Chapter 27 PLANETS OF THE SOLAR SYSTEM
Read pp 685-7 and 695-708.
27.1 Formation of the Solar System (key terms: p 685)
The Nebular Hypothesis (Laplace, French mathematician 1796)
The sun and planets condensed at about the same time out of a rotating cloud of gas and dust
approx. 5 billion years ago. (Rotating cloud from which sun &planets formed is called the solar
nebula.) Center became hotter and denser due to forces from collisions and gravity. When
temp=107 oC, hydrogen fusion began &Sol (THE SUN) formed. The sun contains 99% of the
mass of the former solar nebula. (Fig 1 shown Orian nebula-not yet a solar system.)
Formation of the Planets (Fig 2 shows steps.)
Small bodies from which planet forms=planetesimals. Clumping planetesimals for larger bodies
called protoplanets. Clumping protoplanets and planetesimals eventually may become planets
and moons.
Inner Planets (Mercury, Venus, Earth, Mars)
Contain large %’s of heavy elements (ex: iron, nickel). Inner planets lost their dense gases
because at the temperature of the gases, gravity wasn’t strong enough to hold onto them; solar
radiation may have blown or boiled away gases. As densest materials sank, layers formed. Inner
planets are smaller, denser, rockier than outer planets.
Outer Planets (Jupiter, Saturn, Uranus, Neptune; NOT Pluto)
Colder B/C they are further from the sun, so they didn’t lose their gaseous atmospheres. (Gases
include helium, hydrogen, water (ice), methane (ice), ammonia (ice). Outer planets are gas
giants; Jupiter is 24%as dense as Earth, but has diameter 11x larger than Earth.
The Different Planet—Pluto (NOT considered a planet by the astronomy community)
Pluto is smaller than Earth’s moon. It is an ice ball made of frozen gases and rocks.
27.3 The Inner Planets
Mercury: 1
Venus: 2
Closest to the Sun
Orbits in 88 days
Rotates 1x in 58 days
Many craters/cliffs
No significant
atmosphere
Day temp=427oC
Night temp=-173oC
P695/fig1
Second from Sun
Orbits in 225 days
Rotates 1x in 234 days
Atmosphere=96%CO2!
SO2 in upper
atmosphere strongly
reflects light;
Commonly called the
evening star or morning
star.
Average temp=464 oC
Surface has basalt and
granite rocks
Has mountains, BIG
volcanoes, lava plains,
sand dunes p696/fig 2
Earth: 3
Mars: 4
Third from Sun
Orbits in 365 ¼ days
Rotates 1x in 1 day
Has one moon
LIQUID water@
surface
Atmosphere is 80%N2,
16%O2. Also CO2,
H2O.
Supports life. Geology
similar to Venus; but
plate tectonics prevent
formation of huge
volcanoes.
Fourth from the Sun
(50% further from Sun
than Earth)
Orbits in 687 days
Rotates 1x in 24h37min.
Geologically active in
past. Has mountains, BIG
volcanoes,erosion/canyons
No significant atmosphere
Temperatures range from
20oC nr equator (summer)
to -130oC nr poles(winter)
P698/fig 4
P699/fig 5
27.4 The Outer Planets (Gas Giants; see fig 1) (Pluto is not a planet.) [ starts p 701]
Gases : ________________ and _________________
Core: _________________ and __________________
Rings=_______________________________________ from _____________________
Jupiter: 5
Saturn: 6
Uranus: 7
Neptune: 8
Largest planet in solar
system (SS)
Orbits in 12 yrs
Rotates 1x in 9h50min
Has 60 moons, 4which
are the size of planets
Has thin rings
Hydrogen, helium are
92%of atmosphere
Rocky iron core?
Not enough mass to
become a star
Bands=organic
molecules, methane,
ammonia, water (g)
patterns due to rotation.
Avg. temp=-160oC
HUGE storms (Great
red spot=BIG hurricane)
Galileo probe:
winds>540 km/h
Jupiter’s internal heat
affects weather more
than solar heat.
P702/fig 2
Moons: Gannymede,
Callisto, Europa, Io
Orbits in 29.5 yrs
Rotates in 10hr30min
Rings=2x Saturn’s dia
Rings made of ice /dust
fr comets&other bodies
60+moons, Titan=1/2dia
of earth
Atmosphere similar to 5
Least dense planet in SS
Bands parallel to
equator due to rotation
Equatorial bulge
NASA Cassini (7/1/04)
And ESA Huygens
probes sent to study
Saturn and its moons.
P 704/fig 4
Orbits in 84 yrs
Rotates in 17 hr
3rd largest planet
at least 24 moons, 11
rings
Rotational axis is nearly
parallel to orbital plane
(Voyager 2 discovery,
1986)
Atmosphere is mostly
hydrogen, helium; bluegreen=methane
Avg cloud t =-214oC;
planet may be much
hotter
(Core temp7000oC)
Hubble Space Telescope
used to study Uranus
p705/fig 5
Orbits in 164 yrs
Rotates in 16h
Similar to Uranus (size,
mass)
At least 8 moons, 4
rings(?)
Discovered because it
affected Uranus orbit
(gravity)
Atmosphere is mostly
hydrogen, helium,
methane. (Bands)
Great dark spot=storm
the size of Earth
(1000 km/h winds)
Avg cloud t=-225oC
P706/fig 6
Pluto (discovered 1930): p707/fig 7
Orbits the Sun in elongated, tilted ellipse. Atmosphere=nitrogen. Surface=methane, rocks, ice.
