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Introduction to
the Solar System
11 July 2005
AST 2010: Chapter 6
Some Basics Facts
The solar system
consists of
The Sun
Nine planets
More than a hundred (100) satellites of the
A large number of smaller bodies
Asteroids and comets
Cosmic dust
Countless grains of broken rock
11 July 2005
AST 2010: Chapter 6
Planetary Orbits in the Solar System
11 July 2005
AST 2010: Chapter 6
Inner Solar System
The Sun and the four planets closest to it
belong to what is called the inner solar system
Thus the inner planets are
These 4 inner planets
are also called rocky or
terrestrial planets
The Earth and the other
3 are similar in that they are all composed
primarily of rocks (mostly silicates, made of
silicon and oxygen) and metals (mostly iron)
11 July 2005
AST 2010: Chapter 6
Terrestrial or Rocky Planets
The Earth and the other 3 terrestrial planets
have solid surfaces that bear the record of their
geological history in the forms of craters,
mountains, and volcanoes
are said to have oxidized chemistry because they
are largely composed of oxygen compounds
have relatively high densities
have relatively slow rotation
have no rings and few satellites
Venus has an unusual direction of rotation about
its axis, compared to the other planets
It spins on its axis backwards, in a retrograde
11 July 2005
AST 2010: Chapter 6
Outer Solar System
The part of the solar system outside the inner
solar system is called the outer solar system
Thus, the planets in the outer solar system are
Jupiter, Saturn,
Uranus, and
Neptune are
several times larger than the Earth and hence
are called the giant planets
They are also called the jovian planets
11 July 2005
AST 2010: Chapter 6
Jovian Planets
Jupiter, Saturn,
Uranus, and
are composed
primarily of
hydrogen and helium
have deep atmospheres
are said to have reduced chemistry because they
are largely composed of hydrogen and its
have relatively low densities
have relatively rapid rotation
have extensive ring system and many satellites
11 July 2005
AST 2010: Chapter 6
Mass of Members of the Solar System
11 July 2005
AST 2010: Chapter 6
The Nine Planets (1)
The planets’ sizes are not to scale
11 July 2005
AST 2010: Chapter 6
The Nine Planets (2)
Outer planets
Inner planets
The planets’ sizes are to scale
Main Facts about the Planets
(Natural) Satellites
Only Mercury and Venus do not have a moon
or (natural) satellite
There are more than 100 satellites known
More are being discovered on a regular basis
The largest of the satellites are as big as a
small planet, including
our Moon
the four largest moons of Jupiter
called the Galilean satellites after their discoverer
the largest moons of Saturn and Neptune
respectively called Titan and Triton
11 July 2005
AST 2010: Chapter 6
Rings (1)
Each of the jovian planets has rings
made up of countless small bodies
They range in size from
grains of dust to
terrestrial mountains
Each ring system orbits
its planet at its equator
The rings of Saturn are
the best known, the
widest, and by far the
easiest to see
11 July 2005
AST 2010: Chapter 6
Rings (2)
Recent missions near the jovian planets
showed that the rings
have complex shapes
are influenced by the pull of the planets'
Jupiter’s rings
Uranus’ rings
Neptune’s rings
These are
rocky and metallic objects, quite modest in size
important members of the solar system
found in great number between the orbits of Mars and
believed to be remnants of the initial population of the
solar system
Some of the smallest satellites of the planets are
thought to be asteroids
captured by the
gravitational pull of the
The two moons of Mars
are nowadays believed
to have such an origin
Asteroids Mathilde, Gaspra, and Ida
11 July 2005
AST 2010: Chapter 6
These are another class of small bodies
composed in part of ice, made of frozen gases
such as water, carbon dioxide, carbon
monoxide, and methyl alcohol
Comets are also believed to be remnants from
the formation of the solar system
With rare exceptions, comets orbit the Sun in
distant, cooler regions
Their orbits are large and very eccentric
11 July 2005
AST 2010: Chapter 6
Recent Comet Visits
Cosmic Dust
The solar system contains countless grains of
broken rock which one refers to simply as
cosmic dust
Comic-dust particles constantly collide with
the planets and become trapped by the
planets’ gravitational pull
Millions of these hit the Earth's atmosphere
every day
When these particles enter the atmosphere,
they burn up, producing brief flashes of light
that we see as shooting stars or meteors
