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An Introduction to Astronomy
Part VI: Overview and Origin of the
Solar System
Lambert E. Murray, Ph.D.
Professor of Physics
The Sun is the Dominant
Influence in our Solar System
The Sun’s gravity
controls the motion of
all objects in the solar
system.
 Heat and the Solar
Wind from the Sun
have had a controlling
influence on the
atmospheres and
surface environment
of the planets and their
moons.

The Sun
 The
mass of the Sun is about 700 times the
mass of all the rest of the solar system
combined.
 It is composed mostly of Hydrogen (71%)
and Helium (27%), but also contains a small
fraction of nearly all the other elements.
Principle Components of the
Solar System
The solar system is composed of
– Eight Planets and their respective moons:
The
Terrestrial Planets
The Jovian Planets
– Minor Planets (Dwarf Planets)
– Asteroids
– Comets
Planetary Orbits
All planets orbit the Sun in the same direction as
the Sun rotates (counter-clockwise as viewed from
above the ecliptic), in nealy circular orbits, and
nearly all within 3 degrees of the ecliptic.
 The rotation of most planets is also in a counterclockwise direction, with the exception of Uranus
(and Pluto), which are tilted on their sides, and
Venus which is “flipped over”.
 Most moons orbit their respective planets in the
same direction – although the outer moons of
Jupiter are an exception, and most orbit in the
equatorial plane of the planet. (Our own Moon, an
exception, actually orbits nearer the ecliptic than
the equator.)

Orbital Motion of the Planets
The Terrestrial Planets
 These
planets are small, solid planets
composed of rocky (silicate) material mixed
with a few heavier elements and have a
relatively thin, or no atmosphere.
– All these planets have a density of about 5.5
g/cc, with the exception of Mars (3.94 g/cc).
– How do we know the density of the planets?
The Jovian Planets
These are large planets, with thick gaseous
atmospheres composed mostly of hydrogen and
helium gases and hydrogen compounds. Their
interior may be composed of frozen liquids and
gases such as carbon dioxide, water ice, frozen
ammonia and methane (icy compounds).
 All these planets have a density of 1.6 g/cc or less,
with Saturn being the least dense (0.69 g/cc)
 Each of the Jovian planets has a noticeable ring
and a large number of moons:

–
–
–
–
Jupiter
Saturn
Uranus
Neptune
~ 60
~ 30
~ 20
~ 10
Jupiter’s Retrograde Moons
Are these
moons captured
asteroids?
What is Pluto?
Is it Really a Planet?
It is spherical!
Observations

The outer planets appear to have compositions
similar to the Sun, while the inner planets are
rocky.
– However, the composition of the inner planets are
consistent with the composition of the Sun if hydrogen
and helium were removed!
The age of the rocks on Earth, and the Moon and
the ages of meteorites appear to be similar (the
older rocks of the Earth are about 4 billion years
old.)
 The age of our fusion fueled Sun is consistent with
the age of these rocks implying that they were all
formed at about the same time.

Planetary Debris
 In
addition to the sun, the planets, and the
major moons, there are additional pieces
that make up our solar system
– Asteroids
– Comets
– Meteors
 We
will look at these more closely later.
Comets
Meteors
The Formation of the Solar System:
The Solar Nebula Hypothesis

Gravitational collapse of an interstellar dust/gas
cloud
– Formation of a disk of gas and dust
– Formation of a fusion fueled star
Heating and condensation within the nebular disk.
 Accretion and further heating of planetesimals.
 The end of accretion – formation of craters.
 Creation of atmospheres

– Outer planets simply “captured” their atmosphere
– Inner planets atmosphere arise from volcanic eruptions
releasing gases trapped in the rocky structure of the
planet
Interstellar
Dust/Gas
Cloud
Solar Wind Blowing Away Dust
and Gas
A Question on Planet Formation
The prevailing theory of planet formation is based
upon accretion of “debris” from the solar nebula.
This theory requires a long time to build up a
planet the size of Jupiter.
 Most observed extra-solar planets are about the
size of Jupiter, and many of these are believed to
be formed in “stellar nurseries” where there is a
large flux of high energy particles that would
“blow away” much of the “debris” left from star
formation, thus limiting the time for development
of such a large planet if accretion is the dominant
process. Perhaps there is more to the story?

End of Part VI