Download Solar System Formation

Solar System Formation
Solar System Formation
Question: How did our solar system and other planetary systems form?
“Comparative planetology” has helped us understand
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Compare the differences and similarities among the objects in our solar system
Figure out what physical processes could have led to them
Then construct a model of how our solar system formed based on this
------This model must explain our own solar system…
…but might or might not explain other planetary systems
If not, modify the model to accommodate discrepancies
That is the scientific process
Let’s look at the solar system characteristics comparative planetology has to work with…
Solar System Formation -- Characteristics of Our Solar System
1. Large bodies have orderly motions and are isolated from each other
– All planets and most moons have nearly circular orbits going in the same
direction in nearly the same plane
Solar System Formation -- Characteristics of Our Solar System
1. Large bodies have orderly motions and are isolated from each other
– All planets and most moons have nearly circular orbits going in the same
direction in nearly the same plane
– The Sun and most of the planets rotate in this same direction as well
Solar System Formation -- Characteristics of Our Solar System
1. Large bodies have orderly motions and are isolated from each other
– All planets and most moons have nearly circular orbits going in the same
direction in nearly the same plane
– The Sun and most of the planets rotate in this same direction as well
– Most moons orbit their planet in the direction it rotates
Solar System Formation -- Characteristics of Our Solar System
2. Planets fall into two main categories
– Small, rocky terrestrial planets near the
Sun
– Large, hydrogen-rich jovian planets far
from the Sun
Solar System Formation -- Characteristics of Our Solar System
2. Planets fall into two main categories
Solar System Formation -- Characteristics of Our Solar System
3. Swarms of asteroids and comets populate the solar system
– Asteroids are concentrated in the asteroid belt
Solar System Formation -- Characteristics of Our Solar System
3. Swarms of asteroids and comets populate the solar system
– Asteroids are concentrated in the asteroid belt
– Comets populate the regions known as the Kuiper belt and the Oort cloud
Solar System Formation -- Characteristics of Our Solar System
4. Several notable exceptions to these general trends stand out
– Planets with unusual axis tilts
– Surprisingly large moons
– Moons with unusual orbits
Solar System Formation -- Characteristics of Our Solar System
…which any successful theory must account for…
1. Large bodies in the solar system have orderly motions and are isolated from each
other
– All planets and most moons have nearly circular orbits going in the same
direction in nearly the same plane
– The Sun and most of the planets rotate in this same direction as well
– Most moons orbit their planet in the direction it rotates
2. Planets fall into two main categories
– Small, rocky terrestrial planets near the Sun
– Large, hydrogen-rich jovian planets farther out
• The jovian planets have many moons and rings of rock and ice
3. Swarms of asteroids and comets populate the solar system
– Asteroids are concentrated in the asteroid belt
– Comets populate the regions known as the Kuiper belt and the Oort cloud
4. Several notable exceptions to these general trends stand out
– Planets with unusual axis tilts
– Surprisingly large moons
– Moons with unusual orbits
Solar System Formation – The Nebular Theory
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The nebular theory is the best current explanation of our solar system
It is not a new idea…
…some well-known 18th-century philosophers suggested it:
– Emanuel Swedenborg
– Immanuel Kant
Like all scientific theories, it is still being refined and improved
Solar System Formation – The Nebular Theory
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It starts with cold interstellar clouds of gas and dust
These clouds are mostly hydrogen and helium from the Big Bang
But they contain heavier elements that were not formed in the Big Bang
Astronomers call these “metals” (even though they’re not necessarily classified as such)
Where did these heavier elements come from?
They came from stars!
