Overview of the Solar System
• The solar system consists of the Sun, eight known
planets (+ Pluto!), many satellites of the planets, and
a large number of interplanetary bodies (comets,
asteroids, meteoroids, interplanetary dust particles)
• Much information has been gathered by telescopes
and space probes
– Images reveal surface features (geology) and atmospheric
– Spectroscopic information (analysis of light from excited
atoms) provide insight on atmospheric composition
– Radio and infrared measurements collect data on
planetary surface temperatures
– NASA’s Cassini probe currently in orbit around Saturn
– Pluto fly-by mission (NASA’s New Horizons) launched in
2006 (will have closest approach to Pluto in about 7.5 yrs)
Overview of the Solar System
• The solar system has an overall disk-like structure
– The orbital planes of most planets are closely aligned with
each other
– The orbits of most planets are ellipses with small
eccentricities (nearly circular)
• All planetary orbits, and virtually all satellite orbits,
revolve in the same direction
– Counterclockwise as seen from above the north pole
• The direction of rotation for nearly all planets and
satellites is in the same sense as the orbital motions
– Exceptions are Venus, Uranus, and Pluto
Planetary Orbits
(inner planets)
(outer planets)
Overview of the Solar System
• Terrestrial planets: Mercury, Venus, Earth, Mars
4 innermost planets
Relatively small and dense with rocky surfaces
Average densities range from 3.5 – 5.5 g/cm3
Small abundance of light gases such as H and He
High abundance of metals and rocky materials
Few or no satellites
• Giant planets/gas giants/Jovian planets: Jupiter,
Saturn, Uranus, Neptune
Large outer planets
Low density, no solid surface
Average densities range from 0.7 – 1.7 g/cm3
Light gases (like H and He) are prolific (similar to Sun and
– Many satellites and ring systems
The Planets
• Overview of the planets and Sun (not to scale):
Relative Sizes of the Planets
• Picture of the Sun and the planets, made to scale:
(courtesy of Calvin J. Hamilton)
Scale Model of the Solar System
• Distance scales are used frequently on maps
– i.e. 1 inch = 10 miles
• Consider a scale model of the solar system where 1
inch = 1 million miles
– Diameter of Sun = 860,000 miles ≈ 1 million miles = 1 inch
on this scale (size of golf ball or ping pong ball)
– See next slide for table of average distance of each planet
from Sun, including distances using this scale model
– The planets out to Mars are much more closely spaced
than are Jupiter and the planets beyond it
– Solar system is almost entirely empty space
– Nearest star (Proxima Centauri) would be about 400 miles
away on this scale
• Approximately the distance from Columbus to Philadelphia
Scale Model of the Solar System (1 inch = 1 million miles)
True Distance
(million miles)
Distance (feet)
Scaled Distance
3 feet
5 feet 6 in.
8 feet
12 feet
13 yards
25 yards
50 yards
75 yards
100 yards
Scale Model of the Solar System
• Other distances on the scale of 1 inch = 1 million
– Earth – Moon orbital diameter: about ½ inch
– Diameter of Jupiter: about 1/10 of an inch (2.5 mm)
– Diameter of Earth: 1/100 of an inch (0.25 mm)
• Not realistic to represent planet sizes with this scaling
• Another alternative scale: 1 foot = 1 million miles
Sun = 1 foot across (basketball size)
Jupiter and Saturn = 1 inch across (golf ball size)
Uranus and Neptune = ¼ inch across (large peas)
Earth and Venus = 1/10 inch across (mustard seed)
Mercury = 1/3 size of Earth, Mars = ½ size of Earth
Nearest star would be 4000 miles away
Theory of Solar System Formation
• Catastrophic theories
– Solar system formed as the result of a singular
cataclysmic event caused by external forces
– In this view, planetary systems are rare
• Evolutionary theories
– Solar system was the result of natural internal processes
accompanying the formation of the Sun
– In this view, planetary systems orbiting other stars would
be common
• Themes from both models are used today
– Overall process of planet formation is thought to be
evolutionary, and planetary systems are predicted to be
– Many details of formation are thought to be the result of
singular, catastrophic events
Theory of Solar System Formation
• Solar system is believed to have originated from a
rotating, condensing cloud of interstellar gas and dust
• The cloud then collapsed under its own gravity
– Localized regions of high density formed (?)
– Blast wave from exploding star caused compression (?)
