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James T. Shipman
Jerry D. Wilson
Charles A. Higgins, Jr.
Chapter 16
The Solar System
• Astronomy – the scientific study of the universe
beyond Earth’s atmosphere
• Universe – everything, all energy, matter, and
• The Milky Way– one of 50 billion galaxies
scattered throughout the universe
• Solar System – contains our Sun and 9 planets
• Sun – supplies the energy for nearly all life on
the planet earth
Electromagnetic Spectrum
Astronomers are interested in studying the full
range of electromagnetic spectrum
coming from space
• Much of the incoming solar radiation does not
make it to the Earth’s surface – due to
atmospheric absorption
• Electromagnetic radiation that will pass through
the Earth’s atmosphere can be studied using
ground-based detectors
• Other regions of the electromagnetic spectrum
must be detected by space-based instruments
• The Hubble Space Telescope is a good example of
an instrument outside Earth’s atmosphere
The Solar System
• The solar system - complex system of moving
masses held together by gravitational forces
• Sun is center
• Sun is the dominant mass
• Revolving around the sun – 8 major planets with
over 170 moons, 4 dwarf planets, and 1000’s of
other objects (asteroids, comets, meteoroids,
Section 16.1
The Solar System
• Geocentric Model – early belief that the Earth
was motionless and everything revolved around
• Claudius Ptolemy (A.D. 140)
• Heliocentric Model – a Sun-centered model
• Nicolaus Copernicus (1473-1543)
Section 16.1
Johannes Kepler (1571-1630)
• German mathematician and astronomer
• Kepler’s 1st Law – Law of Elliptical Orbits – All
planets move in elliptical orbits around the Sun
with the Sun as one focus of the ellipse
• An ellipse is a figure that is symmetric about two
unequal axes
Section 16.1
Drawing an Ellipse
• An ellipse has two foci, a major axis, and a
semimajor axis
• In discussing the
Earth’s elliptical orbit,
the semimajor axis is
the average distance
between the Earth
and the Sun =
Astronomical Unit
(AU) = 1.5 x 108 km
Section 16.1
Kepler’s Second Law
• Law of Equal Areas – An imaginary line (radial
vector) joining a planet to the Sun sweeps out
equal areas in equal periods of time
Section 16.1
The speed of a revolving planet varies
• Perihelion – the closest point in a planet’s orbit
around the Sun, speed is the fastest
• Perihelion occurs for Earth about January 4
• Aphelion – the
farthest point in a
planet’s orbit around
the Sun, speed is
the slowest
• Aphelion occurs for
Earth about July 5
Section 16.1
Kepler’s Third Law
• Harmonic Law – the square of the sidereal
period of a planet is proportional to the cube of
its semimajor axis
• T2 = k R3
• T = period (time of one revolution)
• R = length of semimajor axis
• k = constant (same for all planets) = 1y2/AU3
Section 16.1
Major Planet Classification
and Orbits in Our Solar System
• Sun – 99.87% of the mass of solar system
• Of the remaining 0.13%, Jupiter is > 50%
• Planets with orbits smaller than Earth are
classified as “inferior”
• Planets with orbits larger than Earth are
classified as “superior”
Section 16.2
• All planets revolve (orbit) counterclockwise (prograde
motion) around the Sun as observed from the north pole.
Each planet also rotates counterclockwise on its axis
except Venus and Uranus (retrograde motion).