Solar System Formation – The Nebular Theory
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Stars make heavier elements from lighter ones through nuclear fusion
Solar System Formation – The Nebular Theory
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Stars make heavier elements from lighter ones through nuclear fusion
The heavy elements (the “metals”) mix into the interstellar medium when the stars die
Solar System Formation – The Nebular Theory
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Stars make heavier elements from lighter ones through nuclear fusion
The heavy elements (the “metals”) mix into the interstellar medium when the stars die
New stars form from the enriched gas and dust, and the cycle continues
Solar System Formation – The Nebular Theory
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Stars make heavier elements from lighter ones through nuclear fusion
The heavy elements (the “metals”) mix into the interstellar medium when the stars die
New stars form from the enriched gas and dust, and the cycle continues
And at the same time stars are forming …planetary systems can form
Here’s how it works…
Solar System Formation – The Nebular Theory
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A large cloud -- a nebula perhaps 1 light year across -- floats in space
Solar System Formation – The Nebular Theory
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A large cloud -- a nebula perhaps 1 light year across -- floats in space
…WHY?... Local density increase
The cloud begins to collapse
Solar System Formation – The Nebular Theory
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A large cloud -- a nebula perhaps 1 light year across -- floats in space
The cloud begins to collapse -- local density increase
Conservation of angular
…WHY?...
As it collapses it begins to spin faster
momentum
Solar System Formation – The Nebular Theory
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A large cloud -- a nebula perhaps 1 light year across -- floats in space
The cloud begins to collapse -- local density increase
As it collapses it begins to spin faster -- conservation of angular momentum
…WHY?...
Collision and motion effects
And as it spins faster, it flattens out
Solar System Formation – The Nebular Theory
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A large cloud -- a nebula perhaps 1 light year across -- floats in space
The cloud begins to collapse -- local density increase
As it collapses it begins to spin faster -- conservation of angular momentum
And as it spins faster, it flattens out -- collision and motion effects
At the same time, it begins to heat up in the center
…WHY?...
Conversion of gravitational
potential energy into thermal
energy
Solar System Formation – The Nebular Theory
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A large cloud -- a nebula perhaps 1 light year across -- floats in space
The cloud begins to collapse -- local density increase
As it collapses it begins to spin faster -- conservation of angular momentum
And as it spins faster, it flattens out -- collision and motion effects
At the same time, it begins to heat up in the center -- conversion of potential to thermal energy
When it gets hot enough, a star forms in the center
And in the disk around the forming star, planets can form
What type of planets can form depends on what the cloud is made of
Solar System Formation – The Nebular Theory
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This is what our own cloud—the solar nebula—was made of
But how do we know this?
Solar System Formation – The Nebular Theory
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This is what our own cloud—the solar nebula—was made of
But how do we know this? This is how…
…the absorption line spectrum of the Sun
It tells us the composition of the gas on the surface of the Sun
Solar System Formation – The Nebular Theory
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This is the composition of the Sun’s surface gas – its atmosphere
We think the solar nebula had the same composition
But is it reasonable to say this?
Solar System Formation – The Nebular Theory
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After all, the solar nebula collapsed 4.6 billion years ago
The Sun’s been making new atoms with nuclear fusion ever since
Wouldn’t this change the composition of the Sun’s atmosphere?
The answer has to do with where the new atoms are being made…
Solar System Formation – The Nebular Theory
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The fusion reactions making new atoms generate the energy that gives us sunlight
The critical question is, Where are these fusion reactions taking place?
