Download Formation of the Solar System Chapter 8

Formation of the Solar System
Chapter 8
To understand the formation of the solar system one
has to apply concepts such as:
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Conservation of angular momentum
Conservation of energy
The theory of the formation of the solar system need to
explain:
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The pattern of the sense of rotation of planets
The plane of the orbits of planets
The sense of rotation of satellites
The different composition of terrestrial, Jovian and dwarf planets
Some of the patterns we can find in the solar system:
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All planets orbit the Sun counterclockwise (as seen from the north pole)
Nearly all the planetary orbits lie in a plane
Almost all planets rotate in the same direction with their axes perpendicular
to the orbital plane
Most satellites revolve around planets in the same direction that the planet
rotates on its axis.
But there are some exceptions
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Venus rotates backwards (Rotational axis tilted close to 189 degrees)
Uranus rotates on its side (Rotational axis tilted close to 90 degrees)
Most small moons do not share the orbital plane of the planet
Triton (Satellite of Neptune) orbit the planet in opposite sense
Earth is the only terrestrial planet with a large moon
A model for the formation of the solar system has to account for:
• Different composition of planets (rocky, gaseous, icy)
• Existence of many asteroids and comets
Angular Momentum
• Objects rotating around a point have angular momentum.
• Simplest case ( a small sphere orbiting a larger mass)
L=mxvxr
L :angular momentum
m: mass of small sphere
v: velocity of the small sphere
r :separation between the small sphere and the larger object
• Conservation of angular momentum  if r changes, v must change
(ice skaters)
But the value of L remains constant
The nebular hypothesis (and the rejected
collision theory)
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The idea that the solar system was born from the collapse of a cloud of dust and gas
for proposed by Immanuel Kant (1755) and by Pierre Simon Laplace 40 years later.
During the first part of the 20th century, some proposed that the solar system was
the result of a near collision of the Sun with another star. Planets formed from
debris of the collision. But we know now that collision (or near collisions) between
two stars are very, very rare.
Considering that collision are rare, the proposed idea of the collision may explain a
unique event on how our planetary system formed but not how other planetary
systems formed.
During the rest of the 20th century, new ideas and theories about the formation of
stars (and possible planets) made this collision theory obsolete and was discarded
In 1995, the first exoplanet (planets orbiting other stars) was discovered
Many more planets have been found so far in the solar neighborhood ( close to
1000 confirmed and more than 2000 possible ones). It is clear now that formation
of planets is not a rare event.
Any theory about the formation of planetary system must explain the formation of
planets, not as a single unique and rare event but more like a common event in the
Galaxy
The nebular theory
• Stars are born from the collapse of an interstellar cloud of
dust and gas.
• Planets form as part of the process of the formation of a star
• As part of the formation of a star, a proto planetary disk forms
around the star
• Planets are formed from the collapse of the material of a
proto planetary disk
• The basic and simple idea suggested by Kant and Laplace
needed a lot of modifications before it became the actual
nebular theory for the formation of the solar system and
other planetary system
Where did all the dust and gas that formed the nebula
originated?
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All the dust and gas comes from “recycled” material in the Galaxy.
The first original stars in the Galaxy were formed from H and He.
The rest of the heavier elements were synthesized in the interior of the stars
as part of the process of energy generation inside the star (fusion,
conversion of lighter elements into heavier elements)
The most massive stars are able to generate the heaviest elements. They
end their life in a spectacular explosion called supernova.
All that material is thrown into the interstellar space and contaminate the
original clouds of H and He gas.
Heavier elements and lighter elements combine and form molecules. These
molecules form aggregates which become dust particles.
Gas and dust form clouds of “contaminated” material.
From that material new stars are formed, already containing heavier
elements.
The fusion process inside of these stars and the evolution of these stars
continues enriching the material in the clouds of dust and gas.
The Orion nebula
An example of an interstellar cloud of gas and dust where new stars are being
born
A schematic representation of the process of contamination of
interstellar clouds
The cloud of gas that gave birth to our solar system resulted from the recycling of
material through many generations of stars within our galaxy.
Nebular Hypothesis
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Event such as the impact of a shock wave from the
explosion of a supernova or the passage of a compression
wave in the galaxy, (spiral density wave) will triggers the
gravitational collapse (collapse due to its own gravity) of
the cloud
Gravitational force
Radiation pressure
Gravitational force
Radiation pressure
Centrifugal force
Nebular Hypothesis
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• The cloud collapse into a
disk
• This is called a
protoplanetary disk.
• Planets form from the
material in the disk
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The collapse cause an increase in density
Increased density -> increased gravity -> more material gets
sucked in -> center heats up
Because of conservation of angular momentum, as the
nebula collapses, it decreases it radius and it will spin faster
The density and temperature in the center increases
Gravitational potential energy is converted into kinetic
energy
As the temperature keeps increase in the center , it will reach
the fusion point (It needs to reach about 10 million K in the
case of the Sun to convert H into He).