– As cloud collapsed, it flattened into a disk because of its
rotation (“solar nebula”)
• As the cloud collapsed, the compression was most
rapid at the center
– Central concentration became dense enough to form
molecular hydrogen, and heating occurred (cooling slowed
by molecular hydrogen)
– Further gravitational collapse slowed by gas pressure
– Temperature increased enough for nuclear fusion to occur
– Process took about 100 million years (beginning of Sun)
Theory of Solar System Formation
• Material from the solar nebula was still condensing
slowly after Sun was formed
• Gradually, the gas in the nebula began to condense
into planetesimals through process called accretion
– Temperature highest in the inner regions of the nebula
(radiation from Sun)
• Prevented light gases from condensing into solid form
• Heavier elements still condensed into solids
• Inner concentrations were smaller as a result
– Temperature was much cooler in the outer regions
• All elements condensed and incorporated
• Outer concentrations were thus larger
• Larger concentrations attracted more matter through gravity,
including that which formed disks around them (giving rise to
moons and rings)
– Collisions between planetesimals formed larger bodies
(giving rise to planets)
Theory of Solar System Formation
• Other, later, collisions thought to cause some irregularities
– Late collisions between planetesimals thought to cause unusual tilts of
Venus, Uranus, and possibly Pluto
– Formation of the Moon is thought to be the result of a collision between
Earth and a very large planetesimal
– Mercury may have lost much of its outer portion due to a collision
– Many craters are visible on planets and satellites resulting from
collisions with leftover debris in young solar system
• Some planetesimals did not help form planets
– Tidal forces exerted by young Jupiter formed asteroids between Jupiter
and Mars
– Pluto is thought of as a planetesimal that was never included into one
of the larger planets
– More minor planetary bodies are thought to be in Kuiper belt disk that
exists from beyond Uranus to 50 AU from Sun (source of comets)
– Oort cloud exists at 100,000 AU (another source of comets)
• Solar system is thought to be about 5 billion years old
• Planetology is the comparative study of Earth and
the other planets
– Several general processes and principles apply to all
– All planets began from same material, but various
processes have defined and altered their individual
• For example, relative amount of atmosphere on
each planet (and the Moon) can be understood in
terms of a common “kinetic theory” of gases
• Another example: amount of volcanic activity
– Understood in terms of common theory on heat transfer
and cooling mechanisms inside planets
• By comparing information from other planets to that
from Earth, we learn more about Earth as well
Internal Structures of Planets
• Differentiation causes relatively heavy elements to
sink toward the center of each planet
– Requires a fluid medium
– Differentiation in terrestrial planets has helped to create a
layered structure
• Thin outer crust
• Intermediate-density (semi-rigid) mantle
• Dense nickel-iron core (which is partially fluid in some cases)
– Differentiation in gas giants has created a small, solid core
beneath fluid layers that make up most of the interior
• Internal circulation, driven by planetary rotation,
occurs in any planet that has fluid zones in its interior
– Occurs in the core of the terrestrial planets (if at all)
– Complex in interiors of the gas giants
Internal Structures of Planets
• Magnetic fields and particle belts are manifestations
of internal circulation
– Internal fluid motions in a planet can create electrical
– Electrical currents create a magnetic field
– Fluid motions in Earth’s nickel-iron core have created a
significant magnetic field
– Mercury has weak magnetic field despite a slow rotation –
thus it probably has a large, partially molten core
– All of the gas giants have intense magnetic fields due to
internal circulation
– A planetary magnetic field can trap electrically charged
particles in belts or zones surrounding the planet
Surface features of Terrestrial Planets
• Slow-flowing motions in the mantle are responsible
for tectonic activity (movement of crustal plates)
– Associated with earthquakes and volcanic activity
– Flows in the mantle may be caused by convection
(overturning motion caused by heated bubbles rising in a
gravitationally confined fluid)
– On Earth, tectonic activity causes continental drift
– On Venus, crustal plates are locked together (no drifting)
– Tectonic activity is responsible for major structures of the
crusts of terrestrial planets (mountains, continental
• Several conditions influence surface geology
– Composition of crust determined by original composition of
the planet (after differentiation) and volcanism
– Erosion by groundwater
– Collisions with other bodies in space
Planetary Atmospheres
• A balance between gains and losses of various
constituents controls atmospheric composition
– Gains
• Accretion of gases during and after planetary formation
• Venting of gases from the interior
• Chemical and biological processes that occur on the surface
– Losses
• Escape of gases into space (determined by mass of
atoms/molecules and temperature of the atmosphere)
• Chemical and biological reactions at the surface
• Similar factors in all planets control the circulation of
an atmosphere
– Convection creates high- and low-pressure zones and
vertical flowing motions
– Planetary rotation converts simple convective flows into
rotary flow systems (creates both calm and stormy weather)

Scale Model of the Solar System