Solar System Formation - University of North Floridan00006757/astronomylectures/ECP4e... ·...

Solar System Formation

Question: How did our solar system and other planetary systems form?

“Comparative planetology” has helped us understand

• Compare the differences and similarities among the objects in our solar system

• Figure out what physical processes could have led to them

• Then construct a model of how our solar system formed based on this

-------

• This model must explain our own solar system…

• …but might or might not explain other planetary systems

• If not, modify the model to accommodate discrepancies

• In other words, carry out the scientific process

-------

• Let’s look at the solar system characteristics comparative planetology has to work with…

Solar System Formation

1. Large bodies have orderly motions and are isolated from each other

– All planets and most moons have nearly circular orbits going in the same

direction in nearly the same plane

Solar System Formation -- Characteristics of Our Solar System




– The Sun and most of the planets rotate in this same direction as well






– And most moons orbit their planet in the direction it rotates


2. Planets fall into two main categories

Small, rocky “terrestrial” planets near the Sun

Large, hydrogen-rich “jovian” planets far from

the Sun




3. Swarms of asteroids and comets populate the solar system

– Asteroids are concentrated in the asteroid belt




– Comets populate the regions known as the Kuiper belt and the Oort cloud


4. Several notable exceptions to these general trends stand out

– Planets with unusual axis tilts

– Surprisingly large moons

– Moons with unusual orbits


1. Large bodies in the solar system have orderly motions and are isolated from each

other




– Most moons orbit their planet in the direction it rotates


– Small, rocky terrestrial planets near the Sun

– Large, hydrogen-rich jovian planets farther out

• The jovian planets have many moons and rings of rock and ice



– Comets populate the regions known as the Kuiper belt and the Oort cloud

4. Several notable exceptions to these general trends stand out





…which any successful theory must account for…

• The nebular theory is the best current explanation of our solar system

• It is not a new idea…

…the philosophers Emanuel Swedenborg and Immanuel Kant suggested it in the 1700s

• And like all scientific theories, it is still being refined and improved

Solar System Formation – The Nebular Theory

• It starts with cold interstellar clouds of gas and dust

• These clouds are mostly hydrogen and helium from the Big Bang

• But they contain heavier elements that were not formed in the Big Bang

• Astronomers call these “metals” (even though they’re not necessarily metallic elements)

• Where did these heavier elements come from?

• They came from stars!


• Stars make heavier elements from lighter ones through nuclear fusion



• The heavy elements (the “metals”) mix into the interstellar medium when the stars die



• The heavy elements (the “metals”) mix into the interstellar medium when the stars die

• And then new stars form from the enriched gas and dust

• And the cycle continues


• And at the same time stars are forming…

…planetary systems can form

• Here’s how it works…


• A large cloud -- a nebula perhaps 1 light year across -- floats in space



• The cloud begins to collapse


…WHY would this happen?...

Local density increase


• The cloud begins to collapse -- local density increase

• As it collapses it begins to spin faster



Conservation of angular momentum



• As it collapses it begins to spin faster -- conservation of angular momentum

• And as it spins faster, it flattens out



Collisions




• And as it spins faster, it flattens out -- collisions

• At the same time, it begins to heat up in the center



Conversion of gravitational

potential energy into thermal

energy




• And as it spins faster, it flattens out -- collisions

• At the same time, it begins to heat up in the center -- conversion of potential to thermal energy

• And when it gets hot enough, a star forms in the center

• And in the disk around the forming star, planets can form

• What type of planets can form depends on what the cloud is made of…


• This is what our own cloud—the solar nebula—was made of

• But how do we know this?


• This is what our own cloud—the solar nebula—was made of

• But how do we know this? This is how…

• …the absorption line spectrum of the Sun

• It tells us the composition of the gas on the surface of the Sun


• This is the composition of the Sun’s surface gas – its atmosphere

• We think the solar nebula had the same composition

• But a skeptic might say, is it reasonable to say this?


• After all, the solar nebula collapsed 4.6 billion years ago

• The Sun’s been making new atoms with nuclear fusion ever since

• That’s how it generates the energy that gives us sunlight

• Wouldn’t this change the composition of the Sun’s atmosphere?

