1. Overview of the Solar System Its gross features and theories of how it formed. At the heart of science is an essential tension between two seemingly contradictory attitudes – an openness to new ideas, no matter how bizarre or counterintuitive they may be, and the most ruthless skeptical scrutiny of all ideas, old and new. - Carl Sagan Final Exam: Dec. 13, 11AM-1PM HW due Friday. Topic of Essay II due Friday. There will be only the 1 st Sky Journal, worth 5% of the final grade. The HW is adjusted to 35% of the final grade.

2. Reconnaissance of the Solar System so far Sun, planets, moons, asteroids, comets, dust, gas.

3. Overview of the Planets

4. Orbital Characteristic Review

5. Eccentricity measures the flattening of the ellipse

6. The Orbits of some Planets are Slightly Inclined to the Ecliptic Plane

7. Orbits of the Planets

8. Graph of Semi-major axes

9. Rotation of the Planets

10. Summary of Orbital Characteristics Planets orbit in nearly the same plane (the ecliptic plane), inclinations are small. Planets orbit in the same direction with small eccentricities. The direction is that which the sun rotates. Most of the planets spin in the same direction that they orbit. Venus, Uranus and Pluto are exceptions.

11. Sizes and Densities of the Planets H 2 O has a density of 1 gram/cc Silicate rocks ~ 3-4 grams/cc Metals ~5-7 grams/cc

12. Composition of the Solar System C O N

13. Some Nomenclature Astronomers classify materials according to their tendency to exist as gases, ices, or rocks at Earth-like temperatures and pressures. Gases: Elements - H, He, Ar, Ne, other noble gas. Molecules - H 2, He, Ar, Ne, … Ices: Elements – O, C, N. Molecules – H 2 O, CH 4, NH 3, CO, CO 2, … Rocks: Elements, Fe, Si, O, Mg, Ni, … Minerals – Silicates, Sulfides, Metals, …

14. Classification of Planets Terrestrial Planets: Mercury, Venus, Earth, Mars Mostly rock, radii of several thousand kilometers, densities of ~5 grams/cc. These are the first 4 planets out from the Sun. Jovian Planets: Jupiter, Saturn, Uranus, Neptune Radii of tens of thousands of kilometers, densities of 0.7-1.76 grams/cc composition similar to the Sun but with extra “heavy” elements (carbon, oxygen, nitrogen, etc.). Flotsom and Jetsom of the Solar System: Comets, Asteroids, Kuiper Belt Objects, Pluto. Radii from tens (or smaller) to hundreds of kilometers. Density ~ 0.5-2 grams/cc (with exceptions). Composed of ice and rock.

15. Inner vs Outer Planets Mars Jupiter

16. Composition Trends BodyRocky(%)Icy(%)Gaseous(%) Sun0.31.298.5 Terrestrial Planets 70300 Jupiter2593 Saturn61480 Uranus255817 Neptune276211 Zelik, P 358

17. The Nebula Hypothesis The solar system (planets, satellites, asteroids, comets, etc.) formed along with the Sun 4.5 Byr ago from the gravitational collapse of an interstellar cloud of gas and dust. The planets and Sun formed from the same reservoir of interstellar matter and are therefore composed of primarily the same elements. As the cloud collapsed under the force of gravity it began to spin rapidly and then flattened into a plane. This explains why the solar system is a relatively flat plane and why the planets orbit in the same direction and tend to rotate in the direction that they orbit. The collapsing cloud of gas and dust was hottest near the Sun and coolest far from the Sun. The local temperature determined which compounds solidified from the gas phase as a function of distance from the Sun. This explains why the inner planets are composed mostly of rock and the outer planets have large complements of ice. Ice forming elements are more abundant than rock forming elements so planets in the outer solar system are larger. In fact they are so large that their gravitational fields were able to capture the H and He in the cloud. The gravity of the inner planets is too weak to hold on H and He. This explains why the outer planets are gaseous and the inner planets are rocky.

18. Accretion of the Planets

19. Why Planets Spin the Way That They Do Click here to see movie

20. Condensation in the Solar Nebula High Density Low Density