Dating the Universe

Lecture 6: Formation of the Solar System

Astro 202Prof. Jim Bell (jfb8@cornell.edu)

Spring 2008

But first...

Graded Paper 1 returned today...

Paper 2 is due at beginning of class on Feb. 14Don’t wait until the last minute!

Paper 3 to be posted online next Tuesday 2/12

How did the Solar System Form?

!Observational Constraints– Planetary motions

– Planet, asteroid, comet compositions

– Meteorite and cosmic dust ages

– Other solar systems?

!The Solar Nebula Model

– Formation of the Terrestrial Planets

– Formation of the Giant Planets• Chemical Condensation ("Lewis") Model

!Planetary EvolutionAstro 202 4

Observational Constraints

!For a theory of the formation of our solar

system to be viable, it must be consistent with

our presently-available data

!Theories must fit 3 classes of constraints:

(1) Motions: The observed spin and orbital motions

of the planets, asteroids, and comets

(2) Composition: The measured or inferred

compositions of planets, asteroids (meteorites),

and comets

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"Motion" Constraints

! All of the planets orbit the Sun in roughly the same plane (the

ecliptic), which is very close to the Sun's equatorial plane

! The orbits of the major planets are nearly circular

! Planets, asteroids, and most comets circle the Sun counter-

clockwise as viewed from above (exceptions: some comets)

! The Sun and most of the planets generally spin in this same

counterclockwise direction and have very small tilts relative to the

plane of the solar system (exceptions: Venus, Uranus, Pluto)

! Nearly all the planets and asteroids spin with the same period,

roughly 20±10 hours per "day" (exceptions: Venus, Pluto)

! The Sun contains 99.9% of the mass of the solar system, but the

planets contain 99.7% of the conserved angular momentum

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Top view

Edge on

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Compositional Constraints

! The compositions of the planets vary with distance from the Sun

– The inner planets are dense and rocky/metal-rich

– The outer planets are primarily gas (H, He) and ices

– Asteroids in the main belt are transitional

– Comets and KBOs are primarily ices

! Most of Jupiter and Saturn, and the atmospheres of Uranus and Neptune, have approximately the same composition as the Sun

! Many meteorites appear to be chemically different (more "pristine") than planets and moons

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Age Constraints

!The primitive (unaltered) carbonaceous and ordinary chondrite meteorites ALL have ages of approximately 4.5 billion years

!Tiny fractions of some very primitive meteorites and cosmic dust particles have ages even slightly older than this

!The oldest rocks from the Moon are about 4.4 billion years old

!More details in next 4 lectures...

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Major Characteristics of our Solar System: Summary

(1) Large bodies have orderly motions

(2) Planets fall into two main categories • Small, rocky inner planets

• Giant, gas- and ice-rich outer planets

(3) Swarms of asteroids and comets abound!

(4) There are notable exceptions to the trends• Tilt of Uranus, tilt of Pluto's orbit, spin of Venus, orbit of

Triton, ...

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Historical Models for the Formation of the Solar System! Catastrophic Models

– A large comet collided with the Sun and ejected material that cooled to form the planets [Buffon, 1745]

– Another star made a close pass by the Sun, tidally pulling material out that cooled to form the planets [Bickerton, 1880]

! Nebular Models– Sun and planets formed from a vast nebula of gas

[Descartes, 1644] that swirled around and condensed into planets [Kant, 1755]

– Nebula contracted into a fast spinning disk, shedding "rings" of material that formed the planets [Laplace, 1796]

! All have problems with the basic physics or matching some of the constraints, but nebular models "best"

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The Solar Nebula Theory

! The leading model today is called the solar nebula theory

– Previous generations of stars lived and died before our solar

system was formed ("cosmic recycling")

– The violent death of a previous star or stars formed an enormous

cloud of hydrogen, helium, tiny amounts of heavier elements, and

interstellar dust particles in this part of our galaxy

– This cloud collapsed under its own gravity and spun into a disk

– Most of the mass fell into the center of the disk and formed the Sun

– A tiny fraction of the leftovers formed the planets, moons, etc.

