Download The Solar System

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The Solar System
The study of the solar system is the most local aspect of
astronomy, which includes:
 the eight traditional planets
 ―dwarf planets‖ and ―plutoids‖
 smaller icy planetesimals beyond Neptune (Kuiper
Belt objects)
 planets’ moons and rings
 comets
 asteroids
 meteoroids
 the Sun—Sol
The ―sol‖ar system contains only one star. It does not
include any other stars or astronomical phenomena.
General Description of Solar System:
 The sun is the dominating influence, both
gravitationally and radiatively.
 All major bodies orbit in the same approximate
plane—all planets within 7 from ecliptic.
 All planets and most major bodies orbit in the same
direction—CCW as seen from north.
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 Most planets rotate in the same direction (CCW as
seen from north), with a broad range of axial tilts.
 Solar system is fairly dirty—billions of asteroids,
meteoroids, and comets.
The eight traditional planets fit into one of two categories
based on their general characteristics.
Terrestrial planets: ―Earth-like‖
 Small planets—mostly solid
 High densities (4 – 5.5 gm/cm3)
 All have rocky, solid surfaces
 Closely spaced orbits (< 1.5 AU from sun)
Jovian planets: ―Jupiter-like‖
 Large gaseous worlds—―gas giants‖
 Low densities (0.7 – 1.7 gm/cm3)
 Have no solid surfaces
 Extensive moon and ring systems
 Widely spaced orbits (5.2 – 30 AU)
Note that Pluto does not fit into either category
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These descriptions give us clues to the origin of the Solar
System…
Any theory (model) for the SS formation must explain:
 CCW rotation and revolution of planets
 Coplanar orbits of planets
 Difference between terrestrials and Jovians
Nebular Theory: Solar system formed from a
contracting ―cloud‖ of interstellar gas and dust.
 Many molecular clouds distributed throughout plane
of Milky Way
 Dense, cool parts of clouds can be triggered to
collapse by density or shock wave.
 As cloud contracts under its own gravity, the
conservation of angular momentum flattens it into a
disk
 Original spin was CCW—planets revolve CCW
 As nebula contracts, temperature rises—hottest at
core
 Eventually temperature and density are high enough
in core of nebula for nuclear fusion.
 Sun is “born”
Not all material in disk is incorporated into Sun.
Why not?
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 Leftover material begins to cool and condense.
 Distance from newly formed sun determines which
molecules condense first—condensation sequence.
 Molecules that can condense at high temps (metals
and rocks) make up the majority of the composition
of the terrestrials.
 Molecules that can only condense at low temps (ices)
had to be several AUs from the sun to condense.
 As molecules condensed and collided with one
another in the nebula, they fused together—
accretion.
 Within a relatively short time span (100,000 years,
according to computer simulations) trillions of
planetesimals were formed by accretion.
 Planetesimals near center of forming solar system
were rocky and metallic, those farther out (> several
AUs) were icy.
 As planetesimals continued to collide and grow,
gravity began to play a dominant role in accretion—
spacing.
 Eventually, the four terrestrial planets we see today
―won out‖ in the inner solar system.
 The cores of the Jovian planets were formed from icy
planetesimals in the outer solar system.
 Meanwhile, the newly formed sun had driven all
remaining gas into the outer solar system.
Once the icy cores of the Jovians grew large enough,
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they could gravitationally attract and hold gases (H
and He).
 Formation process of Jovians created mini ―solar‖
systems of moons around planets.
 After approximately 100 million years from initial
contraction, the solar nebula had condensed and
accreted into the sun and 8 major planets.
 Still billions of planetesimals left over.
Where are they?
The solar nebula theory predicts that planet formation
should be a common byproduct of star formation.
Over 300 “extra-solar” planets discovered so far!
Most discovered via the radial velocity method: uses the
Doppler Effect to detect unseen planets