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The Solar SystemThe Solar System
A planet by any other name ….
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Six Planets
What is the Solar System?
• Answer: The system of objects in the solar neighborhood (near the Sun)
• What are these objects?
One Star
Nine Planets
Dozens of moons
Thousands of asteroids
Trillions of comets
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Discovered Planets
• All planets through Saturn known since the ancients – all you have to do is look up to see them
• Uranus in 1781 by William Herschel– “Georgium Sidus” after George III of
England
• Neptune in 1846 by Johann Galle using predictions by Urbain Jean Joseph Leverrier and John Couch Adams
• Pluto in 1930 by Clyde Tombaugh at Lowell Observatory
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Planets
• The first step to studying planets?• Compare and contrast• What are important
quantities?• You have:
– A stick– A tree– A car– A house
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Planetary Properties
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Concept Test
• Which of the following is a true statement about density?a. A stone and a boulder have the same
density.b. A bowling ball and a soccer ball have the
same density.c. A boulder has a higher density than a
pebble.d. A soccer ball has a higher density than a
billiard ball.e. None of the above.
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Density and Mass
• What is mass?– Mass is similar to weight, it measures how
much stuff an object is made of– Example: A bowling ball and a soccer ball
are about the same size, but have different masses
• What is density?– Density is mass per volume. It helps to tell
you what kind of stuff an object is made of– Example: A log and a tree have different
masses (and sizes), but the same density because they are made of the same stuff
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Terrestrial Planets
• Close to the sun• Small
– Mass– Radius
• High density– Primarily rocky– Solid surface
• Weak magnetic field• Few moons• No rings
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Terrestrial Planets
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Inte
riors
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Jovian Planets• Far from the sun• Large
– Mass– Radius
• Low density– Primarily gaseous– No solid surface
• Strong magnetic fields
• Many moons• Many rings
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Jovian Planets
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Interiors
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Orbits
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Planetary Rings
• All the “Jovian planets” have rings
• These are not solid, but composed of millions of tiny particles of ice and dust
• Rings have structure: gaps and spokes
Composition
Size
Jupiter Dust Small grains
Saturn Water ice < house size
Uranus Carbonaceous
Large particles
Neptune
dark, unknown
unknown, small
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What About Pluto?
• Pluto does not easily fit into either category– Far from the sun (jovian)– Small (terrestrial)– Neither rocky nor gaseous (icy)– One moon– No rings
• It is similar in composition to some moons in the outer solar system and its orbit is similar to a group of objects called “Kuiper Belt Objects” or KBOs
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Charon• Largest of any moon in relation to the
planet it orbits (1/2 the size of Pluto)• Pluto and Charon are tidally locked to
each other (always show the same face)• Charon discovered in 1978 by
astronomers at the US Naval Observatory
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Concept Test
• Which of the following is NOT a reason why many astronomers think Pluto should not be classified as a planet. a. It is smaller than some moons.b. It has an orbit similar to many Kuiper Belt
Objects (KBO).c. Pluto is more similar in shape to irregular
asteroids and comets.d. Pluto is more similar in composition to
many moons and KBOs.e. Its orbit is far more elliptical and inclined
to the Sun relative to the other planets.
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KBOs
• Belt of icy objects outside Neptune’s orbit.
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Quaoar
• a = 43 AU• Orbit nearly
circular (e = 0.04)
• P = 285 years.
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Sedna
• a = 479 AU• Orbit (e = 0.84)• Aphelion = 76 AU• Perihelion = 884
AU– Currently = 90 AU
• P = 10,500 years
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Sedna
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“Xena”• 2003 UB313• A = 68 AU• E = 0.44• P = 560 years• Coming soon: mass
and density
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Homework #6Homework #6
• For Wednesday 20-Sept: Read article and answer questions in handout.