• The answer has to do with where the new atoms are being made…


• The sunlight-generating fusion reactions happen in the Sun’s core

• The core is in the Sun’s center, far from the surface

• So the surface layers should be essentially unchanged

• And their composition should be very similar to the solar nebula the Sun formed from


• So it seems reasonable that the Sun’s atmosphere is similar to the solar nebula


• The key to the nebular theory is the condensation temperature of these materials

• That’s the temperature at which they condense into solid form

• The nebula was initially very cold, so everything except H and He was in solid form

• But it heated up as it collapsed…

• …and the temperature was different at different distances from the center



• This image shows a graph of a modeled temperature profile of the solar nebula…

…along with an artist’s rendition of the nebula

• The temperature was hottest in the center, and went down away from the center

• There was a mixture of metals, rocks, and hydrogen compounds throughout the nebula

• These could only be solid where the temperature was below their condensation temperature

• So different chemical components of the nebula condensed at different distances

• A mixture of solid rock and metal existed out to about 4.5 AU from the center

• At 4.5 AU, the temperature dropped low enough for hydrogen compounds to condense, too

• The boundary between where they could and could not condense is called the “frost line”, “snow line”, or “ice line”


• The frost line was located between the present-day orbits of Mars and Jupiter


• Once materials condense into solid form they can stick together

• This is called “accretion”

• And it launches the next step in planet formation…

• “Core accretion”


• Small clumps grow like

snowballs until they

become planetesimals

the size of moons

• The planetesimals collide

and coalesce until

planets are born

• This suffices to explain

terrestrial planet

formation, but jovian

planets require adding an

extra layer to the

process...literally


• Jovian planets also begin

by core accretion

• But this happens in the

outer solar system,

beyond the frost line,

where there is 3x more

solid material available

• So the cores get much

bigger (10-15 times the

mass of Earth)


• Unlike terrestrials, the jovian

cores gather gas from the

nebula and retain it

• This is because:

• They are more massive…

• stronger gravity

• It is colder…

• lower escape speeds

for gas

• The result is a “gas giant” -- a

jovian planet


• There is an alternative to the core accretion model…

…“disk-instability"

• In it, cool gas beyond the frost line collapses directly into jovian planets…

…much like the solar nebula collapsed to form the solar system

• This takes much less time than the "core-accretion model"

• And this makes it consistent with claims that some jovians form faster than would be

possible by core-accretion


• It is not known for certain whether jovian planets form by core accretion or disk instability

• Perhaps they form one way in some circumstances and the other way in others…

…The main difference is in the way the process begins

• Once it starts, the nebular gas forms an accretion disk

• This disk swirls around the growing jovian planet in the same direction that the planet

orbits the Sun due to conservation of angular momentum

• And in that accretion disk, moons form around the jovian planet like planets formed in

the solar nebula around the Sun

• The process of jovian

and terrestrial planet

formation was finalized

by the infant Sun

• As the Sun became a

star, a strong solar wind

blew out from it…

…and cleared the

remaining nebular gas

away…

…thus halting the

growth of the planets

from the solar nebula


A successful theory must explain our solar system

So how does this one do?

How Does the Nebular Theory Do?

• Large bodies in the solar system have orderly motions and are isolated from each

other :





• Planets fall into two main categories:

– Small, rocky terrestrial planets near the Sun

• No rings and few, if any, moons

– Large, hydrogen-rich jovian planets farther out

• Rings of rock and ice and many moons

• Swarms of asteroids and comets populate the solar system:


– Comets in the Kuiper belt and the Oort cloud

• Several notable exceptions to these general trends stand out:




• Large bodies in the solar system have orderly motions and are isolated from each

other:



The planets and moons orbit in the direction that the solar nebula was spinning






• Large bodies in the solar system have orderly motions and are isolated from each other


– Large, hydrogen-rich jovian planets far from the Sun, with rings of rock and ice

and many moons

– Small, rocky terrestrial planets near the Sun with no rings and few, if any, moons




– Large, hydrogen-rich jovian planets far from the Sun, with rings of rock and ice

and many moons

Outside the frost line, lower temperatures led to condensation of hydrogen

compounds (ices) along with metals and rocks

Cores large enough to capture gas could form

Moons made of rock and ice formed in the swirling jovian nebula around each

growing jovian planet

Rings appear when some of those moons get torn apart by tidal forces

– Small, rocky terrestrial planets near the Sun with no rings and few, if any, moons

Inside the frost line, higher temperatures meant that only metals and rocks

could condense, providing less than 1/3 as much material and leading to small,

rocky cores

The smaller cores and higher temperatures prevented gas capture, and moon

and ring formation



• Planets fall into two main categories


– Asteroids mainly in the asteroid belt


• The asteroids in the asteroid belt are a “frustrated planet”

• The Trojan asteroids are planetesimals that became locked in gravitational

"wells" caused by the gravity of Jupiter and the Sun













• The icy planetesimals that formed beyond the frost line near Jupiter and Saturn

were thrown in random orbits, forming the Oort Cloud







• Those that formed beyond Neptune were relatively unaffected, and make up the

Kuiper Belt







• Those that formed near Uranus and Neptune were flung into the inner solar

system, and some provided water for Earth and other terrestrial planets

How Does the Nebular Theory Do?• Large bodies in the solar system have orderly motions and are isolated from each other



• Several notable exceptions to these general trends stand out:




