Download Chapter 8: Formation of the solar system 8.1 The Search for Origins

Chapter 8: Formation of the solar system
8.1 The Search for Origins
What properties of our solar system must a formation theory explain?
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In chapter 7 we discussed four features of our solar system and these features hold the key to
testing any hypothesis that claims to explain the origin of the solar system
If the hypothesis fails to explain even one of the four features then it cannot be correct and if it
successful explains all four we might reasonably assume it is on the right track
The four criteria of success of solar system formation theory”
1. it must explain the patterns of motion we discussed in chapter 7
2. It must explain why planets fall into two major categories: small, rocky, terrestrial planets,
near the sun and large hydrogen-rich jovian planets farther out.
3. It must explain the existence of huge numbers of asteroids and comets and why these objects
reside primarily in the regions we call the asteroid belt, the Kuiper belt, and the Oort cloud
4. It must explain the general patterns while at the same time making allowances for exceptions
to the general rules, such as the odd axis tilt of Uranus and the existence of Earth’s large Moon
The theory can gain additional support ex. This theory can make predictions about other solar
systems
What theory best explains the features of our solar system?
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18th C scientists Immanuel Kant proposed that our solar system formed from the gravitational
collapse of an interstellar cloud of gas
40 years later Pierre Simon Laplace put forth the same idea independently because an
interstellar cloud is usually called a nebula, their idea became known as the nebula hypothesis
The nebula hypothesis remained popular throughout the 19th C
By the early 20th C scientist had found a few aspects of our solar system that the nebular
hypothesis did not seem to explain well
During much of the first half of the 20th C the nebular hypothesis faced stiff competition from a
hypothesis proposing that the planets represent debris from a near collision between the sun
and another star
According to this close encounter hypothesis the planets formed from blobs of gas that had
been gravitational pulled out of the Sun during the near collision
Today the close encounter hypothesis has been discarded because it could not account for
either the observed orbital motions of the planets or the neat division of the planets into two
major categories (terrestrial and jovian)
Moreover, the close encounter collision between our sun and another star given the vast
separation between star system in our region of the galaxy, the chance of such an encounter is
so small that it would be difficult to imagine it happening even once in order to form our solar
system and it could not account for the many other planetary system that we have discovered in
recent years
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The modification of the nebular theory has much evidence accumulated in favour of the nebular
hypothesis that it achieved the status of a scientific theory ----the nebular theory
The nebular theory: scientist found that the nebular hypothesis offered natural explanations for
all four general features of our solar system
8.2 The Birth of the Solar System
Where did the solar system come from?
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The nebular theory begins with the idea that our solar system was born from a cloud of gas
called the solar nebular that collapsed under its own gravity
But where did this gas come from? It was the product of billions of years of galactic recycling
that occurred before the sun and planets were born
Universe as a whole thought to have been born in the big bang which produced two chemical
elements: hydrogen and helium. Heavier elements were produced later by massive stars and
released into space when the stars died and then the heavier elements mixed with other
interstellar gas that formed new generations of stars
Although recycling within the galaxy has probably gone on for the most of the 14 billion year
history only a small fraction of the original hydrogen and helium has been converted into heavy
elements
Spectroscopy shows that old stars have a smaller proportion of heavy elements than younger
ones just as we should expect if they were born at a time before many heavy elements had been
manufactured
What caused the orderly patterns?
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The solar nebular began as a large spherical cloud of very cold and very low density gas
This gas was probably o spread out perhaps over a region a few light years in diameter that
gravity alone may not have been strong enough to pull it together and start its collapse
Instead, the collapse may have been triggered by a cataclysmic event such as the impact of a
shock wave from the explosion of a nearby star (a supernova)
Once the collapse started the law of gravity ensured that it would continue….remember that the
strength of gravity follows an inverse square law with distance because the mass of the cloud
remained the same as it shrank the strength of gravity increased as the diameter of the cloud
decreased since gravity pulls inward in all directions explains why the sun and the planets are
spherical
Heating, Spinning and Flattening
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As the solar nebula shrank in size three important processes altered its density, temperature,
and shape changing it from a large diffuse (spread-out) cloud to a much smaller spinning disk
Heating: the temperature of the solar nebular increased as it collapsed such heating represents
energy conservation in action. As the cloud shrank its gravitational potential energy was
converted to the kinetic energy of individual gas particles falling inward.