– Some day (maybe 5 billion years from now) the Sun will die and

redistribute most of this matter back into an interstellar cloud...

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Cosmic Recycling...

Ours may not

be the first

solar system to

occupy this

part of the

galaxy...

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The Solar Nebula

Theory

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The "Standard Model":

From gas clouds to gas giants...

"accretion"

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The Violent Young Sun

Original nebula may have had ~2 solar

masses of material; half of it lost by the

young Sun's strong "bipolar outflow"

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How long did this take?

!FAST according to computer models

– Contraction, spin-up of original cloud of gas takes only a few million years

– Condensation of dust and settling of the dust into a rotating disk may have taken only ~ 100 years!

– Clumping of dust by self-gravity into 1-5 km planetesimals may have taken only ~ 1000 years!

– Accretion of planetesimals into ~1000 km objects may have occurred over only ~1 million years

– Growth of a few objects to even larger Moon, Mars, or Earth sizes in only about 100 million years

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The Cosmic Calendar

(from Carl Sagan’s Cosmos)

Putting the age of the solar

system into perspective...

Our galaxy forms

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Implications...

!The original catastrophic theories assumed that the Sun formed first and the planets formed later, by accident

– Implies that planets are rare

!Nebular theories assume that the Sun and planets form together

– Implies that planetary systems are common

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Testing the Solar Nebula Theory

! How do we decide if this is right? (we weren't there!)

– Do the models rely on the laws of physics and chemistry or

do they require miracles or ad hoc assumptions?

– Do the models accurately predict the present nature and

distribution of the planets?

– Are there clues in the compositions of the planets or comets

or meteorites that could help to support this model?

– Can we look elsewhere in the galaxy for evidence for this

kind of process of solar system formation?

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Compositional Evidence

• The solar nebula theory

predicts a specific

chemical condensation

sequence as the hot

gaseous nebula cooled...

• In general, this sequence

is consistent with the

observed changes in

planetary composition

with solar distance

• Often called the "Lewis

Model"

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Compositions of Meteorites

• A few special "primitive"

carbonaceous chondrite meteorites

contain grains of dust with ages

greater than 4.5 billion years

• Samples of the Sun's "parent body"?

• Confirms that meteorites (and by

inference, asteroids and comets)

provide the best way to study the

"building blocks" of the solar system

• Some of these ancient stones also

have complex organic molecules

and even amino acids

Allende

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Observational Evidence:

Nebulae...

Star-forming Regions...

Orion

Dusty disks around nearby stars!

"Brown Dwarfs" and

Planets around other stars!

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Chemical Condensation Model(the "Lewis" Model)

• The solar nebula theory

predicts a specific

chemical condensation

sequence as the hot

gaseous nebula

cooled...

• Model worked out by

astronomer John Lewis

and others (for example,

in the March 1974

Scientific American)

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• Only rocks and metals could condense out near the Sun

• Farther away, lower temperatures allow lighter condensates

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Assumptions...

!Pressure, Temperature of the nebula

– From a different model...

!Composition of the nebula

– Solar: 75% H, 24% He, 1% everything else

!Phase diagrams of nebula materials

– When do solids condense from the gas?

!Equilibrium chemistry

– Slow solid/gas reactions, predictable products

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Predictions of the Lewis Model

!Rocky inner planets, icy outer ones– Yes

!Mercury will have a large metallic core– Yes

!H2O increases from Venus to Earth to Mars

– Maybe, but interiors not well understood...

!Volatile-rich materials in the asteroid belt– Consistent with some asteroid spectra & meteorites

! "Unprocessed" outer satellites half rock, half ice– Consistent with Ganymede and Callisto data

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But there are problems...

!Pluto & Triton should be almost all ices

– But densities near “rockier” 2 g/cm3 ???

!The Moon

– Few volatiles, depletion of Fe and Ni

– Implies formation inside Mercury's orbit!

– But Moon rocks not primarily metal oxides!

!Oddballs due to exceptional circumstances?

– Moon, Venus, Uranus, Pluto, ...