Unusual (backward) orbits indicate captured objects

The unusual axis tilts can be explained by giant impacts during

the “Era of Heavy Bombardment”

• The “surprisingly large moon” is our own

• It is unlikely that it formed at the same time as Earth

because its density is lower

• But Earth is too small to have captured it

• It too can be explained by a giant impact

solarsystemformation-nebulartheory 10.htm

Summary of Nebular Theory

Summary of Nebular Theory

• There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen

compounds)

Summary of Nebular Theory• There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)

• Initially the nebula was very cold, and all of the dust was in the form of solid particles



• The nebula began to contract, spin faster and faster, flatten out, and heat up




• As it heated, the dust particles vaporized





• The nebula was hottest in the center






• The farther away from the center, the cooler it got







• Different types of dust resolidified at different distances from the center depending on

their condensation temperatures







• Different types of dust resolidified at different distances from the center depending on their condensation temperatures

• Close to the center only rock and metal dust was able to condense









• Far from the center, beyond the “frost line”, hydrogen compounds could also condense










• The solid particles stuck together (“accreted”), forming bigger and bigger clumps until

they were the size of planets










• The solid particles stuck together (“accreted”), forming bigger and bigger clumps until they were the size of planets

• Inside the frost line, where only rock and metal could condense, small terrestrial planets

formed











• Inside the frost line, where only rock and metal could condense, small terrestrial planets formed

• Beyond the frost line, hydrogen compounds as well as rock and metal could condense,

and much larger jovian planet cores could form












• Beyond the frost line, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores

could form

• The jovian cores were massive enough, and the temperatures cold enough, to attract

and retain gas from the surrounding nebula, becoming our “gas giant” planets













could form

• The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding

nebula, becoming our “gas giant” planets

• When the Sun matured into a star, the solar wind blew out the remaining gas and

arrested the development of the planets













could form



• When the Sun matured into a star, the solar wind blew out the remaining gas and arrested the development of the planets

• Planetesimals still remained, and these collected into the asteroid belt, Kuiper belt, or

Oort cloud—or were captured by planets as moons—or collided with the planets, in

some cases altering their axis tilts













could form



• When the Sun matured into a star, it emitted a strong solar wind that blew out the remaining gas and arrested the development

of the planets

• Planetesimals still remained, and these collected into the asteroid belt, Kuiper belt, or Oort cloud—or were captured by planets

as moons—or collided with the planets, in some cases altering their axis tilts

• It was 4.6 billion years ago that our solar system formed

• But how do we know this?...

• From radiometric dating, using radioactive isotopes

• Every element exists as a mixture of isotopes

• Some isotopes, like 14C, are radioactive

• Every radioactive isotope has its own half-life

• If a sample has a certain amount of radioactivity, after one half-life it will have half as much

• With radiometric dating, you estimate the initial amount of radioactivity in a sample, and determine its age from the amount that’s left

When did all this happen, and how do we know?

• Carbon-14 (14C) provides a familiar example of

radiometric dating

• It’s used to date mummies, archaeological artifacts, and

the like

• The diagram shows how it works…

• 14C is useful for dating things up to ~60,000 years old

• But its half-life of ~5700 years is too short to be useful

in measuring the age of our solar system

When did all this happen?


• One isotope whose half-life is long enough is potassium-40 (40K)

• 40K decays to argon-40 (40Ar) with a half-life of 1.25 billion years

• 40K is found in rock along with 40Ar from its decay

• If the rock is melted, the 40Ar escapes as a gas

• When the rock cools and resolidifies, it contains 40K, but no 40Ar

http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/courseware/Chronology/movies/Melting.html


• One isotope whose half-life is long enough is potassium-40 (40K)

• 40K decays to argon-40 (40Ar) with a half-life of 1.25 billion years

• 40K is found in rock along with 40Ar from its decay

• If the rock is melted, the 40Ar escapes as a gas

• When the rock cools and resolidifies, it contains 40K, but no 40Ar

• So by measuring the ratio of 40Ar to 40K in a piece of rock, you can determine how long

it’s been since the rock solidified

When did all this happen?• How can 40K be used to date the formation of the solar system?

• The solar system formed from the solar nebula, a vast cloud of gas and (solid) dust

• The solid (cold) dust particles initially contained both 40K and 40Ar

• But as the nebula contracted and heated, the dust vaporized, and the 40Ar was released

• When the dust condensed to solid form again, it contained 40K, but not 40Ar

• If rocks accreted from this dust could be found unchanged, their age would be the age of the

solar system

• This is a type of meteorite called a “chondrite”

• Chondrites have not melted since they accreted

from the nebular dust when the solar system

formed

• So whatever 40Ar they contain has appeared

since then

When did all this happen?• How can 40K be used to date the formation of the solar system?