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These particles crashed into one another converting the kinetic energy of their inward
fall to the random motions of thermal energy. The sun formed in the center where the
temperatures and densities were highest
Spinning: The solar nebula rotated faster and faster as it shrank in radius. This increase in
rotation rate represents conservation of angular momentum in action. The clouds shrinkage
made fast rotation inevitable
o The rapid rotation helped ensure that not all material in the solar nebula collapsed
into the center: the greater the angular momentum of a rotating cloud the more
spread out it will be
Flattening: the solar nebula flattened into a disk. This flattening is a natural consequence of
collisions between particles in a spinning cloud. The random motions of the original cloud
therefore become more orderly as the cloud collapses changing the clouds original lumpy shape
into a rotating flattened disk
o Similarly collisions between clumps of material in highly elliptical orbits reduce their
eccentricities making their orbits more circular
The formation of the spinning disk explains the orderly motions of our solar system today
The planets all orbit the sun in nearly the same plane because they formed in the flat disk
The direction in which the disk was spinning became the direction of the sun’s rotation and the
orbits of the planets
Computer models show that planets would rotate in the same direction they formed which is
why most planets rotate the same way today
The fact that collisions in the disk tended to make orbits more circular explains why mot planets
in our solar system have nearly circular orbits
Collapsing clouds go through heating, spinning, and flattening
The heating that occurs in collapsing cloud of gas means that the gas should emit thermal
radiation primarily in the infrared
Many of these young stars appear to be ejecting jets are thought to result from the flow
material from the disk onto the forming star and they may influence the solar system formation
processes
We expect flattening to occur anywhere that orbiting particles can collide which explains why
we find so many cases of flat disks including the disks of spiral galaxies like the milky way, the
disks of planetary rings and the accretion disks that surround neutron stars and black holes in
close binary star systems
8.3 The formation of Planets
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The planets started to form after the solar nebula had collapsed into a flattened disk of perhaps
200 AU in diameter
Why are there two major types of planets?
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Rocky terrestrial planets formed in the warm inner regions of the swirling disk while jovian
planets formed in the colder outer regions
Condensation: Sowing the Seeds of Planets
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The general process in which solid (or liquid) particles form in a gas is called condensation –we
say that the particles condense out of the gas. These particles start out microscopic in size, but
they can grow larger with time
Different materials condense at difference temperatures
o Hydrogen and helium (98% of the solar nebula). These gases never condense in
interstellar space
o Hyrodgen compounds (1.4% of the solar nebula). Materials such as water (H2O),
methane (CH4) and ammonia (NH3) can solidify into ices at low temperatures (below
about 150 K under the low pressure of the solar nebula)
o Rock (0.4% of the solar nebula). Rocky material is gaseous at very high temperatures,
but condenses into solid bits of mineral at temperatures between about 500 K and 1300
K, depending on the type of rock. (A mineral is a type of rock with a particular chemical
composition and structure)
o Metal (0.2% of the solar nebula). Metals such as iron, nickel, and aluminum are also
gaseous at very high temperatures, but condense into solid form at higher temperatures
than rock –typically in the range of 1000 K to 1600 K
Hydrogen compounds could condense into ices only beyond the frost line –the distance at
which it was cold enough for ices to condense –which lay between the present-day orbits of
Mars and Jupiter
***Temperature differences in the solar nebula led to different kinds of condensed materials at
different distances from the Sun, sowing the seeds for two kinds of planets
o Within the frost line, rocks and metal condense, hydrogen compounds stay gaseous
o Beyond the frost line, hydrogen compounds, rocks, and metals condense
o Within the solar nebula 98% of the material is hydrogen and helium gas that doesn’t
condense anywhere
2 types of planets: planets born from seeds of metal and rock in the inner solar system and
planets born from seeds of ice (as well as metal and rock) in the outer solar system
How did the terrestrial planets form?
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The process by which small “seeds” grew into planets is called accretion
o early in the accretion process, there are many relatively large planetesimals on
crisscrossing orbits.
o As time passes, a few planetesimals grow larger by accreting smaller ones, while others
shatter in collisions
o Ultimately only the largest planetesimals avoid shattering and grow into full-fledged
planets
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As the particles grew in mass, gravity began to aid the process of their sticking together,
accelerating their growth into boulders large enough to count as planetesimals, which
means “pieces of planets”
How did the jovian planets form?
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The young jovian planets were surrounded by disks of gas, much like the disk of the entire solar
nebula but smaller ins ize. According to the leading model, the planets grew as large, ice-rich
planetesimals captured hydrogen and helium gas from the solar nebula.
What ended the era of planet formation?
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Vast majority of hydrogen and helium got swept into interstellar space by some combination of
radiation from the young Sun and the solar wind –a stream of charged particles (such as protons
and electrons) continually blown outward in all directions from the Sun. Although the solar wind
is fairly weak today, observations of winds from other stars show that such winds tend to be
much stronger in young stars
Explaining the Sun’s Rotation
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The young Sun should have rotated rapidly, but today the sun rotates quite slowly. The sun
probably lost angular momentum because of drag between slow-moving charged particles in
the solar nebula and the Sun’s rotating magnetic field
Material in the nebula orbits the sun slowly, in accordance with Kepler’s laws. While the Sun’s
magnetic lines sweep through the nebula faster, because they are tied to the Sun’s rotation
Result: the sun’s rotation slows down because he charged particles in the nebula exert a frag on
the magnetic field
8.4 The Aftermath of Planet Formation
Where did asteroids and comets come from?
