Lecture12 SolarSystem Formation

7/29/2019 Lecture12 SolarSystem Formation

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Summary: Four Major Features of our Solar System


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How did the

solar system form?


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According to the nebulartheory, our solar systemformed from the gravitationalcollapse of a giant cloud of

interstellar gas.

(nebula = cloudin Latin)


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The cloud of gas that gave birth to our solar system

resulted from the recycling of gas through manygenerations of stars within our galaxy.


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Stars are born in molecular clouds Clouds are very cold: ~10-30 K. (273 K = water freezes)

Stars form when gravity overcomes thermal pressure.

Then gas clumps begin to collapse.


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Earliest stages of star birth

Dense cores of gas in thelarger molecular cloudcollapse due to self-gravity.

Cloud heats up as it contracts dueto conservation of energy:gravitational potential energy isconverted to thermal energy (heat).


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Rotation is an importantfactor during the star birth

process (part 1)

As gravity forces a dense core tobecome smaller, it spins fasterand faster.

This is due to conservation ofangular momentum. Dense cores have a small

amount of initial rotation.

As the cores get smaller, theymust spin up to conserveangular momentum.


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Rotation is an importantfactor during the star birth

process (part 2)

Collisions between gas particles incloud gradually reduce randommotions and up+down motions.

Collisions flatten the cloud into a disk.

The result is a rotating protostar witha rotating disk of gas & dust.

The orderly motions of our solarsystem today are a direct result of

the solar systems birth in a

spinning, flattened cloud of gas.


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As gravitycauses cloudto shrink, its

spin increases(conservationof angular

momentum).

Spinning cloudalso flattens

as it shrinks.


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Collisions betweengas particles incloud gradually

reduce randommotions.

Initial gas cloud has

motions of alldifferent ellipticities.But at the end, onlycircular orbitsremain.


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Spinningcloud flattensas it shrinks.


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Orderly motions of our solar systemtoday are a direct result of the solar

systems birth in a spinning,flattened cloud of gas.


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Nearby star-forming

regions have 1000s ofyoung (few Myr) stars.

Most of them (~2/3)

have disks of gas &dust around them,which are thebirthplaces for other

solar systems.

Disks around other stars:

Solar systems in the making


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Disks around other stars:

Solar systems in the making

Plenty of evidence for spinning disks of gas and dust around

other stars, especially around newly formed (few Myr) stars.


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The Formation of Planets

Why are there two types of planets?

Where did asteroids and comets come from?

How do we explain the existence of our Moonand other exceptions to the rules?

When did the planets form?


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Four Major Features of the Solar System

1. Why do large bodies in our solar system haveorderly motions?

2. Why are there two types of planets?

3. Where did the comets and asteroids come from?

4. How can we explain the exceptions to the rulesabove?


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Why are there two types of planet, when all

planets formed from the same nebula?


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Temperatures during

solar system formation

Initial parent molecular cloud is

very cold, only ~10-30 K(273 K = water freezes).

As gravity causes cloud to contract,

it heats up (conservation of energy).

Protostar gets hotter as it contracts,

but still no core hydrogen fusion.

Inner parts of the surrounding disk

are hotter than the outer parts.


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Inner parts of

disk are hotter

than outer

parts.

Rock can besolid at muchhigher

temperaturesthan ice.


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The Frost Line The distance in solar nebula where ices can form (~150 K).

Inside the frost line: too hot for hydrogen compounds.

Outside the frost line: cold enough for ices to form.

Between the present-day orbits of Mars & J upiter, at ~3 AU.


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Condensation & agglomeration Outside the frost line, ices form by condensation

(gas phase --> solid phase directly).

Inside the frost line, this also happens for metals & rocks.

Below ~1600 K, gaseous metals & some types of rocks condenseinto small solid particles (grains). This is near Mercurys orbit.

Other types of rock condense at lower temperatures.

Small grains then grow by agglomeration, namely by

colliding & sticking to make bigger particles. Collisionsare actually very gentle.

Sticking due to electrostatic forces (static electricity), not due togravity for such small particles.


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Formation of the terrestrial planets Tiny solid particles stick to formplanetesimals

(pieces of planets)

Gravity draws planetesimals together to formplanets. This process of assembly is calledaccretion.

Planetesimals grow from few kilometers in size to100s of km in ~few Myr.

Further growth inhibited by destructive collisionsbetween planetesimals. Only the largest onessurvive to form planets.


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Growth of planetesimalsjust like for the terrestrial

planets (condensation,agglomeration, andaccretion).

But beyond the frost line,both solid rock and solidices are present.

This allows the coresofthe J ovian proto-planetsto grow to much larger

masses (~10 MEarth).

Formation of the gas giants: Part 1


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Gravity of the massiverocky/icy J ovian cores

draws in H and He gasesfrom the solar nebula(gas accretion).

Allows the J ovian planetsto achieve much largerfinal sizes & masses thanthe terrestrial planets.

(Remember, 98% of thesolar nebula was

gaseous H & He.)

Formation of the gas giants: Part 2


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Why are there two types of planets?

J ovian planets are more massive b/c they

formed outside the frost line. Hydrogen compounds condense to form ICES.

Solid cores of J ovian planets grow large enough to

accrete H & He gas from the solar nebula via gravity. Outer solar system (>3 AU) forms gas giant planets.

Terrestrial planets form where only metals & rockare solid, too hot to form ices.

Inner solar system forms only rocky planets.


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Massive moons of J ovian planets form in miniature disks,analogous to planets forming in the solar nebula.


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3. Where did the comets and asteroids come

from?



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Comets & asteroids = leftover planetesimals.

Asteroids: rocky b/c they formed inside the frost line.Comets: icy b/c they formed outside the frostline.

The early S.S. had many more of these,

causing many collisions in the first ~500 Myr.

May have also delivered water to the Earth.


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Evidence that planets formed from small piecescrashing into each other is seen in impact craters

MercuryMeteor crater, Arizona


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The End of Planet Formation

Rocks, metals & ices go intosolid bodies (planets,asteroids & comets).

H & He gas amount to 98%

of the mass of the nebula. Some gas locked up in the

J ovian planets.

The rest is blown away by the

young solar wind orevaporated by Suns radiation.


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4. How can we explain the exceptions to therules above?


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Earths moon wasprobably created whena big (Mars-sized)

planetesimal slammedinto the newly formingEarth roughly 30-50Myr after solarsystem formation.

Other giant impacts

may be responsible forother exceptions likeUranus extreme tilt.

Formation of the Moon


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Figure 6.27

Review ofnebular

theory


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When did the planets form? We cannot directly

know a planets age.But we can find theages of its rocks.

Ages of rocks are

determined byanalysis of its variousatoms and isotopes.

We use radioactivedecay of elementsinto other elementsas a clock.


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When did the solar system form?(a.k.a. how old is this rock?)

Age dating of rocks from

planets tells us the timesince the rock solidified.This time is younger thanthe age of the Solar

System formation. Age dating of meteorites

that are unchanged sincethey condensed and grewin the solar nebula tell usthe solar system is about4.6 billion years old.

Lecture12 SolarSystem Formation

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