeds up
Same reason a skater
speeds up as she
pulls in her arms.
Collapse of the Solar Nebula
Why would it
develop planets?
•
Collisions of material
form “planetary
seeds”
•
Gravity, Static
Electricity, Accretion
Why would it
develop planets?
•
Temperature rises in
the center; cooler on
the outer edges
•
Conservation of
Energy
Why would it
develop two
kinds of planets?
•
Temperature varies
from inside to
outside
•
Condensation
Sequence Theory
Why two major types of planets?
•~ 99% of material is Hydrogen & Helium gas
•~ 1% Small bits of Dust
(Carbon, Silicates, Iron, etc.)
Inside the frost line: Too hot for hydrogen
compounds to form ices
Very Little Material to grow solid planets!
Outside the frost line: Cold enough for ices of
Hydrogen (H20, NH3, CH4) to form
HUGE amounts of material to grow planets from
Observational Evidence
BETA Pictoris – a Hint at Stars with
Planetary Disks?
Computer Model Evidence
Computer Model Evidence
Computer Model Evidence
Additional Evidence

Jupiter & Saturn as “miniature” solar
systems
Moon systems mirror planet systems?
 Show trends in density of moons from inside
to outside

Moons of jovian planets form in miniature disks.
Additional Evidence

Jupiter & Saturn as “miniature” solar
systems
Moon systems mirror planet systems?
 Show trends in density of moons from inside
to outside


Disks of Matter around other stars!
Disks Around Other Stars
•
Observations of disks around other
stars support the nebular hypothesis.
Tiny solid
particles stick to
form
planetesimals.
Summary of the Condensates in the Protoplanetary Disk
Gravity draws
planetesimals
together to form
planets.
This process of
assembly
is called
accretion.
Summary of the Condensates in the Protoplanetary Disk
Accretion of Planetesimals
•
Many smaller objects collected into just
a few large ones.
Formation of Jovian Planets
•
Hydrogen-rich ices could also form small
particles outside the frost line.
•
Larger planetesimals and planets were able to
form from these abundant elements
•
The gravity of these larger planets then was
able to draw in surrounding H and He gases.
Where did asteroids and
comets come from?
Asteroids and Comets
•
•
•
Leftovers from the accretion process
Rocky asteroids inside frost line
Icy comets outside frost line
Asteroids Play A Role in Solar
System Evolution!

Leftover
planetesimals
bombarded other
objects in late
stages of solar
system formation.

Craters on EVERY
solid surface!
Comets Play A Role in Solar
System Evolution!

Water may have
come to Earth by
way of collisions
with icy comets &
planetesimals from
the outer solar
system.
Explaining existence of our
Moon & other exceptions?
Captured Moons
•
The unusual moons of some planets
may be captured planetesimals.
Giant Impact Theory for
Earth’s Moon’s Formation
Giant impact stripped matter from
Earth’s crust
Stripped matter began to orbit
Then accreted into Moon
More Giant Impacts?

Giant impacts might
also explain the
different rotation axes
of some planets like
Uranus & Venus
Review
of
Nebular
Theory
condensation
sequence
Thought Question
How would the solar system be different if the
solar nebula had cooled with a temperature
half its current value?
A.
Jovian planets would have formed closer
to the Sun.
B.
There would be no asteroids.
C.
There would be no comets.
D.
Terrestrial planets would be larger.
Thought Question
How would the solar system be different if the
solar nebula had cooled with a temperature half
its current value?
A.
B.
C.
D.
Jovian planets would have formed closer
to the Sun.
There would be no asteroids.
There would be no comets.
Terrestrial planets would be larger.
Thought Question
Which of these facts is NOT explained by
the nebular theory?
•
•
•
•
There are two main types of planets: terrestrial
and jovian.
Planets orbit in the same direction and plane.
Asteroids and comets exist.
There are four terrestrial and four jovian planets.
Thought Question
Which of these facts is NOT explained by
the nebular theory?
•
•
•
•
There are two main types of planets: terrestrial
and jovian.
Planets orbit in the same direction and plane.
Asteroids and comets exist.
There are four terrestrial and four jovian
planets.
When did the planets form?

Cannot find age of a planet, but can
find ages of rocks on a planet.

Determine age of rock through careful
analysis of proportions of various
atoms and isotopes within it.

Radioactive Dating
Radioactive Decay
Some isotopes
decay into
other nuclei.
 A half-life is
the time for half
the nuclei in a
substance to
decay.

Dating the Solar System
Age dating of meteorites
that are unchanged since
they condensed and
accreted tells us that the
solar system is about 4.6
billion years old.
Multiple Pieces of Evidence!
Radiometric dating tells us that the oldest
moon rocks are ~4.4 billion years old.
•
•
The oldest meteorites are ~4.55 billion
years old.
•
Planets probably formed
~4.5 billion years ago.