– The answer: In its core
– And that’s in the Sun’s center, far from the surface
So the surface layers should be essentially unchanged
And their composition should be very similar to the solar nebula
Solar System Formation – The Nebular Theory
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So it seems reasonable to say that the composition of Sun’s atmosphere is the same as the
composition of the solar nebula
Solar System Formation – The Nebular Theory
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The key to the nebular theory is the condensation temperature of these materials, at which
they will condense into solid form
The nebula was initially very cold, so everything except H and He was in solid form
But it heated up as it collapsed…
…and the temperature was different at different distances from the center
Solar System Formation – The Nebular Theory
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This graph shows a modeled temperature profile of
the solar nebula
The temperature was hottest in the center, and went
down away from the center
There was a mixture of metals, rocks, and hydrogen
compounds throughout the nebula
These could only be solid where the temperature
was below their condensation temperature
So different chemical components of the nebula
condensed at different distances
A mixture of solid rock and metal existed out to about
4.5 AU from the center
At 4.5 AU, the temperature dropped low enough for
hydrogen compounds to condense, too
The boundary between where they could and could
not condense is called the “frost line”, “snow line”, or
“ice line”
Solar System Formation – The Nebular Theory
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The frost line was located between the present-day orbits of Mars and Jupiter
Solar System Formation – The Nebular Theory
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Once materials condense into solid form they can stick together
This is called “accretion”
And it launches the next step in planet formation…
“Core accretion”
Solar System Formation – The Nebular Theory
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Small clumps grow like
snowballs until they
become planetesimals
the size of moons
The planetesimals collide
and coalesce until
planets are born
This suffices to explain
terrestrial planet
formation, but jovian
planets require adding an
extra layer to the
process...literally
Solar System Formation – The Nebular Theory
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Jovian planets also begin
by core accretion
But this happens in the
outer solar system,
beyond the frost line,
where there is 3x more
solid material available
So the cores get much
bigger (10-15 times the
mass of Earth)
Solar System Formation – The Nebular Theory
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Unlike terrestrials, the jovian
cores gather gas from the
nebula and retain it
This is because:
• They are more massive…
• stronger gravity
• It is colder…
• lower escape speeds
for gas
The result is a “gas giant” -- a
jovian planet
Solar System Formation – The Nebular Theory
• There is an alternative to the core accretion model
• “disk-instability"
• Cool gas beyond the frost line collapses directly into jovian planets
• This takes much less time than the "core-accretion model"
• And this makes it consistent with claims that some jovians form faster than would be
possible by core-accretion
Solar System Formation – The Nebular Theory
• It is not known for certain whether jovian planets form by core accretion or disk instability
• Perhaps they form one way in some circumstances and the other way in others
• The main difference is in the way the process begins
• Once it starts, the nebular gas swirls in an accretion disk around the growing jovian
planet
• In that accretion disk, moons would form around the jovian planet like planets formed in
the solar nebula around the Sun
Solar System Formation – The Nebular Theory
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The process of jovian
and terrestrial planet
formation was finalized
by the infant Sun
As the Sun became a
star, a strong solar wind
blew out from it
This cleared the
remaining nebular gas
away
And this halted the
growth of the planets
from the solar nebula`
A successful theory must explain our solar system
So how does this one do?
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each
other :
– All planets and most moons have nearly circular orbits going in the same
direction in nearly the same plane
– The Sun and most of the planets rotate in this same direction as well
– Most moons orbit their planet in the direction it rotates
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Planets fall into two main categories:
– Small, rocky terrestrial planets near the Sun
• No rings and few, if any, moons
– Large, hydrogen-rich jovian planets farther out
• Rings of rock and ice and many moons
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Swarms of asteroids and comets populate the solar system:
– Asteroids are concentrated in the asteroid belt
– Comets in the Kuiper belt and the Oort cloud
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Several notable exceptions to these general trends stand out:
– Planets with unusual axis tilts
– Surprisingly large moons
– Moons with unusual orbits
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each
other:
– All planets and most moons have nearly circular orbits going in the same
direction in nearly the same plane
The planets and moons orbit in the direction that the solar nebula was
spinning
– The Sun and most of the planets rotate in this same direction as well
Conservation of angular momentum
– Most moons orbit their planet in the direction it rotates
Conservation of angular momentum
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories:
– Large, hydrogen-rich jovian planets far from the Sun, with rings of rock and ice
and many moons
– Small, rocky terrestrial planets near the Sun with no rings and few, if any, moons
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories:
– Large, hydrogen-rich jovian planets far from the Sun, with rings of rock and ice
and many moons
Outside the frost line, lower temperatures led to condensation of hydrogen
compounds (ices) along with metals and rocks
Cores large enough to capture gas could form
Moons made of rock and ice formed in the swirling jovian nebula around each
growing jovian planet
Rings appear when some of those moons get torn apart by tidal forces
– Small, rocky terrestrial planets near the Sun with no rings and few, if any, moons
Inside the frost line, higher temperatures meant that only metals and rocks
could condense, providing less than 1/3 as much material and leading to small,
rocky cores
The smaller cores and higher temperatures prevented gas capture, and moon
and ring formation
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories
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Swarms of asteroids and comets populate the solar system:
– Asteroids mainly in the asteroid belt
• The asteroids in the asteroid belt are a “frustrated planet”
• The Trojan asteroids are planetesimals that became locked in gravitational
"wells" caused by the gravity of Jupiter and the Sun
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories
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Swarms of asteroids and comets populate the solar system:
– Asteroids mainly in the asteroid belt
