Welcome to the Solar System
“How vast those Orbs must be, and how
inconsiderable this Earth, the Theater upon
which all our mighty Designs, all our
Navigations, and all our Wars are transacted, is
when compared to them. A very fit
consideration, and matter of Reflection, for those
Kings and Princes who sacrifice the lives of so
many People, only to flatter their Ambition in
being Masters of some pitiful corner of this small
Spot.”
Christiaan Huygens (1629 -- 1695)
Dutch Astronomer and Scholar
Reading for Solar System: Look up solar system – tell me about the
references - read them for more understanding. (or ask in class)
The Layout of the Solar System
• Large bodies in the Solar System have orderly motions
–
–
–
–
planets orbit counterclockwise in same plane
orbits are almost circular
the Sun and most planets rotate counterclockwise
most moons orbit counterclockwise
The Layout of the Solar System
• Planets fall into two main categories
– Terrestrial (i.e. Earth-like)
– Jovian (i.e. Jupiter-like or gaseous)
Mars
Neptune
Terrestrial
Jovian
The Layout of the Solar System
• Swarms of asteroids and comets populate
the Solar System
A Few Exceptions to the Rules…
• Uranus is tilted on its side.
• Venus rotates “backwards” (i.e.
clockwise).
• Triton orbits Neptune “backwards.”
• Earth is the only terrestrial planet with a
relatively large moon.
The Sun – King of the Solar System
• How does the Sun influence the planets?
– Its gravity regulates the orbits of the planets.
– Its heat is the primary factor which determines
the temperature of the planets.
– It provides practically all of the visible light in
the Solar System.
– High-energy particles streaming out from the
Sun influence planetary atmospheres and
magnetic fields.
A Brief Tour of the Solar System -- Motions
A Brief Tour of the Solar System – Composition
The planets are tiny compared to the distances between them (a
million times smaller than shown here), but they
exhibit clear patterns of composition and motion.
The patterns are far more important and interesting than numbers,
names, and other trivia
Recall scale of solar system
Planets are tiny compared to distances
between them…
Sun is the size of a CD (~12 cm)
– then:
Earth – 15 m
Mars – 23 m
Jupiter – 78 m
Saturn –143 m
Uranus – 287 m
Sun
• Over 99.9% of solar system’s mass
• Made mostly of H/He gas (plasma)
• Converts 4 million tons of mass into energy each second
Mercury
• made of metal and rock; large iron core
• desolate, cratered; long, tall, steep cliffs
• very hot and very cold: 425°C (day), –170°C (night)
Venus
• nearly identical in size to Earth; surface hidden by thick clouds
• hellish conditions due to an extreme greenhouse effect:
• even hotter than Mercury: 470°C, both day and night
• atmospheric pressure equiv. to pressure 1 km deep in oceans
• no oxygen, no water, …
• perhaps more than any other planet, makes us ask: how did it end
up so different from Earth?
Earth and
Moon to scale
Earth
• An oasis of life
• The only surface liquid water in the solar system; about 3/4 of
surface covered by water
• A surprisingly large moon
Mars
• Looks almost Earth-like, but don’t go without a spacesuit!
• Giant volcanoes, a huge canyon, polar caps, more…
• Water flowed in the distant past; could there have been life?
Jupiter
• Much farther from
Sun than inner 4
planets (more than
twice Mars distance)
• Also very different in
composition: mostly
H/He; no solid surface.
• Gigantic for a planet:
300  Earth mass;
>1,000  Earth
volume.
• Many moons, rings…
Moons can be as
interesting as the
planets themselves,
especially Jupiter’s
4 large “Galilean
moons” (first seen
by Galileo)
• Io (shown here): active volcanoes all over
• Europa: possible subsurface ocean
• Ganymede: largest moon in solar system — larger than Mercury
• Callisto: a large, cratered “ice ball” with unexplained surface features
Saturn
• Giant and gaseous
like Jupiter
• most spectacular
rings of the 4 jovian
planets
• many moons,
including cloudcovered Titan
• currently under
study by the Cassini
spacecraft
Saturn
Rings are NOT
solid; they are
made of
countless small
chunks of ice
and rock, each
orbiting like a
tiny moon.
Artist’s conception
Saturn
Cassini probe
arrived July 2004
(Launched in 1997)
Uranus
• much smaller than
Jupiter/Saturn, but still
much larger than Earth
• made of H/He gas,
hydrogen compounds
(H2O, NH3, CH4)
• extreme axis tilt —
nearly tipped on its
“side” — makes
extreme seasons during
its 84-year orbit.
• moons also tipped in
their orbits…
Neptune
• Very similar to
Uranus (but much
smaller axis tilt)
• Many moons,
including unusual
Triton: orbits
“backward”; larger than
Pluto.
Pluto
• A “misfit”: far from Sun like large jovian planets, but much
smaller than any terrestrial planet.
• Comet-like composition (ices, rock) and orbit (eccentric, inclined
to ecliptic plane, long -- 248 years).
• Its moon Charon is half Pluto’s size in diameter
• Best current photo above; New Horizons mission launch 2006,
arrival 2015…
What features of our solar system
provide clues to how it formed?
The Sun, planets, and large moons orbit and
rotate in an organized way
counterclockwise
seen from above
the north pole)
Terrestrial planets are small, rocky, and close to the Sun.
Jovian planets are large, gas-rich, and far from the Sun.
(What about Pluto?)
Rocky asteroids
between Mars & Jupiter
Icy comets in vicinity of
Neptune and beyond
Asteroids and comets
far outnumber the
planets and their moons
A successful theory
of solar system
formation must
allow for exceptions
to general rules
Summary: Four Major Features of our Solar System
What theory best explains the
features of our solar system?
According to the
nebular theory our
solar system formed
from a giant cloud of
interstellar gas
(nebula = cloud)
Where did the solar system come
from?
The cloud of gas that gave birth to our solar system resulted
from the recycling of gas through many generations of stars
within our galaxy.
What caused the orderly patterns of
motion in our solar system?
Start with a cloud of gas and
dust…As gravity forced the
cloud to become smaller, it
began to spin faster and
faster
As gravity forced the cloud
to become smaller, it began
to spin faster and faster
Conservation of angular
momentum
As gravity
causes cloud to
shrink, its spin
increases
Conservation
of angular
momentum
I  Mr
2
I   constant
Collisions flatten the cloud into a
disk.
The orderly motions of our solar
system today are a direct result of
the solar system’s birth in a
spinning, flattened cloud of gas.