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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.