ASTR 1101-001
Spring 2008
Joel E. Tohline, Alumni Professor
247 Nicholson Hall
[Slides from Lecture26]
Chapter 8: Principal Topics
• How old is the Solar System?
• Nebular Hypothesis + Planetesimals + Core
Accretion: A model that explains how the solar
system acquired its key structural properties.
– Directions and orientations of planetary orbits
– Relative locations of terrestrial and Jovian planets
– Size and compositions of planets
• Observational evidence for extrasolar planets
Chapter 8: Principal Topics
• How old is the Solar System?
• Nebular Hypothesis + Planetesimals + Core
Accretion: A model that explains how the solar
system acquired its key structural properties.
– Directions and orientations of planetary orbits
– Relative locations of terrestrial and Jovian planets
– Size and compositions of planets
• Observational evidence for extrasolar planets
Nebular Hypothesis
• General idea: Gravitational collapse of a rotating, diffuse
interstellar gas cloud leads naturally to a “central star +
rotating disk” configuration  the “solar nebula”
– The planets form from material in the disk
– This is why the planets all orbit the Sun in the same direction
and in very nearly the same “ecliptic” plane
• There is observational evidence that similar nebular
structures exist in regions of our Galaxy where new stars
are presently forming
Nebular Hypothesis
• General idea: Gravitational collapse of a rotating, diffuse
interstellar gas cloud leads naturally to a “central star +
rotating disk” configuration  the “solar nebula”
– The planets form from material in the disk
– This is why the planets all orbit the Sun in the same direction
and in very nearly the same “ecliptic” plane
• There is observational evidence that similar nebular
structures exist in regions of our Galaxy where new stars
are presently forming
Nebular Hypothesis
• General idea: Gravitational collapse of a rotating, diffuse
interstellar gas cloud leads naturally to a “central star +
rotating disk” configuration  the “solar nebula”
– The planets form from material in the disk
– This is why the planets all orbit the Sun in the same direction
and in very nearly the same “ecliptic” plane
• There is observational evidence that similar nebular
structures exist in regions of our Galaxy where new stars
are presently forming
Chemical Composition
• The chemical composition of the ‘solar nebula’
reflects the chemical composition of the original
diffuse, interstellar gas cloud
– 71% hydrogen; 27% helium
– 2% (“trace” amounts) of all other chemical elements
• The sun’s atmospheric composition still reflects
this mixture
• Once the rotationally flattened disk has formed,
heavy elements (all the “trace” elements heavier
than hydrogen and helium) settle gravitationally
toward the mid-plane of the disk
Chemical Composition
• The chemical composition of the ‘solar nebula’
reflects the chemical composition of the original
diffuse, interstellar gas cloud
– 71% hydrogen; 27% helium
– 2% (“trace” amounts) of all other chemical elements
• The sun’s atmospheric composition still reflects
this mixture
• Once the rotationally flattened disk has formed,
heavy elements (all the “trace” elements heavier
than hydrogen and helium) settle gravitationally
toward the mid-plane of the disk
Chemical Composition
• The chemical composition of the ‘solar nebula’
reflects the chemical composition of the original
diffuse, interstellar gas cloud
– 71% hydrogen; 27% helium
– 2% (“trace” amounts) of all other chemical elements
• The sun’s atmospheric composition still reflects
this mixture
• Once the rotationally flattened disk has formed,
heavy elements (all the “trace” elements heavier
than hydrogen and helium) settle gravitationally
toward the mid-plane of the disk
Planetesimals
• In the mid-plane of the ‘solar nebula’ disk, heavy
elements began to collide and stick together – initially
forming “dust” particles, then agglomerating into larger
clumps of debris we refer to as “planetesimals”
• A few planetesimals experienced runaway growth,
resulting in the formation of ‘rocky’ planets or ‘rocky’
planet cores
• What about the much more abundant lighter elements,
such as hydrogen and helium?
– In the cold, outermost regions of the solar nebula, a gaseous
“atmosphere” became gravitationally attracted to the ‘rocky’
planet core  formation of Jovian planets
– In the hot, innermost regions of the solar nebula, an extended
gaseous atmosphere did not “stick” because the gas was hot
enough to ‘evaporate’ from the ‘rocky’ planet core  formation of
terrestrial planets
Planetesimals
• In the mid-plane of the ‘solar nebula’ disk, heavy
elements began to collide and stick together – initially
forming “dust” particles, then agglomerating into larger
clumps of debris we refer to as “planetesimals”
• A few planetesimals experienced runaway growth,
resulting in the formation of ‘rocky’ planets or ‘rocky’
planet cores
• What about the much more abundant lighter elements,
such as hydrogen and helium?
– In the cold, outermost regions of the solar nebula, a gaseous
“atmosphere” became gravitationally attracted to the ‘rocky’
planet core  formation of Jovian planets
– In the hot, innermost regions of the solar nebula, an extended
gaseous atmosphere did not “stick” because the gas was hot
enough to ‘evaporate’ from the ‘rocky’ planet core  formation of
terrestrial planets
Planetesimals
• In the mid-plane of the ‘solar nebula’ disk, heavy
elements began to collide and stick together – initially
forming “dust” particles, then agglomerating into larger
clumps of debris we refer to as “planetesimals”
• A few planetesimals experienced runaway growth,
resulting in the formation of ‘rocky’ planets or ‘rocky’
planet cores
• What about the much more abundant lighter elements,
such as hydrogen and helium?
– In the cold, outermost regions of the solar nebula, a gaseous
“atmosphere” became gravitationally attracted to the ‘rocky’
planet core  formation of Jovian planets
– In the hot, innermost regions of the solar nebula, an extended
gaseous atmosphere did not “stick” because the gas was hot
enough to ‘evaporate’ from the ‘rocky’ planet core  formation of
terrestrial planets
Planetesimals + Core Accretion
• In the mid-plane of the ‘solar nebula’ disk, heavy
elements began to collide and stick together – initially
forming “dust” particles, then agglomerating into larger
clumps of debris we refer to as “planetesimals”
• A few planetesimals experienced runaway growth,
resulting in the formation of ‘rocky’ planets or ‘rocky’
planet cores
• What about the much more abundant lighter elements,
such as hydrogen and helium?
– In the cold, outermost regions of the solar nebula, a gaseous
“atmosphere” became gravitationally attracted to the ‘rocky’
planet core  formation of Jovian planets
– In the hot, innermost regions of the solar nebula, an extended
gaseous atmosphere did not “stick” because the gas was hot
enough to ‘evaporate’ from the ‘rocky’ planet core  formation of
terrestrial planets
Result…
• Jovian planets reside in the outer region of the
solar system whereas terrestrial planets reside
in the inner region primarily because of
temperature differences between these two
regions in the solar nebula
• Rocky debris (dust, planetesimals, asteroids)
remains scattered about the solar system; this
debris continues to collide with and “accrete”
onto the planets and their “moons”
Result…
• Jovian planets reside in the outer region of the
solar system whereas terrestrial planets reside
in the inner region primarily because of
temperature differences between these two
regions in the solar nebula
• Rocky debris (dust, planetesimals, asteroids)
remains scattered about the solar system; this
debris continues to collide with and “accrete”
onto the planets and their “moons”
Chapter 8: Principal Topics
• How old is the Solar System?
• Nebular Hypothesis + Planetesimals + Core
Accretion: A model that explains how the solar
system acquired its key structural properties.
– Directions and orientations of planetary orbits
– Relative locations of terrestrial and Jovian planets
– Size and compositions of planets
• Observational evidence for extrasolar planets