Section 16.2
Planet Classification
• Terrestrial planets – Mercury, Venus, Earth,
• High percent of more massive (non-gaseous)
• Jovian planets - Jupiter, Saturn, Uranus, and
• High percent of less massive gaseous elements
Section 16.2
The Solar System -- drawn to scale
with the eight major planets
Section 16.2
A Planet’s Period
• Sidereal Period – the time interval between two
successive conjunctions as observed from the
• Mercury’s sidereal period is 88 Earth days
• Synodic Period – the time interval between two
successive conjunctions of the planet with the
Sun as observed from Earth
• Mercury’s synodic period is 116 Earth days
Section 16.2
Mercury shown in inferior and
superior conjunctions with Earth
Section 16.2
Planet Earth
• The Earth is the third planet from the sun, and is
a solid, spherical, rocky body with oceans and
an atmosphere
• Large amounts of surface water in all three
phases – solid, liquid, and gas – exist on Earth
• An oxygen-containing atmosphere, temperate
climate, and living organisms all make Earth a
unique planet
Section 16.3
Composition of the Earth
• Atmosphere – 21% oxygen
• Earth’s crust – over 90%, by volume, of the
rocks/minerals are oxygen!
• We live in an oxidized environment
• Examples of very common minerals at the
Earth’s surface include:
• Quartz – SiO2, Calcite – CaCO3, Feldspar – KAlSi3O8
• Note that most common minerals have oxygen
(O) in their formula
Section 16.3
Earth’s Shape
• The planet Earth is not a perfect sphere, but
rather an oblate spheriod
• Flattened at the poles
• Bulging at the equator
• Due to rotation about its axis
• Pole Diameter is about 43 km less than the
Equatorial Diameter
• Since the Earth has an average diameter of 12,900
km this difference is only a small fraction
Section 16.3
• Albedo – the fraction of the incident sunlight
reflected by an object
• Earth’s albedo is 33%
• Moon’s albedo is 7% (from Earth the moon is
the 2nd brightest object in the night sky)
• Venus’ albedo is 76% (3rd brightest is sky)
• Since the Moon is so close to Earth it is brighter
than Venus
Section 16.3
Earth Motions
• Daily Rotation on its axis (daily cycle)
• Rotation – spin on an internal axis
• Annual revolution around the sun (annual cycle)
• Revolution – movement of one mass around another
• Precession – the slow change of the earth’s
rotational axis (now at 23.5o) – see chapter 15
Section 16.3
Earth’s Rotation on its Axis
• Not generally accepted until 19th century
• Very difficult to prove???
• 1851, experiment designed by French engineer,
Jean Foucault
• The Foucault Pendulum – very long pendulum
with a heavy weight at the end
• Basically, the Foucault pendulum will swing back
and forth as the Earth moves under it
Section 16.3
Foucault Pendulum
The pendulum does not
rotate with reference to
the fixed stars.
Experimental proof of the
Earth’s rotation
Section 16.3
• Parallax – the apparent motion, or shift, that
occurs between two fixed objects when the
observer changes position
• Parallax can be seen with outstretched hand
• The motion of Earth as it revolves around the
Sun leads to an apparent shift in the positions of
the nearby stars with respect to more distant
Section 16.3
Stellar Parallax
• The observation of
parallax is
indisputable proof that
the Earth revolves
around the Sun.
• In addition, the
measurement of the
parallax angle is the
best method we have
of determining the
distance to nearby
Section 16.3
Aberration of Starlight
• A 2nd proof of Earth’s orbital motion
• Telescopic observations of a systematic change
in the position of all stars annually
• Due to the motion of the Earth around the Sun
• Angular discrepancy between the apparent
position of a star and its true position, arising
from the motion of an observer relative to the
path of the beam of light observed
• This is similar to what you see when driving in the
Section 16.3
Aberration of Starlight
• This discrepancy is very small and is measured
in a -- parsec
• Parsec  parallax + second
• Recall that a circle contains 360o. Each degree
is divided into 60 minutes, and each minute into
60 seconds
• Therefore 1 second = 1/3600 degree
• Parsec = the distance to a star when the star
exhibits a parallax of 1 second.