– Comets in the Kuiper belt and the Oort cloud
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories
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Swarms of asteroids and comets populate the solar system:
– Asteroids mainly in the asteroid belt
– Comets in the Kuiper belt and the Oort cloud
• The icy planetesimals that formed beyond the frost line near Jupiter and Saturn
were thrown in random orbits, forming the Oort Cloud
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories
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Swarms of asteroids and comets populate the solar system:
– Asteroids mainly in the asteroid belt
– Comets in the Kuiper belt and the Oort cloud
• Those that formed beyond Neptune were relatively unaffected, and make up the
Kuiper Belt
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories
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Swarms of asteroids and comets populate the solar system:
– Asteroids mainly in the asteroid belt
– Comets in the Kuiper belt and the Oort cloud
• Those that formed near Uranus and Neptune were flung into the inner solar
system, and some provided water for Earth and other terrestrial planets
How Does the Nebular Theory Do?
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Large bodies in the solar system have orderly motions and are isolated from each other
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Planets fall into two main categories
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Swarms of asteroids and comets populate the solar system:
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Several notable exceptions to these general trends stand out:
– Moons with unusual orbits
Unusual (backward) orbits indicate captured objects
– Planets with unusual axis tilts
The unusual axis tilts can be explained by giant impacts during
the “Era of Heavy Bombardment”
– Surprisingly large moons
• The “surprisingly large moon” is our own
• It is unlikely that it formed at the same time as Earth
because its density is lower
• But Earth is too small to have captured it
• It too can be explained by a giant impact
Summary of Nebular Theory
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen
compounds)
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
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Initially the nebula was very cold, and all of the dust was in the form of solid particles
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
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The nebula began to contract, spin faster and faster, flatten out, and heat up
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
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As it heated, the dust particles vaporized
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
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The nebula was hottest in the center
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
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The farther away from the center, the cooler it got
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
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Different types of dust resolidified at different distances from the center depending on
their condensation temperatures
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
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Close to the center only rock and metal dust was able to condense
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
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Far from the center, beyond the “frost line”, hydrogen compounds could also condense
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
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The solid particles stuck together (“accreted”), forming bigger and bigger clumps until
they were the size of planets
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets
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Inside the frost line, where only rock and metal could condense, small terrestrial planets
formed
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets
Inside the frost line, where only rock and metal could condense, small terrestrial planets formed
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Beyond the frost line, hydrogen compounds as well as rock and metal could condense,
and much larger jovian planet cores could form
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets
Inside the frost line, where only rock and metal could condense, small terrestrial planets formed
Beyond the frost line, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores
could form
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The jovian cores were massive enough, and the temperatures cold enough, to attract
and retain gas from the surrounding nebula, becoming our “gas giant” planets
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets
Inside the frost line, where only rock and metal could condense, small terrestrial planets formed
Beyond the frost line, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores
could form
The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding
nebula, becoming our “gas giant” planets
When the Sun matured into a star, the solar wind blew out the remaining gas and
arrested the development of the planets
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets
Inside the frost line, where only rock and metal could condense, small terrestrial planets formed
Beyond the frost line, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores
could form
The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding
nebula, becoming our “gas giant” planets
When the Sun matured into a star, the solar wind blew out the remaining gas and arrested the development of the planets
Planetesimals still remained, and these collected into the asteroid belt, Kuiper belt, or
Oort cloud—or were captured by planets as moons—or collided with the planets, in
some cases altering their axis tilts
Summary of Nebular Theory
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There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)
Initially the nebula was very cold, and all of the dust was in the form of solid particles
The nebula began to contract, spin faster and faster, flatten out, and heat up
As it heated, the dust particles vaporized
The nebula was hottest in the center
The farther away from the center, the cooler it got
Different types of dust resolidified at different distances from the center depending on their condensation temperatures
Close to the center only rock and metal dust was able to condense
Far from the center, beyond the “frost line”, hydrogen compounds could also condense
The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets
Inside the frost line, where only rock and metal could condense, small terrestrial planets formed
Beyond the frost line, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores
could form
The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding
nebula, becoming our “gas giant” planets
When the Sun matured into a star, it emitted a strong solar wind that blew out the remaining gas and arrested the development
of the planets
Planetesimals still remained, and these collected into the asteroid belt, Kuiper belt, or Oort cloud—or were captured by planets
as moons—or collided with the planets, in some cases altering their axis tilts
When did all this happen, and how do we know?
