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Introduction to the
Solar System
The bright star Antares embedded in
dust and gases
The Formation of the Solar
System
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Through the course of history
there have been many theories
about the solar system
Early Greeks thought the Earth
was the center of the solar
system, with the stars, sun and
moon revolving around the
Earth – the Geocentric Model
In 1543, a scientist proposed
that the sun was the center of
the solar system and the
planets revolved around it –
the Heliocentric Model
Origin of the Solar System
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Modern astronomers believe that the sun and
planets condensed out of a nebula or large
cloud of gas and dust.
This idea is named the Nebular Hypothesis. It
was first presented by the German philosopher
Immanuel Kant in the late 1700’s.
Such clouds have been observed around stars
other than our sun (e.g., Beta Pictoris)
The Formation of the Solar System
Our solar system began as a rotating
gas cloud or nebula that collapsed
toward its center under the influence of
gravity.
The condensing and contracting
caused the cloud to begin to rotate,
as it rotated the center became
dense and the temperature reached
about 10 million K
A condensation formed at the center,
which is called a protostar.
The extremely high temperatures
allowed for a process called nuclear
fusion to occur
A flattened disk of matter surrounded
the protostar, which began to shine
and become a star, our sun.
The Formation of our Solar System
The rising temperature from the sun
removed the gas from the inner regions,
leaving dust and larger debris
•Inner planets formed from solid
debris
•Outer planets retained original gases
Planets established dominance in their
regions of the solar system.
After almost all of the remaining gas, dust,
and small debris was collected by the larger
objects, the solar system took on the form
we recognize today.
Other Star Systems Forming
•We can look at young star systems
developing today.
•The planets orbiting these stars are
formed from the surrounding disks of
gas and dust, called protoplanetary
disks or proplyds.
Proplyd in the Orion
Nebula
Solar System Composite
http://www2.jpl.nasa.gov/galileo/sepo/education/nav/ss2.gif
1.
Mercury
2.
Venus
3.
Earth
4.
Mars
5.
Jupiter
6.
Saturn
7.
Uranus
8.
Neptune
Common Properties of
Planet Orbits in Our
Solar System
As viewed from above, all
of the planets orbit the Sun
in a counterclockwise
direction.
The planets orbit in nearly
the same plane (ecliptic). All
planets except Pluto have an
orbital inclination of less than
7°.
Inner Solar System
http://www.nineplanets.org/overview.html
Outer Solar System
http://www.nineplanets.org/overview.html
Pluto’s Odd Orbit
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Pluto’s orbit, or
plane of revolution,
is tilted by 17° to
the general solar
system orbits
(ecliptic)
Pluto can also cut
across Neptune’s
orbit (but they can
never collide)
Ecliptic Plane
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Plane of the Ecliptic: The orbits of the planets
are mostly in the same plane.
This plane is called the ecliptic and is defined by
the plane of the earth’s orbit.
The exception is Pluto, which is tilted quite a bit
in comparison to the rest of the planets.
The ecliptic plane is a remnant of the original,
rotating nebular disk that formed the sun and
planets
Ecliptic: Artist’s View
Motions
Directions of Motion: The planets orbit in a
counterclockwise direction around the sun
(when looking down upon the solar system
from the sun’s north pole).
 All the planets, except for Venus, Uranus,
and Pluto, rotate in the same direction as
their orbits.
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Orbits
The planets nearest to the Sun (Mercury,
Venus, Earth, and Mars) are relatively
close together, while those farther away
(Jupiter, Saturn, Uranus, and Neptune) are
more spread out.
 Most of the planets are in nearly circular
orbits.
