From Cosmic Dust to Planets
Thematic Questions about Planet Formation
• Origin of the solar system and planetary systems
• When and how did our solar system form?
• How do planets form? How common are they?
• What are the major sequential processes?
• Characteristics of individual planets and moons
• How do they differ? How are they similar?
• What factors govern these differences?
• History of planets and their moons
• How have the planets changed through time?
• What is their long-term fate?
G302 Development of the Global Environment
From Cosmic Dust to Planets
Thematic Questions about the Planets
• Origin of the solar system and planetary systems
• When and how did our solar system form?
• How and why were the planets formed?
• How common are planets in the universe?
• Characteristics of individual planets and moons
• How do they differ? How are they similar?
• What factors govern these differences?
• History of planets
• How have the planets changed through time?
• What is their long-term fate?
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Solar System Formation
Presumed Sequence of Planetary Formation
• Loss of volatiles within inner nebula
• Clustering of planetesimals in various orbits
• Gradual accretion, clustering into planets
• Planets heated by impact of bombardments
• Major impacts frequent in early history
• Compressed by gravity as mass increases
• Heated by decay of radioactive isotopes
• Internal heat depends on composition
• Separation into layered structure while molten
• Depends on density characteristics
G302 Development of the Global Environment
Formation of the Solar System
Origins of the Sun and Planets
• Nature of the Solar System
• Sequence and timing of its formation
• Compositional features
• Characteristics of interstellar material
• Types of meteorites, solar composition
• Planetary accretion: source materials
• Spatial variations in the solar system
• Nature of the planets and their moons
• Compositional characteristics and differences
• Individual features and moons
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Formation of the Solar System
Nebula of Gas and Dust
• Rotating and coalescing clouds
• Forces: gravity and gas pressure
• Gas cloud collapses, spins faster
Gas pressure: favors
cloud expansion
supernova
nebula
Gravitation force:
favors cloud collapse
Cloud spins more rapidly as it
collapses because angular
momentum is conserved
birthplace for stars
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Formation of the Solar System
Subsequent Stage Leading to Planets
• Rotating nebula flattens, thin disc formed
• Local instabilities favor gravitational collapse
• Collapse creates local centers of contraction
Angular momentum conserved
side view
protosun
side view
top view
protoplanets
collapsing
protoplanet
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Protoplanetary Disks
Hubble Images
protoplanetary
disks
• Gas clouds in nebulae
• Protoplanetary disks
(proplyds) in Orion Nebula
Orion Nebula
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Formation of Planetesimals
Dispersed
material in disc
Accretion and
bombardment creates
larger bodies
side view
Planetesimals form,
which aggregate
into planets
Simulation of
Planetesimal
Formation: Prelude to
Planet Formation
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Extrasolar Planetary Systems
Confirmed Examples:
• Stars of comparable size to the Sun
Mercury
Earth
Venus
Mars
0.6 MJup
Selected
Examples
8.1 MJup
3.5 MJup
1
Orbital distance
2
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Extrasolar Planetary Systems
Growing Evidence for Planetary Systems
• At least 119 planets have been confirmed
• 7 stars with 2 planets, 2 with 3 planets
• Mass range: 0.16 - 17.1 Jupiter mass
• 13 Jupiter mass is sufficient for 2H burning
• Several (>12) protoplanetary disks confirmed
• 21 stars without planets confirmed
0.75 MJup
Upsilon Andromedae
2 MJup
Orbital distance
1
4 MJup
2
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Composition of Planets
Relative Abundance of Elements in the Sun
• Major constituents hydrogen and helium
Relative abundance (Si = 106)
10 10
10 8
hydrogen
helium
oxygen
silicon iron
10 6
10 4
10 2
10 0
10 -2
• Comparability to planets?
• Evidence about timing of
formation?
• Relationship to meteorites
boron
lead
beryllium
bismuth
thorium
10
20
30
40
50
60
70
Element (Atomic ) Number
80
uranium
90
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Timing of Solar System Formation
Evidence from Formation of Planetesimals
• Initial condensation of solid particles
• Differentiation associated with their accretion
• Post-accretion differentiation associated with
impacts, melting or thermal metamorphism
• Addressed by investigation of meteorites,
remnants from the early solar system
Types of Meteorites
• Definitions based on composition and texture
• Irons, stony-irons, achondrite, chondrites
• Carbonaceous chondrites
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Meteorites: Major Types
Irons and Stony-Irons
• Irons: primarily iron and nickel (core of Earth)
• Stony-Irons: mixture of iron and silicate
minerals (various types)
iron
Stony-iron
G302 Development of the Global Environment
Meteorites: Major Types
Chondrite and Achondrites
• Chondrites: similar to materials of inner planets
• Vast majority of meteorites are chondrites
• Achondrites: similar to terrestrial basaltic rocks
• Fine grained, homogenous
chondrite
achondrite
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Meteorites: Major Types
Carbonaceous Chondrites
• Carbonaceous Chondrites: similar to composition
of the Sun minus volatiles
• Contain up to 5% carbon, including organic
molecules
Carbonaceous chondrites
Chondritic texture: blebs that
have never been molten
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Sun vs. Carbonaceous Chondrites
107
106
Solar Photosphere
105
Similarity in
Elemental
Composition
Ivuna
104
103
102
10
Carbonaceous chrondites vary in
composition. CI are the most primitive,
including Ivuna and Orgueil, which may
be fragments of a comet
1
10-1
10-2
10-3 10-2 10-1 100
10
102 103
104
105 106
CI Carbonaceous Chrondrites
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Planetary Characteristics
Classification of Planets
• Position in solar system
relative to Earth, which
affects their aspects
and phase (i.e.