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It was 4.6 billion years ago that our solar system formed
But how do we know this?...
From radiometric dating, using radioactive isotopes
Every element exists as a mixture of isotopes
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Some isotopes, like 14C, are radioactive
Every radioactive isotope has its own half-life
If a sample has a certain amount of radioactivity, after one half-life it will have half as
much
With radiometric dating, you estimate the initial amount of radioactivity in a sample,
and determine its age from the amount that’s left
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When did all this happen?
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Carbon-14 (14C) provides a familiar example of
radiometric dating
It’s used to date mummies, archaeological artifacts, and
the like
The diagram shows how it works…
14C is useful for dating things up to ~60,000 years old
But its half-life of ~5700 years is too short to be useful
in measuring the age of our solar system
When did all this happen?
• One isotope whose half-life is long enough is potassium-40 (40K)
• 40K decays to argon-40 (40Ar) with a half-life of 1.25 billion years
• 40K is found in rock along with 40Ar from its decay
• If the rock is melted, the 40Ar escapes as a gas
• When the rock cools and resolidifies, it contains 40K, but no 40Ar
http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/courseware/Chronology/movies/Melting.html
When did all this happen?
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One isotope whose half-life is long enough is potassium-40 (40K)
40K decays to argon-40 (40Ar) with a half-life of 1.25 billion years
40K is found in rock along with 40Ar from its decay
If the rock is melted, the 40Ar escapes as a gas
When the rock cools and resolidifies, it contains 40K, but no 40Ar
So by measuring the ratio of 40Ar to 40K in a piece of rock, you can determine how long
it’s been since the rock solidified
When did all this happen?
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How can 40K be used to date the formation of the solar system?
The solar system formed from the solar nebula, a vast cloud of gas and (solid) dust
The solid (cold) dust particles initially contained both 40K and 40Ar
But as the nebula contracted and heated, the dust vaporized, and the 40Ar was released
When the dust condensed to solid form again, it contained 40K, but not 40Ar
If rocks accreted from this dust could be found unchanged, their age would be the age of the
solar system
• This is a type of meteorite called a “chondrite”
• Chondrites have not melted since they accreted
from the nebular dust when the solar system
formed
• So whatever 40Ar they contain has appeared
since then
When did all this happen?
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How can 40K be used to date the formation of the solar system?
The solar system formed from the solar nebula, a vast cloud of gas and (solid) dust
The solid (cold) dust particles initially contained both 40K and 40Ar
But as the nebula contracted and heated, the dust vaporized, and the 40Ar was released
When the dust condensed to solid form again, it contained 40K, but not 40Ar
If rocks accreted from this dust could be found unchanged, their age would be the age of the
solar system
• This is a type of meteorite called a “chondrite”
• Chondrites have not melted since they accreted
from the nebular dust when the solar system
formed
• So whatever 40Ar they contain has appeared
since then
• Radiometric dating using 40Ar/40K shows that
chondrites formed 4.6 billion years ago
• The age determined using other isotopes is
similar, and this gives us confidence that it is
correct
Is ours the only solar system?
