Comparative Planetology
• Comparative Planetology is the comparing and
contrasting of different worlds to describe and categorize
them
• Important Properties:
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Distance to the Sun
Orbital Period
Radius
Mass
Rotation Period
Density
Solar System Layout
• All planets travel in elliptical orbits with the Sun
at one focus
• Most eccentricities are low
• Most lay in the same orbital plane
• Mercury and Pluto are the exceptions
– Each has significant eccentricity and don’t lie in the
orbital plane
Terrestrial Planets
• Properties of all terrestrial planets
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Within about 1.5 AU of the Sun
Relatively small
Relatively high density
Rocky Composition
Solid Surfaces
• Some Differences
– Atmospheres are very different
– Some have moons
– Surface conditions very different
Jovian Planets
• Properties of Jovian planets
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Farther away from the Sun
Made almost entirely of gas
Relatively Large
Strong Magnetic Fields
Many Moons
All have Rings
• Some Differences
– Compositions different
– Inner Structure differences
Terrestrial vs. Jovian
Pluto
• What is Pluto?
Interstellar Matter
Interstellar Matter
Interstellar Matter
• Interstellar matter is made of gas and dust
– The gas is mostly individual atoms and small
molecules
– 0.1 – 1.0 nanometers in size
– Gas does not account for all the obscuration of light
• Interstellar dust is made of clumps of atoms and
molecules
– Dust absorbs or scatters light (like headlights in fog)
– Obscuration increases with decreasing wavelength
Interstellar Matter
• Interstellar dust is typically about 100 nm in size
– This makes the dust invisible to radio waves
– The dust is opaque to shorter wavelengths
– “Extinction” is the term for the dimming out of light
• Because dust blocks the shorter wavelengths
more than the longer wavelengths, visible light
loses some of its blue component
– Makes the light appear more red
– “Reddening”
Interstellar Matter
• Notice dust cloud
edges
• Cloud blocks some of the
blue light intensity
Interstellar Matter
• Space is an empty place? A dirty place?
• Average gas density ~ 9 billion atoms per m3
– Better than any vacuum created on Earth
– Earth’s gas density is approximately 1x1025 atoms per m3
– That is a million – billion times more!
• Average dust distribution ~ 1000 particles per km3
• Volume the size of Earth would not fill a coffee cup
Interstellar Matter
• Earth’s atmosphere has 1 dust
particle per 1018 gas atoms
• Space has 1 dust particle per
1012 gas atoms
• If the gas density of space was
equal to Earth, we couldn’t see
our hand in front of our face
• As we look over large distances,
this is significant
Interstellar Matter
Interstellar Medium
• Nebula: “Fuzzy” patches in the sky
• Emission Nebulae: gas clouds hot enough
(thousands of Kelvins) to emit visible light
• Dark Dust Cloud: Cold and dense (relatively)
clouds of dust and gas
Nebulae
Nebulae
Dark Dust Clouds
21-cm Radiation
• How do you observe nebula that are too dense or
cool to emit usable radiation?
• 21-cm wavelength radiation is emitted from cool
atomic Hydrogen
• This long wavelength
allows the radiation
to penetrate dust
Nebular Theory
• The solar system
started as a cloud of
hot gas
• The cloud’s gravity
started to pull it
inward
• As it pulled inward it
started to rotate
Nebular Theory
• Because of the rotation it started
to flatten out
• It started to spin faster as the
cloud shrunk
• Planets formed in cooler outer
regions
Condensation Theory
• Condensation theory adds to the nebular theory by
introducing DUST
• Models show that gas alone would not clump
together
• Dust particles act as the nucleus for larger object
formation
Condensation Theory
Condensation Theory
Angular Momentum
• When objects spin, Newton says they should
keep spinning
• Spinning objects have momentum
• This momentum must be conserved
Angular Momentum
• Angular momentum: L = Iω
• For Conservation of Momentum:
Ii ωi = If ωf
• I is the moment of inertia
– For spheres I = 2/5 MR2
• ω = angular velocity (how fast it rotates)
• If the moment of inertia goes down, then the
angular momentum must go up
Angular Momentum
• A Star in the final stages of its life will shrink and turn
into a neutron star
• The mass of the star is 5x1035 kg
• Its initial radius is 1x109 m
• Its initial ω is 3x10-6 rad/sec (1 revolution in 25 days)
• As it becomes a neutron star, it shrinks to a radius of
20,000 m
• What is it’s new rotation rate?
Angular Momentum
Ii ωi = If ωf
2/5 M Ri2 ωi = 2/5 M Rf2 ωf
Ri2 ωi = Rf2 ωf
ωf = Ri2 ωi / Rf2
ωf = (1x109 m)2 (3x10-6 rad/s) / (2x104 m)2
ωf = 7,500 rad/s
4,300,000 revolutions per hour
Angular Momentum
• A new planet is forming much like ours did
• Its initial mass is 5x1025 kg
• It is initially spinning at a rate of once every 50 hours
• Fast-forward a million years
• Due to collisions with other things, it’s mass has
increased to 7.5x1025 kg
• However, because some of the planet has cooled (and
therefore shrunk) its radius has not changed
• What is the new rate of rotation?
Angular Momentum
Ii ωi = If ωf
2/5 Mi R2 ωi = 2/5 Mf R2 ωf
Mi ωi = Mf ωf
ωf = Mi ωi / Mf
ωf = (5x1025 kg)(1rev/50hr) / (7.5x1025 kg)
ωf = 0.0133 rev/hr
1 revolution every 75 hours
Angular Momentum
Ii ωi = If ωf
• Everything else being equal:
• Rotation slows if:
– Mass Increases
– Radius Increases
• Rotation speeds up if:
– Mass Decreases
– Radius Decreases