appearance in the sky)
• Composition
• Differences between
inner and outer
planets
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Inferior Planets
Inner
(Terrestrial)
Planets
Asteroids
Gas Giants
(Jovian
Planets)
Superior
Planets
Outer
Planets
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Differences between Planets
Physical Characteristics
• Differences in size, position, orbit, axis
• Relationship to history of formation
• Protoplanets: centers of gravitation collapse
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Differences between Planets
Orbital and Axial Characteristics
• Inclination (relative to Earth) and rotation axis
• Inclination: 0.8° (Uranus) to 17.2° (Pluto)
• Axis: 0° (Mercury) to 97.55° (Uranus)
• Eccentricity: 0.007 (Venus) to 0.254 (Pluto)
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Differences between Planets
Chemical Characteristics
• Differences in density and composition
• Relationship to history of formation
• Chemical fractionation within solar system
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Planet-Forming Solids
Physical Characteristics
• Solid Materials
• ices
• oxides
• metal
• Major differences:
• density
• melting point
• Planetary composition
• reflects these
features
Compound
Density
M.P.
(g/cm3)
(°C)
Ices
CH4
NH3
H2O
0.4
0.7
1.0
-184
-78
0
SiO2
Mg2SiO4
2.7
3.2
1710
1200
Fe
7.9
1540
Oxides
Metal
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Solar System Composition
Name
Mass
Diameter
(1027g)
(103km)
Sun
1,990,000
Mercury
0.33
Venus
4.87
Earth
5.97
Mars
0.64
Asteroids 0.0002
Jupiter
1900
Saturn
570
Uranus
88
Neptune
103
4.88
12.11
12.76
6.79
143.2
120
51.8
49.5
Metals
Fe, Ni
%
(1027g)
0.1
50
30
29
10
15
4
7
8
6
0.16
1.46
1.73
0.06
3x10-5
80
40
7
6
Oxides
SiO2,MgO,FeO
%
(1027g)
0.2
50
69
69
90
85
9
14
17
14
0.17
3.36
4.12
1.7x10-4
170
80
15
14
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Solar System Composition
Metals
(Fe, Ni)
Name
Sun
Mercury
Venus
Earth
Mars
Asteroids
Jupiter
Saturn
Uranus
Neptune
%
0.1
50
30
29
10
15
4
7
8
6
(1027g)
0.16
1.46
1.73
0.06
3x10-5
80
40
7
6
Oxides
Ices (H2O,
(SiO2,MgO,FeO) CH4,NH3,H2S)
%
0.2
50
69
69
90
85
9
14
17
14
(1027g)
%
(1027g)
1.2
0.17
3.36
4.12
1
2
1.7x10-4
170
5
80
12
15
60
14
70
Gases
(H2, He)
%
(1027g)
98.5
0.05
0.12
100
70
53
73
82
67
15
10
1550
380
13
10
G302 Development of the Global Environment
Differences between Planets
Loss of Volatiles from Inner Solar System
• Product of Sun formation? (extrasolar systems)
• Condensation, specific event or effect?
• Evidence from K/U ratios (potassium/uranium)
• Volatile/less volatile element
• Venus, Earth and Mars all in range 5-20 x103
• CI carbonaceous chondrites: 4-10 x104
• Inner planets depleted in volatiles
• Inner nebula purged of volatile elements?
• Effect of intense early solar activity
• Solar flares, solar winds?
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Volatile Elements in Meteorites
Chondrites
10
Concn. Chondrites/
Concn. Carbonaceous Chondrites
• Carbonaceous
chondrites
contain higher
proportions of
volatile elements
• Evidence of loss
of volatiles
during baking
process
1
10-1
10-2
Mg
Line for equal abundance
RbMn P
As
K Li
Sb
Na
F Se
Cu
Sn
Ag Ga
Te
S
Zn Hg
I
Br
Cd
Lower
Cl Pb
concentration
in chrondrites
Bi
Ca
Al
Tl
In
10-3
Increasing Volatility
G302 Development of the Global Environment
Earth’s Interior Heat
Radioactive Decay
Total Heatflow
235U
Increasing Heatflow
• Contributions from
different isotopes
vary through time
• General decrease
• Initially 235U and
40K were more
important
• Now 232Th is the
dominant source
of internal heat
40K
232Th
4
3
2
1
Time
(Ga)
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Composition of Earth
Element
Mantle
Relative
and Crust
to C1
Lithium
2.1ppm
Sodium
2040ppm
Magnesium 20.52%
Aluminum
2.02%
Silicon
22.40%
Phosphorus 57ppm
Sulfur
48ppm
Potassium
151ppm
Calcium
2.20%
Titanium
1225ppm
0.87
0.26
1.46
1.57
1.44
0.05
0.0025
0.17
1.58
1.86
Element
Mantle
Relative
and Crust
to C1
Iron
2.1ppm
Nickel
1961ppm
Rubidium 0.39ppm
Strontium 16.2ppm
Thorium 0.0765ppm
Uranium 0.0196ppm
0.22
0.13
0.11
1.42
1.50
1.40
volatiles
volatiles primarily in core
elements primarily in core
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