Section 16.3
Terrestrial Planets
• The terrestrial planets include: Mercury, Venus,
Earth, Mars
• Due to physical/chemical characteristics they
resemble Earth
• All four terrestrial planets are
• Relatively small in size and composed of rocky
material and metals
• Relatively close together and close to Sun
• Have no rings
• Only Earth and Mars have moons
• Only Earth has surface water and oxygen
Section 16.4
• Mercury is the closest planet to the Sun
• Mercury has the shortest period of revolution (88
days), and is the fastest moving
• Mercury was named by the early Greeks after
the swift messenger of the gods
• Temperatures on Mercury range from about
473oC on the side facing the Sun to about 173oC on the dark side
• Due to its small size and closeness to the Sun,
Mercury has practically no atmosphere
• It has a density close to that of Earth.
Section 16.4
Mercury, a Terrestrial Planet
The Messenger
spacecraft mapped the
surface of Mercury. It
was the first
spacecraft to orbit the
Rotates 3 times while circling the Sun twice
Section 16.4
• Venus is the closest planet to Earth
• Venus is the third brightest object in the sky
• Due to its brightness it was named after Venus
the goddess of Beauty
• The surface of Venus cannot be seen from
Earth, due to dense, thick clouds that cover the
• Magellan radar images indicate that the surface
of Venus is composed of black, hot rock
• Most surface rocks appear to be volcanic
Section 16.4
The Atmosphere of Venus
• Venus’ atmosphere is composed of 96% CO2
• It is so dense that the surface of Venus has a
pressure of 90 atm
• The large percent of CO2 in the atmosphere
results in high surface temperatures (477o C)
due the “greenhouse effect”
• Radar images have revealed relatively few
impact craters
• Most of these craters are fairly large, because the
smaller incoming objects are consumed by Venus’s
thick atmosphere
Section 16.4
Venus, a Terrestrial Planet
A radar map of
Venus. The inset is
from the Russian
spacecraft Venera
Atmosphere rotates faster than solid planet.
Retrograde rotation of planet
Section 16.4
• Mars has a red color, as viewed from the Earth,
and was named for the Roman god of war
• The surface of Mars has two outstanding
features that have intrigued scientists for
decades; polar ice caps and extinct volcanoes
• The ice caps are composed of frozen CO2 in the
winter and CO2 vapor with frozen water in the
• The red color is thought to be due to fine grain
iron oxide minerals.
Section 16.4
Martian Volcano – Mt. Olympus
• The largest
volcano in
the solar
system, at 24
km in height,
it is about
three times
that of
Mauna Loa
Section 16.4
Mars at closest approach
Section 16.4
Section 16.4
Mars – Valles Marineris
This canyon on Mars is
4000 km in length and
6 km deep
Geologists think that it
is a crustal fracture
caused by internal
Section 16.4
In 2006 the Opportunity rover reached the edge
of Victoria crater
Section 16.4
The Jovian Planets
• Jupiter, Saturn, Uranus, Neptune
• Much larger than the terrestrial planets
• Composed mainly of hydrogen and helium
• The four Jovian planets have a very low average
density (approximately 1.2 g/cm3)
• All four are thought to have a rocky core
composed of iron and silicates
• Thick layers of frozen methane, ammonia, and
water are found above the core
Section 16.5
Formation of the Terrestrial Planets
• The two least massive elements – H & He –
were the most abundant when the planets
started to coalesce about 5 billion years ago
• Due to the heat from the Sun most of these less
massive elements escaped the gravitational pull
of the inner planets
• Leaving behind more of the massive elements and
resulting in thick rocky cores and higher densities for
the inner planets
Section 16.5
Formation of the Jovian Planets
• The four large outer planets were much farther
from the Sun and therefore much colder
• The Jovian planets retained most of their H and
He which now surround their ice layers and
innermost rocky cores
• As a consequence the Jovian planets have a
much lower average density
Section 16.5
• Largest planet of the solar system, in both volume and
total mass
• Named after the supreme Roman god of heaven
because of its brightness and giant size
• Diameter is 11 times Earth’s -- 318 times more mass
than Earth
• The average density of Jupiter approximately 1.3 g/cm3
• Jupiter is covered with a thin layer of clouds composed
of hydrogen, helium, methane, ammonia, and several
other substances
• It is a fast rotator – taking about 10 hours to rotate.
