Introduction to Comparative Planetology
Chapter 7
Solar System Trivia
1. How many (real) planets are there in the solar system?
2. Which is the largest planet?
3. Which is the smallest planet?
4. Which planet is closest to the Sun?
5. Which planet would appear faintest from outside the solar system?
6. Which planet is most dense?
7. Which planet is less dense than water?
8. Which planet has the most moons?
9. Which planet has the highest surface temperature?
10. Which planet has the most carbon dioxide in its atmosphere?
Our Solar System to an Outsider
Orbits of planets
 Roughly circular
 Roughly in same plane
 Clue to origin of Solar System
Keplerian orbits
 Inner planets move fastest
Which Planets Would Appear Brightest?
Depends on:
 How large the planet is
 How much light it receives from the Sun
 How much light it reflects (albedo)
1. Jupiter
1.00
2. Venus
0.37
4. Earth
0.15
Mars
0.01
3. Saturn
0.19
Uranus
0.01
Neptune
0.003
Mercury
0.05
Two Groups of Planets
Inner planets
 Mercury, Venus, Earth, Mars
Outer planets
 Jupiter, Saturn, Uranus, Neptune
Sizes of the Planets
Brightest does not always mean biggest
 Uranus & Neptune are large but dim (far from Sun)
 Earth and Venus are small but bright (close to Sun)
Terrestrial planets (inner)
 Mercury, Venus, Earth, Mars
 Small, close to Sun
Jovian planets (outer)
 Jupiter, Saturn, Uranus, Neptune
 Large, far from Sun
How Are Terrestrial Planets Different to Jovian?
Terrestrial planets
 Rocky, mountains, craters, valleys, volcanos…
 No atmosphere or thin layer
Jovian planets
 Thick, deep atmosphere
 Is planet mostly gas or liquid?
Prediction:
 Jovian planets have lower densities
Mass
Mass
Density 
4 3
Volume
3 R
 Need mass and radius (or diameter) to test

Masses of Planets
Measure orbits of natural
satellites
 All but Mercury and Venus
have natural satellites
 Kepler’s 3rd Law tells you the
mass
Measure motion of spaceprobes
The Densities of Planets
Air = 1.2 kg/m3
Water = 1000 kg/m3
Concrete = 2000 kg/m3
The Densities of Planets
Air = 1.2 kg/m3
Water = 1000 kg/m3
Concrete = 2000 kg/m3
Terrestrial vs. Jovian Planets
The terrestrial planets are made of
rocky materials and have dense iron
cores, which gives these planets high
average densities.
The Jovian planets are composed
primarily of light elements such as
hydrogen and helium, which gives
these planets low average densities.
The Almost-Planets
More than 140 moons in the Solar system






Earth - 1
Mars - 2
Jupiter - 62+
Saturn - 43+
Uranus - 24+
Neptune - 13+
Seven are comparable to Mercury
 Similar to terrestrial planets
 Either rock or mixed rock & ice
The Ex-Parrots Planets
So there are eight planets, right…?
Right.
Pluto is now an ex-planet, a ‘Dwarf Planet’
 One of many Trans-Neptunian Objects
 Smaller than our Moon
 More than 900 objects in Kuiper Belt beyond Neptune
Beyond Average Densities
Why are Jovian planets less dense than Terrestrial planets?
Why is Saturn less dense than Jupiter?
Why is Callisto less dense than Mercury?
We need to know chemical compositions of planets - what are
they made of?
Spectroscopy in Action!
Kirchhoff’s 3rd Law
A cool, transparent gas in front of a source of a continuous
spectrum produces and absorption line spectrum
Spectroscopy of Titan’s Atmosphere
Methane!
 Absorption line of methane
does not come from Sun or
Earth
 UV+IR spectra tell us about
more compounds in Titan’s
atmosphere
Spectroscopy of Europa’s Surface
Direct Exploration of Other Planets and Moons
Venera 13 on Venus, 1982
Mars Rovers, 2004-present
Huygens Probe
descending on Titan,
2005
Chemical Composition of the Planets
Terrestrial planets (e.g. Mars)
 Heavy elements - iron, oxygen,
silicon, magnesium, nickel, sulfur
Jovian planets (e.g. Jupiter)
 Lightest elements - hydrogen,
helium