Occasionally some of the larger chunks survive
the passage through the atmosphere and land
on Earth to become meteorites
11 July 2005
AST 2010: Chapter 6
Trends in Temperature
The planets and satellites
do not generate their own heat as the Sun does
are heated by the radiant energy of the Sun
Mathematically, the kelvin temperatures
decrease approximately in proportion to the
square root of the distance from the Sun
Thus, the farther a planet or satellite is from
the Sun, the cooler it is
Mercury has a surface temperature of 500 K
Pluto’s temperature is only about 50 K, which is
colder than water’s freezing point (273 K)
11 July 2005
AST 2010: Chapter 6
Earth’s Uniqueness for Life Support
Because of the strong dependence of
temperature on distance from the Sun,
the Earth is the only planet where
the surface temperatures lie between the
freezing and boiling points of water
water can be liquid on the planet’s
Therefore, the Earth is the only known
planet that can support life — at least
life as we know it
11 July 2005
AST 2010: Chapter 6
Physical Appearance
The external appearance of a planet is
determined by
its surface composition
external bombardments on it
geological activity on the planet
11 July 2005
AST 2010: Chapter 6
Planets under Attack!!
Externally, planets are bombarded with a slow
rain of projectiles from space
The impact of the
projectiles on the
surface of the planet
leaves pockmarked
by impact craters of
all sizes
The amount of craters
a planet has on its
surface may provide
clues about its history
11 July 2005
AST 2010: Chapter 6
Shoemaker-Levy 9
A dramatic example:
impacts of large
pieces of Comet
Shoemaker-Levy 9
with Jupiter in
summer 1994
Sequence of images taken by Hubble Space Telescope
11 July 2005
AST 2010: Chapter 6
Geological Activity (1)
The geological activity
on the terrestrial planets
involves the internal
forces that constantly
reshape their surfaces by
buckling and twisting their
building up mountain ranges
creating and erupting volcanoes
Geological activity results from a hot interior
Mountains and volcanoes arise from the buildup
and release of heat escaping from a planet’s core
11 July 2005
AST 2010: Chapter 6
Geological Activity (2)
Each of the planets and satellites is believed to
have been heated at the time of its birth
This primordial heat initially powered extensive
volcanic activity
However, smaller bodies, such as our Moon, soon
cooled off
The larger the body, the more likely it is to retain
its internal heat
Thus, it is more likely to see surface evidence of
geological activity on the larger planets/satellites
The terrestrial planets appear to conform to this
Mercury and our Moon are geologically dead
Earth and Venus are still geologically active
Mars represents an intermediate case
11 July 2005
AST 2010: Chapter 6
The Dating Game
Two popular methods for estimating the age of
a planet or satellite or its surface
Crater counting
Radioactive dating
The crater counting method is based on the
assumptions that
the rate at which asteroids or comets bombard a
planet’s surface is roughly constant for a long time
the planet’s surface has not been reshaped, and the
craters have never been erased, since it
experienced a major change
If these assumptions are satisfied, then the
number of craters will be proportional to the
length of time the surface has been exposed
11 July 2005
AST 2010: Chapter 6
Counting Craters
The number of impact
craters on the surface of
a planet, satellite, or even
The Moon
asteroid may provide clues
about the length of time the surface has
undergone cosmic bombardment since the
surface solidified or geological activity stopped
Radioactive Decays (1)
Radioactivity is a natural phenomenon that
was discovered at the beginning of the 20th
Some atomic nuclei are not stable, but can
spontaneously split apart, or decay, into smaller
Such nuclei are said to be radioactive
For any one radioactive nucleus, the decay
process happens randomly
It is not possible to predict when the decay will
Radioactive decays involve the emission of
particles, such as electrons, or of radiation in
the form of gamma rays
11 July 2005
AST 2010: Chapter 6
Radioactive Decays (2)
The rate at which nuclei decay is most easily
expressed in terms of their half-life
The half-life is a specific time period during which
the chances are fifty-fifty that decay will occur for
any one of a large number of radioactive nuclei
For example, if you had 1 gram of pure radioactive
nuclei of one type whose half-life is 10 years, then
after 10 years you would have ½ gram, after 20
