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The Solar System
09.19.07 / 09.20.07
Essential Questions
What are distinguishing features of solar
system bodies?
 What are Kepler’s laws and how do they
apply to planetary motion?
 What is the nebular hypothesis?

Overview of the solar system
 Solar
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system includes
Sun
Eight planets and their satellites
Asteroids
Comets
Meteoroids
Kuiper Belt
2.8 - 4.6 billion miles away
 Probably tens-of-thousands of rocky, icy
objects
 Includes Pluto
 Discovered circa 1992
 Predicted in 1951 by Gerald Kuiper
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The solar system
The orbits of most asteroids
lie between Mars and Jupiter
Minor members of the
solar system
 Asteroids
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Most lie between Mars and Jupiter
Small bodies – largest (Ceres) is about 620
miles in diameter
Some have very eccentric orbits
Irregular shapes
Origin is uncertain
Image of asteroid 951
(Gaspra)
Minor members of the
solar system
 Comets
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Often compared to large, "dirty snowballs"
Composition
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Frozen gases
Rocky and metallic materials
Minor members of the
solar system
 Comets
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Origin
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Not well known
Form at great distance from the Sun
Comet Hale-Bopp
Minor members of the
solar system
 Meteoroids
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Called meteors when they enter Earth's
atmosphere
A meteor shower occurs when Earth
encounters a swarm of meteoroids
associated with a comet's path
Meteoroids are referred to as meteorites
when they are found on Earth
Overview of the solar system
 A planet's
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orbit lies in an orbital plane
Similar to a flat sheet of paper
The orbital planes of the planets are
inclined
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Planes of seven planets lie within 3 degrees of
the Sun's equator
Mercury's is inclined 7 degrees
Pluto's is inclined 17 degrees (That crazy
Pluto)
Overview of the solar system
 Two
groups of planets occur in the solar
system
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Terrestrial (Earth-like) planets
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Mercury through Mars
Small, dense, rocky
Low escape velocities
Overview of the solar system
 Two
groups of planets occur in the solar
system
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Jovian (Jupiter-like) planets
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Jupiter through Neptune
Large, low density, gaseous
Massive
Thick atmospheres composed of hydrogen,
helium, methane, and ammonia
High escape velocities
Pluto not included in either group (oddball)
The planets
drawn to scale
Evolution of the planets
 Nebular
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hypothesis
Planets formed about 5 billion years ago
Solar system condensed from a gaseous
nebula
 As
the planets formed, the materials
that compose them separated
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Dense metallic elements (iron and nickel)
sank toward their centers
Lighter elements (silicate minerals, oxygen,
hydrogen) migrated toward their surfaces
Kepler’s Laws
Law 1: The planet orbit is ellipses with the
Sun at one of two foci
 Law 2: The line connecting the planet to
the Sun sweeps equal areas in equal time
 Law 3: The periods of planets’ revolutions
is proportional to their distances from the
Sun
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Law 1
http://astro.isi.edu/notes/gravity.html
Law 2
http://burro.astr.cwru.edu/Academics/Astr221/Gravity/kepler2.htm
Law 3
http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html
Eight or Nine?
 Pluto
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Discovered in 1930
Highly eccentric orbit causes it to
occasionally travel inside the orbit of
Neptune, where it resided from 1979
through February 1999
Moon (Charon) discovered in 1978
Pluto and its
moon Charon
as compared
to the size of
Earth
A decision to make
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With the discovery of the Kuiper Belt,
astronomers need to make on of two
choices:
allow the possibility of many, many more
planets
 create a more restrictive definition of planet
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 They
chose this second option
 They then created possible definitions
Potential definition 1
A planet would need to a) revolve around
a star, b) be massive enough to have
formed into a round shape, c) not be a
moon, and d) not be another star
 This option would have resulted in 12
planets, with more possible in the future
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Potential definition 2
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The same as the first with one addition
the planet must have “cleared the
neighborhood around its orbit.”
 Because Pluto’s orbit crosses paths with
Neptune, it would not qualify
 This definition results in 8 planets
 This is the one the International Astronomical
Union chose to accept
 Pluto hasn’t changed, just our definition of
planet
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Light pollution
http://epod.usra.edu/archive/epodviewer.php3?oid=92448
Source: University of Hawaii
Houses cause pollution too
Better lighting
Source: NASA
see also p. 664 in home text
Resources
International Dark-Sky Society
 International Astronomical Union
 Light Pollution Abatement Program (LPAP)
Canada
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