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The strong wind from the young Sun cleared excess gas from the solar nebula, but many
planetismals remained scattered between newly formed planets. These leftovers became
asteroids and comets
Asteroids are the rocky leftover planetesimals of the inner solar system
Today, the total mass of the asteroids in the asteroid belt is only a tiny fraction of the mass of
any terrestrial planet
Comets are the ice-rich leftover planetesimals of the outer solar system’
The nebular theory actually predicts the existence of both the Oort cloud and the Kuiper belt –a
prediction first made in the 1950’s
How do we explain exceptions to the rules?
The Heavy Bombardment
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Planetesimals, vast majority of these collisions in the first few hundred million years of our solar
system’s history, during the period we call the heavy bombardment
How did earth come to have the water that makes up our oceans? The likely answer is that
water, along with other hydrogen compounds, must have been brought to earth and the other
terrestrial planets by the impact of water-bearing planetesimals that accreted farther from the
sun. Either way, the water we drink and the air we breathe probably originated beyond the orbit
of mars
Captured Moons
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How do we explain moons with less orderly orbits, such as those that go in the wrong direction
(opposite their planets rotation) or that have large inclinations to their planet’s equator? These
moons are probably leftover planetesimals that originally orbited the sun but were then
captured into planetary orbit
If friction reduced a passing planetesimal’s orbital energy enough, it could have become an
orbiting moon
Capture process would have worked only on objects of a few kilometers in size
How do we explain the existence of our Moon?
Giant Impacts and the Formation of our Moon
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When one of these planet-size planetesimals collided with a planet the spectacle would have
been awesome. Such a giant impact could have significantly altered a planet’s fate
An impact at a speed and angel that would have blasted rock from earth’s outer layers into
space. This material could have collected into orbit around our planet and accretion within this
ring of debris could have formed the Moon
2 features of the moon’s composition supports this hypothesis
o The moon’s overall composition is quite similar to that of earth’s outer layers –just as
we should expect if it were made from material blasted away from those layers
o Second, the moon has a much smaller proportion of easily vaporized ingredients (such
as water) than Earth
This fact supports the hypothesis because the heat of the impact would have vaporized these
ingredients. As gases, they would not have participated in the subsequent accretion of the moon
Other Giant Impacts
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Giant impacts may also explain many of the other exceptions to the general trends
Numerous giant impacts certainly should have occurred give the number of large leftover
planetesimals predicted by the nebular theory
Was our solar system destine to be?
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Nebular theory accounts for all the major features of our solar system
The entire process of planet formation probably took no more than a few tens of millions of
years, about 1% of the current age of the solar system
Was t inevitable that our solar nebular would form the solar system we see today? Probably not.
The first stages of planet formation were orderly and inevitable according to the nebular theory.
But, the final stages of accretion and giant impacts in particular are inherently random and
probably would not happen again in the same way
Summary of process by which our solar system formed, according to nebular theory
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Contraction of Solar Nebula: as it contracts, the cloud hearts, flattens, and spins faster,
becoming a spinning disk of dust and gas
o A large diffuse interstellar gas cloud (solar nebula) contracts due to gravity
o The sun will be born in the center]planets will form in the disk
Condensation of solid particles: hydrogen and helium remain gaseous, but other material can
condense into solid “seeds” for building planets
o Warm temps allow only metal/rock seeds to condense in inner solar system
o Cold temperatures allow seeds to contain abundant ice in the outer solar system
Accretion of planetesimals: solid “seeds” collide and stick together. Larger ones attract others
with their gravity, growing bigger still
o Terrestrial planets are built from metal and rock
o The seeds of jovian planets grow larger enough to attract hydrogen and helium gas,
making them into giant mostly gaseous planets, moons form in disks of dust and gas
that surround the planets
Clearing the nebula: the solar wind blows remaining gas into interstellar space
o Terrestrial plants remain in the inner solar system
o Jovian planets remaining in the outer solar system
o “leftovers” from the formation process become asteroids (metal/rock) and comets
(mostly ice)
8.5 The Age of the Solar System
How does radioactivity reveal an object’s age?
