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The Solar System
A planet by any other name ….
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What is the Solar System?
• Answer: The system of objects in the solar
neighborhood (near the Sun)
• What are these objects?
One Star
Six Planets
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
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•
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:
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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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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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Composition
Size
Jupiter
Dust
Small grains
Saturn
Water ice
< house size
Uranus
Carbonaceous
Large particles
Neptune
dark, unknown unknown, small
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
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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
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•
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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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2003 UB313
A = 68 AU
E = 0.44
P = 560 years
Coming soon: mass and
density
“Xena”
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Homework #6
• For Wednesday 20-Sept: Read article and answer
questions in handout.
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