Once the central body start generating energy, a new star is
born
Outer, cooler particles suffer repeated
collisions, building planet-sized bodies from
dust grains ( Process called accretion)
The presence of dust is a key element in the
formation of small particles which will stick
together and form planetesimals. Gas
molecules by themselves will not stick and
form planetesimals
Nebular Hypothesis
• Accretion clears a gap near the new star
• Most of the gas is accreted into the central
star and into forming planets
•Young stellar activity produces
high stellar winds, which blows
off any remaining gas and leaves
an embryonic solar system
Nebular Hypothesis
The sequence of the collapse and formation of a planetary system
A better sequence of a collapsing cloud and the
formation of planets
A detailed view of the process of accretion of
planetesimals and formation of planets
Planetary Compositions
The temperature decreases with distance from the Sun and
regulates:
• Which elements actually condense
• Which compounds are formed from the elements
• At what rates the compounds are formed.
Volatile species will only be
stable beyond a point in
which the temperature in
the disk s low enough.
Heavier elements (Like
silicon and its compounds)
can condense at higher
temperatures in the inner
part of the disk
This is why the inner
planets are rock-rich and
the outer planets gas- and
ice-rich
Formation of terrestrial planets
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Terrestrial planets formed from the accretion of smaller bodies called
planetesimals
The process started with small solid particles that condensed from the
dust and gas in the nebula. Even with the high temperatures, the dust was
able to condense. These particles were too small to “stick” together by
gravitational forces. Electrostatics forces may be responsible for them to
stick together .
Once they grew bigger, gravitation was responsible for them to attract
more particles and began forming planetesimal.
Collision were frequent during this stage. Some of the small planets and
planetesimals may have been shattered. Only the largest ones may have
survived and became planets and grew large to form the terrestrial
planets.
The formation of Jovian planets
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The process of accretion also took place at the distance in which the
Jovian planets formed.
But at that distance, condensation of ice was possible due to the lower
temperature.
The planetesimals were formed from condensation of large amounts of
ice, and some metals and rocks.
Planetesimals grew bigger and faster. They grew large enough to attract
and retain H and He
They attracted so much H and He so that the original “seeds” of rock and
metal became small compared with the gas.
Formation of Jovian planets
• Beyond the frost line, planetesimals could accumulate ice
• Hydrogen and other low-mass compounds are more abundant
(98%) than rock/metal (2%) so Jovian planets got bigger, faster.
Outside the “Frost line”, there was more efficiently capture (by their bigger gravitational
pull) of H/He gas before it was dispersed by the Sun’s radiation and solar wind
+ dust particles
Nebular gas collapsed onto rock-ice cores of perhaps 10 Earth masses
Rocky, icy core
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Each young Jovian planet formed its own “miniature” solar nebula out
of the gas around them.
The satellites of the Jovian planets formed out of these gas and dust
disks.
What ended the process of planet formation?
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A large part of the H and He of the original nebula never became part of
the solar system.
Once the Sun formed, it developed a strong solar wind. The solar wind are
charged particles, electrons, and protons ejected by the Sun. Stellar winds
is common in young stars.
The strong solar wind blew away into the interstellar space all the material
including H and He that were not captured in planetesimals and planets.
Clearing of the gas in the disk sealed the fate of the planets: they did not
have more material to accrete.
If gas may have remained longer, the composition of the planets, specially
the terrestrial planets may be different.
This may be the case of other planetary systems!
Lost of angular momentum of the Sun
Magnetic field slow down the rotation of the Sun
Three kinds of planets . . .
• Nebular material can be divided into “gas” (mainly H/He), “ice” and
“rock” (including metals)
• Planets tend to be dominated by one of these three end-members
Gas-rich
Rock-rich
Ice-rich
Ratio
100:1:0.1
Terrestrial (silicate) planets
Venus
Earth
Mars
Mercury
Moon
Io
Ganymede
• Consist mainly of silicates (compounds of silicon oxide) and iron
• Volatile elements (H, He) uncommon in the inner solar system
because of the initially hot conditions. (some were supplied by
comets)
• Satellites like Ganymede have similar structures but have an ice
layer on top (volatiles are more common in the outer nebula)
Gas and Ice Giants
90% H/He
75% H/He
10% H/He
10% H/He
• Jupiter and Saturn consist
mainly of He/H with a rockice core of ~10 Earth masses
• Uranus and Neptune are
primarily ices covered with a
thick He/H atmosphere
• Their cores grew more slowly
and captured less gas
Evidence of planet formation beyond
our Solar System
• Early stages of a planetary system formation can be imaged directly
• Dust disks have large surface area and radiate effectively in the
infrared
Thick disk
Hubble image of a young solar
system. Young star clearing part of the
gas
A proto planetary disk (proplyd) in the
Orion nebula
Proto-planetary Disks
-IR image of a disk around a star
-Images in IR obtained at
wavelengths from 8.7 to 24.3
micrometers
- Dust disk around a young star
-The gas has been blown out of
the system
- The star Beta Pictoris is an
example
An artist’s impression of a young star and its proto
planetary disk in the process of forming planets
The young Sun
solid planetesimals
gas/dust
nebula
How do we explain the existence of our moon?
• Our moon is large compared with the size of
the Earth
•Its composition is not similar to the
composition of the Earth. Its density is lower
and it has less iron.
•It did not form from the same material of the
Earth
•The most accepted theory about the
existence of our moon is the impact of a large
size body (Mars-size) with the Earth.
•The material ejected from the impact may
have accreted and condensed to form the
moon.
• Where did this object came from? Some
proto planets may have been large (Marssize) and one of them may have collided with
the Earth
•The composition of the moon is more similar
to the composition of the Earth crust
•Computer simulations support the impact
theory for the formation of the Moon