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Solar System Orbits (AU)
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The astronomical unit
(AU) is useful in
measuring distances in
the solar system
One AU equals the
average earth-to-sun
distance of 93 million
miles
AU distances allow direct
comparison to the earth
which is equal to 1.0
Planet
Distance in AU
Mercury
0.4
Venus
0.7
Earth
1.0
Mars
1.5
Jupiter
5.2
Saturn
9.5
Uranus
19.2
Neptune
30
Pluto
39.5
Distance in AU (Earth = 1)
40
35
30
25
20
15
10
5
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Revolutions of the Planets*
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Mercury 88 days
Venus 224.7 days
Earth 365.25 days
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Mars 1.88 years
Jupiter 11.86 years
Saturn 29.5 years
Uranus 84 years
Neptune 164.79 years
Pluto 248.32 years
*Earth days and years
Revolution Periods (Earth = 1)
250
200
150
100
50
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Diameters
Planet
Diameter (Earth = 1)
Mercury
0.38
Venus
0.95
Earth
1.0
Mars
0.53
Jupiter
11.21
Saturn
9.45
Uranus
4.01
Neptune
3.88
Pluto
0.18
Diameter (Earth = 1)
12
10
8
6
4
2
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Masses
Planet
Mass (Earth = 1)
Mercury
0.06
Venus
0.81
Earth
1.0
Mars
0.11
Jupiter
317.94
Saturn
95.18
Uranus
14.53
Neptune
17.14
Pluto
0.002
Mass (Earth = 1)
350
300
250
200
150
100
50
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Average Density (kg/m3)
3
Planet
Average Density (kg/m )
Mercury
5430
Venus
5250
Earth
5520
Mars
3950
Jupiter
1330
Saturn
690
Uranus
1290
Neptune
1640
Pluto
2030
Density (Earth = 1)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Rotational Period
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Days (Earth  1)
58.6462
243.0187
0.99727
1.025957
0.41354
0.42637
0.71806
0.67125
6.3872
Rotational Period (Earth = 1)
250
200
150
100
50
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Average Temperature
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Ave Temp (°F)
354°
867°
45°
-81°
-186°
-202°
-337°
-364°
-380°
Average Temperature (Earth = 1)
20.00
15.00
10.00
5.00
0.00
Planet
-5.00
-10.00
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Sizes of the Planets
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In mass, the sun represent 99% of
the solar system
The smallest planet, Mercury, has a
diameter of 3031 mi
Pluto, the previous smallest planet,
has a diameter of 1457 mi
The largest planet, Jupiter, has a
diameter of 88,700 mi
Earth = 7926 mi
Ganymede, the largest moon of
Jupiter, is larger than Mercury, yet
Ganymede is not considered a
planet because it revolves around
Jupiter
http://www.nineplanets.org/datamax.html
Two Basic Groups of Planets
TERRESTRIAL (earth-like)
JOVIAN (Jupiter-like)
Small size, low Mass
Large size, massive
Higher density
Low density
Mostly rock
Mostly gas
Mercury, Venus, Earth, Mars
Jupiter, Saturn, Uranus,
Neptune
Classifications
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Terrestrial or Rocky
planets—Composed
of rock and metal:
Mercury, Venus,
Earth, Mars
Jovian or Gas
planets—Composed
primarily of the gases
hydrogen and helium:
Jupiter, Saturn,
Uranus, Neptune
Classification, cont.
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Small Planets—
Diameters less than
13,000 km: Mercury,
Venus, Earth, Mars
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Giant Planets (Gas
Giants)—Diameters
greater than 48,000
km: Jupiter, Saturn,
Uranus, Neptune
Classification, cont.
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Inner Planets—
Mercury, Venus,
Earth, Mars
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Outer Planets—
Jupiter, Saturn,
Uranus, Neptune
Classification, cont.
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Inferior—Closer to
the sun than earth:
Mercury and Venus
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Superior—Farther
from the sun than
earth: Mars, Jupiter,
Saturn, Uranus,
Neptune
Classification, cont.
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Classical—Known
since prehistoric
times, visible to the
unaided eye:
Mercury, Venus,
Mars, Jupiter, Saturn
Modern—Discovered
in modern times,
visible only with
telescopes: Uranus,
Neptune, (Pluto)
Classification Table
Planet
Rocky or
Gas?
Small or
Giant?
Inner or
Outer?
Inferior or
Superior?
Classical
or
Modern?
Mercury
R
S
I
I
C
Venus
R
S
I
I
C
Earth
R
S
I
N/A
?
Mars
R
S
I
S
C
Jupiter
G
G
O
S
C
Saturn
G
G
O
S
C
Uranus
G
G
O
S
M
Neptune
G
G
O
S
M
Pluto
?
S
O
S
M
New Solar System (> 2006)
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Planets:
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Mercury
Venus
Earth
Mars
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Eris
Ceres (dwarf)
Jupiter
Saturn
Uranus
Neptune
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Pluto (dwarf)
Eris (dwarf)
Makemake (dwarf)
•Two Categories: Planets and Dwarf Planets
(dwarfs beyond Neptune are “Plutoids”)
•Today’s astronomers recognize only 8
planets
Solar System Model
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If the sun were an orange, the earth would be a
grain of sand thirty feet away.
Jupiter would be a cherry pit located one block
from the sun.
Saturn would be another cherry pit located one
block from Jupiter.
Pluto would be a grain of sand 10 blocks from
the sun.
The nearest star to our sun (Alpha Centauri)
would be represented as another orange 2000
miles from the sun.
Jupiter from Fantasia
(Disney)
Planet Names
The planets have been given the Roman
names of gods from ancient Greece.
Roman
 Mercury, Venus, Mars, Jupiter, Saturn
Greek
 Hermes, Aphrodite, Ares, Zeus, Kronos
The modern planets, Uranus, Neptune, and
Pluto, are also Roman gods
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