Section 16.5
Section 16.5
Jupiter’s Great Red Spot (“eye”)
• The Great Red Spot
has erratic movement,
shape, color, and size –
sometimes even
• Likely a huge
“hurricane-like” storm,
lasting hundreds of
Section 16.5
• Distinctive system of three prominent rings
• Rings are inclined 27o to orbital plane
• The rings are thought to be composed of ice and
ice-coated rocks (micrometers  10 m)
• Most spectacular sight that can be viewed from
Earth with a small telescope
• Diameter is 9 times Earth’s -- 95 times more
mass than Earth
• Average density of only 0.7 g/cm3
Section 16.5
Saturn and its rings
Section 16.5
• Discovered in 1781 by William Herschel (17381822), an English Astronomer
• Named after Uranus, the father of the Titans and
the grandfather of Jupiter
• Thin ring system composed of boulder-size
particles (>1m), with very little dust-size
• Average density of only 1.3 g/cm3
Section 16.5
Uranus – unlike the other planets Uranus
revolves around the Sun on its side and rotates
in a retrograde fashion
• Discovered in 1846 by Johann Galle, a German
• Englishman John Couch Adams and Frenchman
U.J.J. Leverrier were mathematicians using
Newton’s law of gravitation
• They noted that Uranus’ motion was disturbed
and predicted the location of another planet –
this is how Galle eventually discovered Neptune
Section 16.5
• Neptune also has a large dark spot similar to
Jupiter’s and thought to be the result of large
wind systems
• Neptune and Uranus are similar in size and in
the composition of their atmospheres
• In many respects these two planets can be
considered twins
Section 16.5
Section 16.5
Designations of Celestial Bodies
International Astronomical Union (IAU)
• In 2006 the IAU adopted the following criteria for
a solar system body to be a planet:
• (1) It must be in orbit about the Sun.
• (2) It must have sufficient mass for self-gravity to
form a nearly round shape.
• (3) It must be the dominant body within its orbit.
• The last statement disqualifies Pluto
Section 16.6
The Dwarf Planets
• Pluto’s orbit takes it inside that of Neptune’s, \
Pluto is not the dominant body of its orbit.
• The IAU established two new categories for
objects that orbit the Sun.
• Dwarf planets is one of the categories
• Pluto, Ceres and Eris are now designated dwarf
Section 16.6
Ceres – First Dwarf Planet from the
Sun Between Mars and Jupiter
• Lies in the asteroid belt between Mars and Jupiter
• Discovered in 1801 by Italian Giuseppe Piazzi and
named after the Roman goddess of agriculture
• Has a diameter of only 940 km and is the smallest dwarf
Section 16.6
Pluto – Second Dwarf Planet
from the Sun Beyond Neptune
• Names for the god of outer darkness
• Average density of only 1.65 g/cm3
• Discovered in 1930, by C.W. Tombaugh
• Investigating discrepancies in the orbital path of
Neptune and Uranus
• Does not resemble either the terrestrial or
Jovian Planets
• Pluto has not been visited by a space probe –
flyby planned in 2015.
Section 16.6
• There are similarities between Pluto and Triton,
of Neptune’s moons
• Some scientists think that both are large
asteroids captured from interplanetary space
• If this is the case, Pluto has maintained its own
orbital path, and Triton was captured by
Section 16.6
Pluto and its Satellites
Section 16.6
The Solar System
Pluto’s orbit is greatly inclined to
the orbital plane of the major planets
Section 16.6
Eris – Third Dwarf Planet from the
Sun Far Beyond Neptune and Pluto
• Previously known as 2003 UB313 (or Xena)
• Officially now named Eris, after the Greek
goddess of chaos and strife.
• Slightly larger and about 3 times farther away
form the Sun than Pluto
• Highly elliptical orbit that takes 560 Earth years
for one revolution.
