Overview of the Solar System

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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
Revision of Essay due April 10
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Reconnaissance of the Solar System so farSun,
planets, moons, asteroids, comets, dust, gas.
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Overview of the Planets
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The Orbits of some Planets are Slightly Inclined
to the Ecliptic Plane
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Eccentricity measures the flattening of the
ellipse
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Orbits of the Planets
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Graph of Semi-major axes
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Rotation of the Planets
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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.

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Sizes and Densities of the Planets
H2O has a density of 1 gram/cc Silicate rocks
3-4 grams/cc Metals 5-7 grams/cc
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Composition of the Solar System
C
O
N
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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, noble gases.
• 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,

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Groups 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.
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Inner vs Outer Planets
Mars
Jupiter
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Composition Trends
Zelik, P 358
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How do we explain all this?

• Disk of planets, rotating in same direction
• Rocky planets near Sun, icy planetary cores and
  moons further out
• Little planets near Sun, big guys further out.
• Big planets have big hydrogen atmospheres

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Accretion of the Planets
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Why Planets Spin the Way That They Do
Click here to see movie
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Condensation in the Solar Nebula
High Density
Low Density
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SummaryThe 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.