Avg temp=-235 oC. Moon=Charon, half of Pluto’s size
Kuiper belt=region beyond Pluto containing small bodies (mostly ice). Quaoar=3/4ths Pluto’s size.
Beyond Kuiper belt: Sedna (discovered 3/04) 3x farther from Earth than Pluto.
Exoplanets= (p708/fig 8)Planets circling stars other than the Sun. Exo=______________________
Hard to observe. Detected due to gravity/Doppler shift of light from stars, larger than Uranus.
http://bill.nineplanets.org/arnett.html (Bill Arnett is a software engineer, but his sources include NASA/JPL)
http://nineplanets.org/overview.html There are moons larger than Pluto. Pluto isn’t considered to be a
planet anymore. There are 2 moons larger than Mercury.
http://www.youtube.com/watch?v=VyQ8Tb85HrU is a video comparison on youtube with animated
models of Ptolomy’s Solar System model (geocentric) and Copernicus’ Solar System model (Heliocentric)
Inner planets are also called terrestrial planets because they are similar to Earth. (Solid rock with metallic
cores. Moons vary from 0-2. Impact craters on surface.
Outer planets are also called Jovian planets (gas giants). Asteroid belt separates inner and outer planets.
http://www.youtube.com/watch?v=KKXvRBfCyHU is a Penn State video about rotation of gas giants.
Read pages 688-694
Formation of Solid Earth
Collisions of planetesimals, compression of inner layers and radioactivity led to Earth’s extremely high temperature
during formation.
Early Solid Earth: Differentiation led to the most dense elements (including iron) sinking to the center to form a
molten iron and nickel core. The thick mantle surrounds the core and contains iron and magnesium-rich rocks. The
thin crust is less sense silica-rich rocks (ex: quartz, sand). Plate tectonics (shape the Earth) are driven by heat
transfer and differences in density below the crust.
Present Solid Earth: Eventually Earth’s surface cooled enough for solid rocks to form. Crust formed during
differentiation. Earth’s surface still changing due to heat from Earth’s interior and impacts and interactions with the
newly formed atmosphere.
Formation of Earth’s Atmosphere
Original Atmosphere: hydrogen and helium-blown away by solar wind? Earth’s magnetic field may not have been
fully developed when Earth formed.
Outgassing released new atmosphere during volcanic eruptions: water vapor, carbon dioxide, nitrogen, methane,
sulfur dioxide, &ammonia. Solar radiation changed some of the water vapor and ammonia to react leading to
release of hydrogen (ended up in space) and oxygen and nitrogen. Some of the oxygen reacted to form ozone in the
upper atmosphere. Ozone layer shields the Earth’s surface from UV radiation. Some of the water on Earth’s surface
came from collisions of comets, asteroids, but most resulted from outgassing.
Present Atmosphere: Cyanobacteria and early green plants appeared and began to use carbon dioxide during
photosynthesis. Oxygen is released as a byproduct of photosynthesis. Fig 5: 78%nitrogen; 21% oxygen; 1% other.
As Earth cooled, water condensed to form rain. Over millions of years the water has cycled from the
atmosphere to the oceans. Ocean’s salt content is due to dissolving rocks which are water soluble.
(Fig 6. When ocean water is heated by the sun, water evaporates leaving salt.) Oceans moderate global
temperatures. Oceans can dissolve carbon dioxide, but there’s a limit.
Early Models of the Solar System (p 691)
Aristotle (Greek philosopher): proposed geocentric solar system. Ptolomy (Greek philosopher) (130) used
epicycles (small circles) to explain why planets “moved backwards” in geocentric orbits around Earth.
Copernicus (Polish astronomer) (1543) proposed a heliocentric solar system. Galileo Galilei (using a telescope)
observed 4 moons orbiting Jupiter; confirmed that objects don’t have to orbit Earth.
Tycho Brahe (Danish astronomer) made detailed observations of the solar system before he died. His assistant,
Johannes Kepler, discovered patterns to the observations and developed 3 laws which explained the motion of
planets.
Kepler’sLaws:
1) Law of Ellipses: Each planet orbits the sun in a path called an ellipse (2 foci, one of which has the Sun at the
center) whose eccentricities differ. (Eccentricity defines how noncircular the ellipse is.)
2) Law of Equal Areas: equal areas are covered in equal amounts of time as an object orbits the sun. (See fig 2,
p693. Halley’s comet moves fast when it’s close to the sun but slowly when it’s farther away. A line from the
center of the sun to the center of an object sweeps through equal areas in equal periods of time. Ex: 2313 days.)
Law of Periods: The cube of the average distance (a) of a planet from the sun is always proportional to the square
of the orbital period (p). Orbital period=how long it takes for a planet to orbit the sun. Ka  p
3
2
The longer the orbital period, the farther the planet is from the sun. (Sample calc for Jupiter@ end of p 693.)
Isaac Newton’s Explanation of Kepler’s Laws

Inertia keeps objects moving with the same speed and direction unless acted on by a force.
The balanced forces of gravity and inertia keep planets in stable orbits. Outer planets are less attracted by
the gravity of the sun than inner planets, so their orbits are curved more gently and they have longer periods
of revolution.
Newton’s law of universal gravitation:
m  m 
F  G 1 2 2 
 d

When distance doubles, F  by factor of____