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Formation of the Terrestrial Planets

! Condensed grains merge into larger and larger

aggregates, forming 1-10 km size planetesimals

! Planetesimals grow by gravitational accretion

! Small number of protoplanets 100-1000 km

(Moon, Mars masses) form

! Continued accretion of planetesimals adds

energy and heat, melting the growing

protoplanets

! Melted planets differentiate and lose most of

their volatiles (gases and ices)

! Terrestrial planets not massive enough to be

able to hold on to light Hydrogen & Helium gas

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Self-Gravity...

R ! 400 km +

R < 400 km

makes a planet?

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Formation of the Giant Planets

! Additional supply of ices and other volatiles in the outer

solar system allowed the protoplanets to grow much

larger, perhaps 5-10 times the mass of the Earth

! Masses large enough to keep Hydrogen and Helium

gravitationally bound to the protoplanet

! Giant protoplanets heated as their cores collapse under

more and more mass: same way stars start to form!

! But the giant planets in our solar system were not

massive enough to initiate fusion of Hydrogen into

Helium, so the cores cooled to their present state

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Solar Systems within Solar Systems

Solar NebulaPutative Jovian Nebula

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Giant Planets vs. Terrestrial Planets

!Why did proto-Jupiter & proto-Saturn grow so big but proto-Earth didn’t?

!Possible solution:

– At lower Jupiter/Saturn temperatures, icy materials could condense out of the nebula

– So: more solid material could condense near Jupiter/Saturn orbit than near Earth

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Jupiter/Saturn vs. Uranus/NeptuneThere appear to have been two possible paths for the

formation of the giant planets in our solar system:

• If the protoplanet grew big enough (5-10 Earth masses)

very early when there was still lots of nebular gas:

- accretes a lot of gas, has a solar-like composition

- Jupiter and Saturn!

• If the protoplanet grew big enough (5-10 Earth masses)

but not until later, when there was less nebular gas:

- accretes only a little gas, less Sun-like composition

- Uranus and Neptune!

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Jupiter Rules!

! If Jupiter formed early enough, it

could have prevented the formation of a planet in

the asteroid belt.

!Why?

! Jupiter's gravity influences nearby planetesimals

! Nearby planetesimal orbits made more eccentric

– Higher eccentricity means higher impact velocities

– Higher impact velocities mean less "sticking"

– Planetesimals cannot accrete and grow very well

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Caveats

!The Chemical Condensation Model is a simplistic view of a complex process!

– Was chemical equilibrium reached

everywhere?

– Many of the "leftovers" that we see today from

solar system formation have undergone

complex evolutionary histories

! Thus it is important to find and study the most

primitive solar system bodies (asteroids, comets)

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Planetary Evolution

!After only 100 to 500 million years, most of the "debris" in the solar system was either swept up by the planets or ejected

!The planets started to cool and assume their present appearances

!Each planet or moon has followed it own evolutionary course, governed by its size, composition, solar distance, and the occasional random catastrophic impact...

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How Do Interiors Get Hot?

Heat In

Heat Out

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Initial "inventory" of internal heat governs a

planet's evolution

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Summary

!Observations of the Sun, planets, asteroids, and

comets can be used to constrain models for the

formation of the solar system

! The currently-favored model is the solar nebula

theory, which postulates that the Sun and planets

were formed from a spinning cloud of gas & dust

! This theory explains many of the observations

– Motions of the planets

– Compositions of the Sun, planets, and meteorites

– Ages of meteorites and cosmic dust particles

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Summary

! The solar system formed about 4.5 billion years ago

– More soon on where that number comes from...

! The leading theory is that it formed rapidly from a spinning cloud (nebula) of gas and dust

! Condensation and accretion of solids occurred quickly: much of the "action" over in ! 108 years

! The outer planets were massive enough (especially Jupiter and Saturn) to accrete gases from the nebula

! But the inner planets were not!

! Jupiter may have prevented the formation of another terrestrial planet in the asteroid belt

Next Time...How do we know how old space rocks are?

Guest: Dr. Matt Hedman

Reading:

Read PBD 7

Check out Matt’s book: