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Lecture 10
Formation of the Solar System
January 6c, 2014
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Orbits of the Planets
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Clues for the Formation of the SS
• All planets orbit in roughly the same plane
about the Sun.
• All planets orbit in the same direction about
the Sun.
• Most planets rotate in the same direction.
• There are distinct differences between Inner
and Outer planets
Formation of the SS
Nebular Theory
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• Interstellar gas and dust contracts due to gravity
–
–
–
–
Size >100 AU and ~2 MSun
Hydrogen and Helium and some trace elements
Started ~4.6 billion years ago.
Gas was very cold.
Gravity pulling
cloud together
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Gravity Collapses Cloud
• Cloud collapses, rotates
faster
– Conservation of Angular
Momentum (e.g. ice skater
pulls arms in, spins faster)
• Central core forms.
• Cloud flattens
– Centrifugal force stretches
gas perpendicular to axis
– No such force to prevent
collapse parallel to axis
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Disk Forms
• Inner parts heated by gravitational collapse
and by the young proto-Sun
• Outer parts very cold
Beta-Pictoris
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Proto-planetary disk – Beta Pictoris
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Proto-planetary Disks – Orion
Nebula
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The Solar Constant
Solar energy received at Earth
S 
 1400 Watts/m2
(per unit time)(per unit area)
• For other planets:
S
Solar constant at distance r (in AU)  2
r
S
– Mars: r = 1.5 AU
S  Mars  
– Neptune: r = 30 AU S  Neptune  
2
 0.44 S
2
 0.001 S
1.5
S
 30 
• It is MUCH colder in the outer regions than
in the inner region
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Jupiter orbits at 5.2 AU. If the Earth receives
1400 W/m2 of sunlight, how much does Jupiter
receive?
A. 37,900 W/m2
B. 7,280 W/m2
C. 269 W/m2
D. 51.8 W/m2
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Jupiter orbits at 5.2 AU. If the Earth receives
1400 W/m2 of sunlight, how much does Jupiter
receive?
A. 37,900 W/m2
B. 7,280 W/m2
C. 269 W/m2
D. 51.8 W/m2
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2
2
SJupiter S rJupiter
rEarth  1 AU 

 2 
  0.037
2
SEarth
S rEarth rJupiter  5.2 AU 
SJupiter  0.037 SEarth  0.037 1400 W/m 2   51.8 W/m 2
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Condensation Temperature
• Condensation occurs when a gas cools and its
molecules stick together to form a liquid or a solid
• Condensation occurs below a critical temperature
that is different for different materials
• Metals and rocks: 1300 K to 1600 K
• Water, methane, ammonia “ices”: 100 K to 300 K
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Planetesimals Form
• Material in cloud clumps and collides
– Form larger bodies through accretion
• Temperature high near proto-Sun.
–
–
–
–
Ices and gases in inner regions are vaporized
Planetesimals cannot hold on to light elements;
Lighter material moves outward
Only heavier elements remain near Sun.
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Planets Form
• Outer planets form from all material
– Planets capture mainly gases (H and He), because
it was cooler in outer regions.
– only a little rock and metal because they have
lower abundance
• Inner planets form from rock and metals.
– Warmer near sun
– Only heavier
elements (rocks
and metals)
remained near
Proto-sun
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Temperature Distribution in the
Solar Nebula
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Planets are Differentiated
• Denser material moves inward in a liquid or
gaseous planet
• Terrestrial planets were molten during early stages.
– Metals found closest to center
– Lighter rocky
material
remains in
outer parts of
planet
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Sun Forms
• Sun obtains sufficiently high pressure and
temperature in core to start fusion
(converts Hydrogen to Helium)
• This is the time it officially becomes a star.
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Solar System Today
• Planets still bombarded by left-over material
• Process took ~100 million years.
• Some debris still leftover (asteroids,
meteoroids, and comets).
• 99.8% of the total mass is in the Sun.
NASA movie
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“Movie” of Solar System Formation
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One explanation of why the planets near the Sun
are composed mainly of rock and iron is that
A. the Sun’s magnetic field attracted all the iron
in the young solar system.
B. the proto-Sun ejected iron-rich material from
its surface.
C. the high temperature near the proto-Sun made
it difficult for ices and gases to condense.
D. the Sun’s gravitational attraction pulled iron
and other heavy material inward.
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One explanation of why the planets near the Sun
are composed mainly of rock and iron is that
A. the Sun’s magnetic field attracted all the iron
in the young solar system.
B. the proto-Sun ejected iron-rich material from
its surface.
C. the high temperature near the proto-Sun made
it difficult for ices and gases to condense.
D. the Sun’s gravitational attraction pulled iron
and other heavy material inward.