Note on Xena: Dr. Mike Brown- CalTech scientist- used data from the
Palomar Observatory to identify it.
Section 16.6
Dwarf planets Haumea and
• Haumea mass 30% of Pluto and orbits at 43 AU.
• Makemake mass 75% of Pluto and orbits at 75
Section 16.6
Outermost reaches of the Solar
• The Kuiper Belt extends just beyond the orbit of
Neptune and into the space of Eris.
• Consists of comet and cometary material and other
small objects – Trans Neptunian Objects
• Many astronomers put the edge of the solar
system to be at about 100 AU.
• Voyager 1, launched in 1977, and in 2004
reached 100 AU. In 2010 it crosses the
boundary of zero solar wind velocity.
Section 16.6
Origin of the Solar System
• Any theory that purports to explain the origin and
development of the solar system must account
for its present form
• According to our best measurements, our solar
system has been in its present state for about
4.5 billion years
• A valid theory for solar system formation – must
be able to explain a number of major properties
of our solar system
Section 16.7
Major Questions
Concerning Solar System
Origin of material?
Forces that formed the solar system?
Isolated planets, circular orbits, in same plane?
Revolution (orbit) in the same direction?
Most Rotate in same direction (except two)?
Terrestrial versus Jovian planets?
Origin of the asteroids?
Origin of comets and meteoroids?
Section 16.7
Formation of the Solar System
• Began with a large, swirling volume of cold
gases and dust – a rotating solar nebula
• Contracted under the influence of its own gravity
– into a flattened, rotating disk
• Further contraction produced the protosun and
eventually accreted the planets
• As particles moved inward, the rotation of the
mass had to increase to conserve angular
momentum (like an ice skater bringing in her
Section 16.7
The Formation of the Solar System
Condensation Theory
Section 16.7
Other Planetary Systems
• Are there other planetary systems in the
• If so, we would expect to find some of these
systems in different stages of formation
• In other words, we should be able to find clouds of
gas and dust, primordial nebula, and protosuns, etc.
• We should also be able to use gravitational
effects to detect small wobbles due to rotational
objects in space
• These are called exoplanets or extra-solar
Section 16.8
Gravitational Effects
• A star with a large planet orbiting about it will
have a small wobble superimposed on its motion
as a result of gravitational effects
• This change in motion (the wobble) is likely to be
very slight, but in some cases may be detected
as a Doppler shift of the star’s spectrum
• As the star approaches the observer, the
wavelengths are compressed (‘blue shift’)
• As the star move away from the observer, the
wavelengths are lengthened (‘redshift’)
Section 16.8
Gravitational Effects
• The amount of wobble can be used to determine
the planet’s mass (related to gravitational pull)
• The wobble’s cycle time can be used to
determine the orbital period
• Once the orbital period is known, Kepler’s third
law (T2= kR3) can be used to determine the
planet’s average distance from the star
Section 16.8
Star Wobble
Due to the gravitational pull of an orbiting planet.
Section 16.6
The wobbling in this illustration is greatly exaggerated!
Section 16.8
Transit method
• A planet passing in front of its star as seen from
• The star’s light will temporarily dim
• The Kepler mission uses the transit method.
Section 16.8
First Planets
Discovered Beyond our Solar System
• In 1992, using the Arecibo Observatory in
Puerto Rico, astronomers reported the discovery
of two objects revolving about a pulsar
• Pulsars are very dense, rapidly rotating stars
• Pulsars have a very precise rotation period
• If the rotation period is disrupted, this would indicate
the presence of an object rotating about the pulsar
• These two objects are the first planets detected
beyond our solar system
Section 16.8
Planets Beyond our Solar System
• There have now been approximately 500
planets detected around other stars and Kepler
is finding more and more.
• These findings strongly indicate the existence of
many other planetary systems in the universe
• Scientists are also searching for signals from
extraterrestrial intelligence (SETI)
• Equipment today is being used to scan wide
frequency ranges over vast areas of the sky
Section 16.8