Comparative Planetology• Comparative Planetology is the comparing and

contrasting of different worlds to describe and categorize them

• Important Properties:– 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

– 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– 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 Rf

2 ωf

Ri2 ωi = Rf

2 ωf

ωf = Ri2 ωi / Rf

2

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