Astr 1050 Wed., Dec. 4, 2002

Today: Chapter 16, The Origin of the Solar System

(We’ll just hit the highlights for Ch. 16-19.)

Chapter 16: Origin of the Solar System

Solar Nebula Hypothesis

Context for Understanding Solar System

Extrasolar Planets

Dust Disks, Doppler Shifts, Transits and Eclipses

Survey of the Solar System

Terrestrial Planets

Jovian Planets

Other “Stuff” including apparent patterns with application to the nebular hypothesis

Patterns in Motion

All planets orbit in almost the same plane (ecliptic)

Almost all motion is counterclockwise as seen from the north:

All planets orbit in this direction

*Almost* all planets spin in same direction

with axes more-or-less perpendicular to ecliptic

Regular moons (like Galilean satellites and our own moon) orbit in this direction too

Planets are regularly spaced

steps increasing as we go outward

Spacing of Planets

Regular spacing of planets on a logarithmic scale

Each orbit is ~75% larger than the previous one

Need to include the asteroids as a “planet”

Solar Nebula Model

Planets form from disk of gas surrounding the young sun

Disk formation expected given angular momentum in collapsing cloud

Naturally explains the regular (counterclockwise) motion

Makes additional explicit predictions

Should expect planets as a regular part of the star formation process

Should see trends in composition with distance from sun

Should see “fossil” evidence of early steps of planet formation

Extra-Solar Planets

Hard to see faint planet right next to very bright star

Two indirect techniques available
(Like a binary star system but where 2nd “star” has extremely low mass)

Watch for Doppler “wobble” in position/spectrum of star

Watch for “transit” of planet which slightly dims light from star

About 100 planets discovered since 1996 See http://exoplanets.org/

Tend to be big (³Jupiter) and very close to star (easier to see)

Characteristics of “Planets”

Two types of planets

Terrestrial Planets: small, rocky material: inner solar system

Jovian Planets: large, H, He gas outer solar system

Small left-over material
provides “fossil” record of early conditions

Asteroids – mostly between orbits of Mars and Jupiter

Comets – mostly in outermost part of solar system

Meteorites – material which falls to earth

Slide 8

Production of the Elements

H, He made in the big bang

Elements up to Fe generated by fusion in stars

expelled back into interstellar medium from red giants and supernova

Elements heavier than Fe require energy to make:
neutron capture

side effect in fusion chains

s process

in supernova explosions

r process

p⁺ capture occasionally important

p process

Timing of the Assembly Process?

Over time inside ⁸⁷Rb one n®p⁺ so ⁸⁷Rb®⁸⁷Sr

Use relative amounts of Rb, Sr to determine age of rocks

Half life: Time it takes ½ of “parent” to decay to “daughter”

Other unstable elements: ⁴⁰K²³⁵U²³⁸U

Ages of old surfaces and meteorites: ~4.5 billion years

Patterns in Composition

Terrestrial Planets

Relatively small

Made primarily of rocky material:

Si, O, Fe, Mg perhaps with Fe cores
(Note – for earth H₂O is only a very small fraction of the total)

Jovian Planets

Relatively large

Atmospheres made of H₂, He, with traces of CH₄, NH₃, H₂O, ...

Surrounded by satellites covered with frozen H₂O

Within terrestrial planets inner ones tend to have higher densities
(when corrected for compression due to gravity)
       Planet Density Uncompressed Density
(gm/cm³)        (gm/cm³)

Mercury     5.44          5.30

Venus     5.24          3.96

Earth     5.50          4.07

Mars     3.94          3.73

(Moon)     3.36          3.40

Equilibrium Condensation Model

Start with material of solar composition material

(H, He, C, N, O, Ne, Mg, Si, S, Fe ...)

Material starts out hot enough that everything is a gas

May not be exactly true but is simplest starting point

As gas cools, different chemicals condense

First high temperature chemicals, then intermediate ones, then ices

Solids begin to stick together or accrete

snowflakes Þ snowballs (“Velcro Effect”)

Once large enough gravity pulls solids together into planetesimals

planetesimals grow with size

At some point wind from sun expels all the gas from the system

Only the solid planetesimals remain to build planets

Composition depends on temperature at that point (in time and space)

Gas can only remain if trapped in the gravity of a large enough planet

The Material Available

Inner solar system dominated by silicate rocks

SiO₂ (quartz) Mg₂SiO₄ Fe₂SiO₄ (olivine) etc.

Outer solar system dominated by H₂, He, ice (H₂O)

Reason for Jovian Planets

Because you cannot condense O by itself (but only in compounds also containing Si, Mg, Fe), you don’t have much material available for making terrestrial planets. You are limited by the low abundance of Si, Mg, Fe: Terrestrial planets are relatively small

Once solid H₂O becomes available you have lots more material

Starting at Jupiter you can make a big enough core from solid H₂O that you can gravitationally hold onto the H and He gas

Growth of the Planetisimals

Once a planetisimal reaches critical size gravity takes over

Growth and Differentiation of Planets

Planet forms from homogeneous mix of material

Planet heats up

“Heat of formation”
(i.e. energy from gravity)

Heat from radioactive decay of U, etc.

Dense material (Fe) sinks to center

Certain “siderophile” elements (like Ni)

Other “lithophile” elements remain behind

Homogeneous model too simple

Final collisions can be big:

Little planetesimals first form bigger ones, then bigger ones collide to form yet bigger ones

Moon may be result of impact of Mars size body as Earth formed (more later)

First material to condense might separate out early

Heterogeneous Growth of Planets

Fe is among the first materials to condense as nebula cools

Might form iron cores before lower temperature materials condenses

Has implications for separation of lower temperature “siderophiles” during later differentiation

Evidence of Assembly Process? Craters

Craters evident on almost all small “planets”

Even larger planets typically have some

Clearing of the Nebula

Radiation pressure (pressure of light)

Will see present day effects in comets

Solar Wind

Strong solar winds from young T Tauri stars

Will see present day effects in comets

Sweeping up of debris into planets

Late Heavy Bombardment

Ejection of material by near misses with planets

Like “gravity assist maneuvers” with spacecraft

Origin of the comets

Patterns and Predictions

Why do different planets have different levels of geologic activity?

Why do different planets have different atmospheres?

What are ages of old “unaltered” planetary surfaces?

Should be similar, and agree roughly with age of Sun

Does composition of asteroids match predictions?

Lower temperature than Mars region: Hydrated silicates, etc.

What types of minerals do we see in meteorites?

What types of ices and minerals do we see in comets?


	Today: Chapter 16, The Origin of the Solar System
	(We’ll just hit the highlights for Ch. 16-19.)


	Solar Nebula Hypothesis
		Context for Understanding Solar System
	Extrasolar Planets
		Dust Disks, Doppler Shifts, Transits and Eclipses
	Survey of the Solar System
		Terrestrial Planets
		Jovian Planets
		Other “Stuff” including apparent patterns with application to the nebular hypothesis


All planets orbit in almost the same plane (ecliptic)
Almost all motion is counterclockwise as seen from the north:
	All planets orbit in this direction
	Almost all planets spin in same direction
		with axes more-or-less perpendicular to ecliptic
	Regular moons (like Galilean satellites and our own moon) orbit in this direction too
Planets are regularly spaced
	steps increasing as we go outward


	Regular spacing of planets on a logarithmic scale
	Each orbit is ~75% larger than the previous one
	Need to include the asteroids as a “planet”


	Planets form from disk of gas surrounding the young sun
		Disk formation expected given angular momentum in collapsing cloud
		Naturally explains the regular (counterclockwise) motion
	Makes additional explicit predictions
		Should expect planets as a regular part of the star formation process
		Should see trends in composition with distance from sun
		Should see “fossil” evidence of early steps of planet formation


	Hard to see faint planet right next to very bright star
	Two indirect techniques available (Like a binary star system but where 2nd “star” has extremely low mass)
		Watch for Doppler “wobble” in position/spectrum of star
		Watch for “transit” of planet which slightly dims light from star









	About 100 planets discovered since 1996 See http://exoplanets.org/
	Tend to be big (³Jupiter) and very close to star (easier to see)


	Two types of planets
		Terrestrial Planets: small, rocky material: inner solar system
		Jovian Planets: large, H, He gas outer solar system

	Small left-over material provides “fossil” record of early conditions
		Asteroids – mostly between orbits of Mars and Jupiter
		Comets – mostly in outermost part of solar system
		Meteorites – material which falls to earth


H, He made in the big bang
Elements up to Fe generated by fusion in stars
	expelled back into interstellar medium from red giants and supernova
Elements heavier than Fe require energy to make: neutron capture
	side effect in fusion chains
		s process
	in supernova explosions
		r process
	p⁺ capture occasionally important
		p process


	Over time inside ⁸⁷Rb one n®p⁺ so ⁸⁷Rb®⁸⁷Sr
	Use relative amounts of Rb, Sr to determine age of rocks
	Half life: Time it takes ½ of “parent” to decay to “daughter”
	Other unstable elements: ⁴⁰K²³⁵U²³⁸U
	Ages of old surfaces and meteorites: ~4.5 billion years


Terrestrial Planets
	Relatively small
	Made primarily of rocky material:
		Si, O, Fe, Mg perhaps with Fe cores (Note – for earth H₂O is only a very small fraction of the total)

Jovian Planets
	Relatively large
	Atmospheres made of H₂, He, with traces of CH₄, NH₃, H₂O, ...
	Surrounded by satellites covered with frozen H₂O

Within terrestrial planets inner ones tend to have higher densities (when corrected for compression due to gravity) Planet Density Uncompressed Density (gm/cm³) (gm/cm³)
	Mercury 5.44 5.30
	Venus 5.24 3.96
	Earth 5.50 4.07
	Mars 3.94 3.73
	(Moon) 3.36 3.40


	Start with material of solar composition material
		(H, He, C, N, O, Ne, Mg, Si, S, Fe ...)
	Material starts out hot enough that everything is a gas
		May not be exactly true but is simplest starting point

	As gas cools, different chemicals condense
		First high temperature chemicals, then intermediate ones, then ices
	Solids begin to stick together or accrete
		snowflakes Þ snowballs (“Velcro Effect”)
	Once large enough gravity pulls solids together into planetesimals
		planetesimals grow with size
	At some point wind from sun expels all the gas from the system
		Only the solid planetesimals remain to build planets
		Composition depends on temperature at that point (in time and space)
		Gas can only remain if trapped in the gravity of a large enough planet


	Inner solar system dominated by silicate rocks
		SiO₂ (quartz) Mg₂SiO₄ Fe₂SiO₄ (olivine) etc.
	Outer solar system dominated by H₂, He, ice (H₂O)


	Because you cannot condense O by itself (but only in compounds also containing Si, Mg, Fe), you don’t have much material available for making terrestrial planets. You are limited by the low abundance of Si, Mg, Fe: Terrestrial planets are relatively small

	Once solid H₂O becomes available you have lots more material

	Starting at Jupiter you can make a big enough core from solid H₂O that you can gravitationally hold onto the H and He gas


	Once a planetisimal reaches critical size gravity takes over


Planet forms from homogeneous mix of material
Planet heats up
	“Heat of formation” (i.e. energy from gravity)
	Heat from radioactive decay of U, etc.
Dense material (Fe) sinks to center
	Certain “siderophile” elements (like Ni)
	Other “lithophile” elements remain behind
Homogeneous model too simple
	Final collisions can be big:
		Little planetesimals first form bigger ones, then bigger ones collide to form yet bigger ones
			Moon may be result of impact of Mars size body as Earth formed (more later)
	First material to condense might separate out early


	Fe is among the first materials to condense as nebula cools

	Might form iron cores before lower temperature materials condenses

	Has implications for separation of lower temperature “siderophiles” during later differentiation


	Radiation pressure (pressure of light)
		Will see present day effects in comets
	Solar Wind
		Strong solar winds from young T Tauri stars
		Will see present day effects in comets
	Sweeping up of debris into planets
		Late Heavy Bombardment
	Ejection of material by near misses with planets
		Like “gravity assist maneuvers” with spacecraft
		Origin of the comets


	Why do different planets have different levels of geologic activity?

	Why do different planets have different atmospheres?

	What are ages of old “unaltered” planetary surfaces?
		Should be similar, and agree roughly with age of Sun

	Does composition of asteroids match predictions?
		Lower temperature than Mars region: Hydrated silicates, etc.

	What types of minerals do we see in meteorites?

	What types of ices and minerals do we see in comets?