• The solar system formed from the solar nebula, a vast cloud of gas and (solid) dust

• The solid (cold) dust particles initially contained both 40K and 40Ar

• But as the nebula contracted and heated, the dust vaporized, and the 40Ar was released

• When the dust condensed to solid form again, it contained 40K, but not 40Ar

• If rocks accreted from this dust could be found unchanged, their age would be the age of the

solar system

• This is a type of meteorite called a “chondrite”

• Chondrites have not melted since they accreted

from the nebular dust when the solar system

formed

• So whatever 40Ar they contain has appeared

since then

• Radiometric dating using 40Ar/40K shows that

chondrites formed 4.6 billion years ago

• The age determined using other isotopes is

similar, and this gives us confidence that it is

correct

Is ours the only solar system?

• Observation of other stars reveals many of them surrounded by disks of dust and gas

• These protoplanetary disks are exactly what the nebular theory predicts

• But until the 1990s, there was no convincing evidence for planets around other stars

• As of today, more than 3500 “extrasolar planets” or “exoplanets” have been confirmed

NASA Exoplanet Archive

The Extrasolar Planet Encyclopaedia

https://exoplanetarchive.ipac.caltech.edu/

http://exoplanet.eu/

Detecting Extrasolar Planets by Radial Velocity

• The first extrasolar planets were found by the radial velocity technique

• This technique depends on the gravitational effect of a planet on its star

• This image shows what would happen

if Jupiter and the Sun were the only

objects in our solar system

• They both would orbit around their

common center of mass (on the

surface of the Sun)


• This image shows the actual path of the Sun

around our solar system’s center of mass

• In a system with more than one planet, the star’s

movement can be quite complicated

• The motion is mainly due to the effects of Jupiter

and Saturn, because they are so massive

• Other stars are affected similarly by their planets


• This back-and-forth motion of the star along the line of sight from Earth causes Doppler-shifting

of its light

• And this can be detected in a light curve


• After recording the light curve, computer modeling is used

to determine how many and what type of planets are there

• This light curve led to the discovery of the first planet

orbiting a Sun-like star – 51 Pegasi

• It is fairly simple, and is consistent with a single planet

• The period of the wobbling gives you the orbital period and

therefore the distance (~0.05AU…how?)

• The magnitude gives you the minimum mass of the planet

(~.5MJupiter…how?)


• This light curve is more complicated


• This light curve is more complicated

• It is consistent with the triple-planet system at right

Detecting Extrasolar Planets by Transit

• In the transit method (used by the Kepler SpaceTelescope), astronomers look for a periodic

decrease in the light from a star

• The decrease indicates that a planet is transiting the star, blocking some of the starlight

• How often and how much the light decreases gives information about the planet’s orbit and size

• Combining this info with radial velocity info can give the density of the planet

Detecting Extrasolar Planets by Imaging

• Planets do not emit their own light, and so are hard to see in telescopes, but a small number of

extrasolar planets have been found this way

• The red object in the image above is the first of them

• It is orbiting a brown dwarf (the brighter object)

Detecting Extrasolar Planets

• A few exoplanets have been found by gravitational microlensing

• In this method, the light from a distant star is bent by the gravity of an intervening star

• If the intervening star has a planet, the planet’s gravity adds to the effect in a recognizable way

• A statistical analysis of planets detected by this technique led to the prediction that each star in

the Milky Way has ~1.6 planets

• You can see a list of all the known extrasolar planets and more at

The Extrasolar Planets Encyclopedia

NASA Exoplanet Archive

http://www.exoplanet.eu/

http://exoplanetarchive.ipac.caltech.edu/index.html


• At one time, most confirmed exoplanets were very large and very close to their star

• This was not because extrasolar systems more like ours do not exist (they do)

• It was simply a reflection of the methods that are used

• They tend to be more sensitive to large planets close to their star


• But the existence of “hot Jupiters” – jovian planets very close to their star – is not

consistent with the nebular theory we have discussed

• Following the scientific method, we need to see if there is some way the nebular theory

can be modified to account for this

• And there is…


• It’s a matter of timing…

• In our own solar system, the waking Sun expelled all the nebular gas and dust

• The strong solar wind produced when fusion was about to start blew it all away

• But if that hadn’t happened, the planets and the nebular disk would interact…


• …and the planets would migrate inward

• The star still blows the nebula away when it finally comes alive

• But a jovian planet that formed beyond the frost line might find itself, after migration,

closer to its star than Mercury is to our Sun

• And the nebular theory lives to fight another day

TRAPPIST-1

TRAPPIST-1

TRAPPIST-1

Date post:	01-Jul-2018
Category:	Documents
Upload:	lekiet
View:	212 times
Download:	0 times

Solar System Formation - University of North Floridan00006757/astronomylectures/ECP4e... ·...

Documents