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By the age of a rock we mean the time since its atoms became locked together in their present
arrangement, which is in cases means the time since the roc last solidified
The method by which we measure the age of a rock is known as radiometric dating
o This method relies on careful measurement of the proportions of various atoms and
isotopes in the rock. Remember that each chemical element is uniquely characterized by
the number of protons in its nucleus. Different isotopes of the same element differ only
in their number of neutrons
Radioactive isotope has a nucleus that can undergo spontaneous change or radioactive decay,
such as breaking apart of having one of the protons turn into a neutron
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We say that potassium-40 is the parent isotope, because it is the original isotope before the
decay and argon 40 is the daughter isotope left behind by the decay process
Parent nuclei half-life, the time it could take for half of the parent nuclei in the collection to
decay
When did the planets form?
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Radiometric dating tells us how long it has been since a rock solidified, which is not the same as
the age of a planet as a whole
The oldest Earth rocks are about 4 billion years old, but even these are not as old as earth itself
because earth’s entire surface has been reshaped through time
Moon’s rocks brought back by the Apollo astronauts date to as far back as 4.4 billion years ago.
Although they are older than earth rocks, these moon rocks must still be younger than the moon
itself. The ages of these rocks also tell us that the giant impact thought to have created the
moon must have occurred more than 4.4 billion years ago
Meteorites that have fallen to earth are our source of such rocks. Many meteorites appear to
have remained unchanged since they condensed and accreted in the early solar system. Careful
analysis of radioactive isotopes in meteorites shows that the oldest ones formed about 4.55
billion years ago, so this time must mark the beginning of accretion in the solar nebula
Because accretion occurred within a few tens of millions of years, earth and the other planets
formed about 4.5 billion years ago. Thus, the age of our solar system is only about a third of the
14 billion year age of our universe
Putting Chapter 8 into Perspective
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The nebular theory of solar system formation gained wide acceptance because if its success in
explaining he major characteristics of our solar system
Most of the general features of the solar system were determined by processes that occurred
very early in the solar system’s history
Chance events may have played a large role in determining how individual planets turned out.
No one knows how different our solar system might be if it started over
We have learned the age of our solar system –about 4 ½ billion years. This age agrees with the
ages we estimate through a variety of other techniques, making it clear that we are recent
arrivals on a very old planet
8.1 The Search for Origins
What properties of our solar system must a formation theory explain? A successful theory must
explain patterns of motion, the differences between terrestrial and jovian planes, the presence of
asteroids and comets, and exceptions to the rules
What theory best explains the features of our solar system? The nebular theory which holds that he
solar system formed from the gravitational collapse of a great cloud of gas and dust, successfully
explains all the major features of our solar system
8.2 The Birth of a Solar System
Where did the solar system come from? The cloud that gave birth to our solar system was the product
of recycling of gas through many generations of stars without our galaxy. This material consisted of 98&
hydrogen and helium, and 2% all other elements combined
What caused the orderly patterns of motion in our solar system? A collapsing gas cloud tends to heat
up, spin faster, and flatten out as it shrinks. Thus, our solar system began as a spinning disk of gas and
dust. The orderly motions we observe today all came from the orderly motion of this spinning disk
8.3 The Formation of Planets
What are there to major types of planets? The two types of planets formed from two different types of
solid “seeds” that condensed from gas in the solar nebula. Within the frost line, temperatures were so
high that only metal and rock could condense; beyond the frost line, cooler temperatures also allowed
more abundant hydrogen compounds to condense into ice
How did the terrestrial planets form? Terrestrial planets formed inside the frost line, where accretion
allowed tiny, solid grains of metal and rock to form into planetesimals that ultimately merged to make
the planets we see today
How did the jovian planets form? Accretion built ice-rich planetesimals in the outer solar system, and
some of these icy planetesimals grew large enough for their gravity to draw in hydrogen and helium gas,
building the jovian planets
8.4 The Aftermath of Planet Formation
Where did asteroids and comets come from? Asteroids are the rocky leftover planetesimals of the inner
solar system, and comets are the ice rich leftover planetesimals of the outer solar system
How do we explain “exceptions to the rules”? Most of the exceptions probably arose from collisions or
close encounters with leftover planetesimals, esp. during the heavy bombardment that occurred early
in the solar system’s history
How do we explain the existence of our moon? Our moon is probably the result of a giant impact
between a Mars size planetesimals and the young earth. The impact blasted material from earth’s outer
layers into orbit, where it reaccreted to form the moon
Was our solar system destined to be? A solar system like ours was probably destined o form from the
collapse of the solar nebula, but individual planets were probably affected by random events that could
have happened differently or not at all
8.5 Age of the Solar System
How does radioactivity reveal an object’s age? Radiometric dating is based on carefully measuring the
proportions of radioactive isotopes and their decay products within rocks. The ratio of the isotopes
changes with time and provides a reliable measure of the rick’s age
When did the planets form? The planets began to accrete in the solar nebula about 4 ½ billion years
ago, a fact we determine from radiometric dating of the oldest meteorites