• Observation of other stars reveals many of them surrounded by disks of dust and gas
• These protoplanetary disks are exactly what the nebular theory predicts
• But until the 1990s, there was no convincing evidence for planets around other stars, now called
extrasolar planets or exoplanets
• As of today, nearly 3500 exoplanets have been confirmed
Detecting Extrasolar Planets by Radial Velocity
• The first extrasolar planets were found by the radial velocity technique
• This technique depends on the gravitational effect of a planet on its star
• This image shows what would happen
if Jupiter and the Sun were the only
objects in our solar system
• They both would orbit around their
common center of mass
Detecting Extrasolar Planets by Radial Velocity
• In a system with more than one planet, the star’s
movement can be complicated
• This image shows the path of the Sun around the
solar system’s center of mass
• The motion is mainly due to the effects of Jupiter
and Saturn, because they are so massive
• Other stars are affected similarly by their planets
Detecting Extrasolar Planets by Radial Velocity
• This back-and-forth motion of the star along the line of sight from Earth causes Doppler-shifting
of its light
• And this can be detected in a light curve
Detecting Extrasolar Planets by Radial Velocity
• After recording the light curve, computer modeling is used
to determine how many and what type of planets are there
• This light curve led to the discovery of the first planet
orbiting a Sun-like star – 51 Pegasi
• It is fairly simple, and is consistent with a single planet
• The period of the wobbling gives you the orbital period and
distance (~0.05AU…how?)
• The magnitude gives you the minimum mass of the planet
(~.5MJupiter…how?)
Detecting Extrasolar Planets by Radial Velocity
• This light curve is more complicated
Detecting Extrasolar Planets by Radial Velocity
• This light curve is more complicated
• It is consistent with the triple-planet system at right
Detecting Extrasolar Planets by Transit
• In the transit method (used by the Kepler SpaceTelescope), astronomers look for a periodic
decrease in the light from a star
• The decrease indicates that a planet is transiting the star, blocking some of the starlight
• How often and how much the light decreases gives information about the planet’s orbit and size
• Combining this info with radial velocity info can give the density of the planet
Detecting Extrasolar Planets by Imaging
• Planets do not emit their own light, and so are hard to see in telescopes, but a small number of
extrasolar planets have been found this way
• The red object in the image above is the first of them
• It is orbiting a brown dwarf (the brighter object)
Detecting Extrasolar Planets
• A few exoplanets have been found by gravitational microlensing
• In this method, the light from a distant star is bent by the gravity of an intervening star
• If the intervening star has a planet, the planet’s gravity adds to the effect in a recognizable way
• A statistical analysis of planets detected by this technique led to the prediction that each star in
the Milky Way has ~1.6 planets
• You can see a list of all the known extrasolar planets and more at
The Extrasolar Planets Encyclopedia
NASA Exoplanet Archive
Detecting Extrasolar Planets
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At one time, most confirmed exoplanets were very large and very close to their star
This was not because extrasolar systems more like ours do not exist (they do)
It was simply a reflection of the methods that are used
They tend to be more sensitive to large planets close to their star
Detecting Extrasolar Planets
• But the existence of “hot Jupiters” – jovian planets very close to their star – is not
consistent with the nebular theory we have discussed
• Following the scientific method, we need to see if there is some way the nebular theory
can be modified to account for this
• And there is…
Detecting Extrasolar Planets
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It’s a matter of timing…
In our own solar system, the waking Sun expelled all the nebular gas and dust
The strong solar wind produced when fusion was about to start blew it all away
But if that hadn’t happened, the planets and the nebular disk would interact…
Detecting Extrasolar Planets
• …and the planets would migrate inward
• The star still blows the nebula away when it finally comes alive
• But a jovian planet that formed beyond the frost line might find itself, after migration,
closer to its star than Mercury is to our Sun
• And the nebular theory lives to fight another day
TRAPPIST-1
TRAPPIST-1
TRAPPIST-1