overview of the solar system its gross features and theories of how it formed. at the heart of...
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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 1st Sky Journal, worth 5% of the final grade. The HW is adjusted to 35% of the final grade.
Reconnaissance of the Solar System so far
Sun, planets, moons, asteroids, comets, dust, gas.
Overview of the Planets
Orbital Characteristic Review
Eccentricity measures the flattening of the ellipse
The Orbits of some Planets are Slightly Inclined to the Ecliptic
Plane
Orbits of the Planets
Planet Semi-major Axis Eccentricity Inclination(A.U.) (degrees) (degrees)
Mercury 0.387 0.205 7.005Venus 0.723 0.007 3.395Earth 1 0.017 0Mars 1.523 0.093 1.851
Jupiter 5.203 0.048 1.305Saturn 9.537 0.054 2.484Uranus 19.191 0.047 0.77Neptune 30.069 0.008 1.769
Pluto 39.481 0.248 17.141
Graph of Semi-major axesPlanetary Orbits
0
5
10
15
20
25
30
35
40
45
Planets
Se
mi-
ma
jor
Ax
is (
A.U
.)
Rotation of the Planets
Planet Rotation Period Axis Tilt(days) (degrees)
Mercury 58.646 0Venus -243.0187 177.3Earth 0.997 23.45Mars 1.026 25.19
Jupiter 0.413 3.12Saturn 0.444 26.73Uranus -0.718 97.86Neptune 0.671 29.58
Pluto -6.387 119.61
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.
Sizes and Densities of the PlanetsRadius Mass Density
(kilometers) (kilograms) (grams/cc)
Sun 695508 1.99E+30 1.409Mercury 2440 3.30E+23 5.43Venus 6052 4.87E+24 5.24Earth 6378 5.97E+24 5.5Mars 3397 6.42E+23 3.94Jupiter 71492 1.90E+27 1.33Saturn 60268 5.69E+26 0.7Uranus 25559 8.68E+25 1.3Neptune 24764 1.02E+26 1.76Pluto 1195 1.30E+22 2.1
H2O has a density of 1 gram/cc
Silicate rocks ~ 3-4 grams/cc
Metals ~5-7 grams/cc
Composition of the Solar System
CO
N
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 - H2, He, Ar, Ne, …
• Ices: Elements – O, C, N. Molecules – H2O, CH4, NH3, CO, CO2, …
• Rocks: Elements, Fe, Si, O, Mg, Ni, …Minerals – Silicates, Sulfides, Metals, …
Classification of PlanetsTerrestrial 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 iceand rock.
Inner vs Outer Planets
Mars
Jupiter
Composition Trends
Body Rocky(%) Icy(%) Gaseous(%)
Sun 0.3 1.2 98.5
Terrestrial Planets
70 30 0
Jupiter 2 5 93
Saturn 6 14 80
Uranus 25 58 17
Neptune 27 62 11
Zelik, P 358
The Nebula HypothesisThe 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.
Accretion of the Planets
Condensation in the Solar Nebula
High Density
Low Density