1. ASTR 1101-001 Spring 2008 Joel E. Tohline, Alumni Professor 247 Nicholson Hall [Slides from Lecture26]

2. 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

3. 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

5. 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

9. 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

10. 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

14. 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

17. 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

18. 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

19. 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

22. 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

24. 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

25. 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

29. 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”

30. 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”

31. 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