years ¼ gram, after 30 years 1/8 gram, and so on
After many half-lives, the original radioactive
nuclei do not disappear, but instead are
replaced by their decay products
The original nuclei are also called parents and the
decay products are called daughters
11 July 2005
AST 2010: Chapter 6
Radioactive Decays (3)
The number of radioactive nuclei in a sample
decreases exponentially
11 July 2005
AST 2010: Chapter 6
Radioactive Elements as Clocks
Radioactive elements can serve as natural
clocks if one can determine how many of the
initial radioactive parents have been replaced
with their daughters
By comparing how much of a radioactive parent
material is left in a rock with how much of its
daughter material has accumulated, one can learn
how long the decay process has been going on and
hence how long ago the rock formed
This method involves the assumption that the
rock being studied has been isolated since its
That is, none of the parent and daughter nuclei in
the rock have leaked out of it or been polluted by
outside contaminants
11 July 2005
AST 2010: Chapter 6
Radioactive Decay Reactions Often Used to Date Rocks
11 July 2005
AST 2010: Chapter 6
Ages of the Moon and Earth
Astronauts that visited the Moon brought back
lunar rock for radioactive age dating
Counting craters had given only relative ages
for different parts of the Moon’s surface
Samples brought back enabled a better dating
of the lunar surface which suggested that the
Moon is an ancient, geologically dead body
Radioactive dating of rocks brought back by
the Apollo mission and of rocks here on Earth
indicated that the two bodies have similar
According to this dating method, both the Earth
and the Moon were formed some 4.5 billion
years ago
11 July 2005
AST 2010: Chapter 6
Origin of the Solar System
An observation of the Sun and the nine planets of
reveals some patterns
The planets all revolve around the Sun in the same
They orbit in approximately the same, flat plane
The Sun spins in the same direction about its own
Astronomers regard these facts as evidence that
the Sun and the planets
formed together as a spinning
system of gas and dust,
referred to as the solar nebula
11 July 2005
AST 2010: Chapter 6
Origin and Composition (1)
The composition of planets provides another
clue about the origin of the solar system
Spectroscopic analysis allows a determination
of what chemical elements are present in the
Sun and the planets
Such an analysis shows that the Sun has the
same hydrogen-dominated composition as
Jupiter and Saturn
This suggests that these three bodies were
formed from the same reservoir of material
11 July 2005
AST 2010: Chapter 6
Origin and Composition (2)
By contrast, spectroscopic analysis shows that
the terrestrial planets and satellites
are relatively deficient in the light gases and the
various ices composed of the common elements
oxygen, carbon, and nitrogen
contain mostly heavy elements like iron and silicon
— these are rare on the Sun and the giant planets
This pattern suggests that the processes
which led to the formation of the inner planets
must have somehow rejected much of the
lighter materials
These appear to have escaped, leaving behind just
a residue of heavy stuff
11 July 2005
AST 2010: Chapter 6
Other Planetary Systems (1)
An additional approach to understanding the
origin of the solar system is to look outside
the solar system for evidence
Many stars in space are much younger than the
Sun, and planet formation might be occurring in
some of these star systems that could be accessible
to direct observation
Astronomers have observed other “solar
nebulas” or circumstellar disks — flattened
spinning clouds of gas and dust surrounding
young stars
These are believed to be examples of what our
solar system may have looked like when it started
to form
11 July 2005
AST 2010: Chapter 6
Other Planetary Systems (2)
Unfortunately, because of the enormous distance
involved, and the fact that planets forming are not
likely to reflect light very efficiently, astronomers have
yet to observe the actual formation of planets in
distant solar nebulas
Astronomers have now developed tools that enable
the observation of planets orbiting distant stars
The technique is still limited to the detection of giant
planets only
Observations nonetheless indicate clearly that other
solar systems have configurations that vastly differ
from the configuration of our solar system
Some have planets with very elliptical orbits
In other cases, giant planets are very near their stars
11 July 2005
AST 2010: Chapter 6