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11. Solar System Formation
Reading: Chapter 8
We now come to the point where we think "How did this solar system come about?". In a way,we are returning to
the thoughts that began the course about what is ourcosmology, our view of the universe, based on the observations
we can make and our ideas of the physical processes that control the organization and behavior of matter. In this
course we have only considered a very small part of the universe, the solar system. On the other hand, it is in our
exploration of the solar system that our observations and understanding have come such a long way since the Greek
astronomers started us on this road of scientific exploration. So, will consider how the basic properties of the solar
system relate to the formation process. As we shall discuss, there is strong evidence that the solar system formed
about 4.5 billion years ago and it took about 500 million years for the planets to form in more or less their current
location and orbits. In subsequent sessions we shall consider how the planets and other solar system material
evolved for the next 4 billion years. The discussion of solar system formation follows the topics:
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Theories under Development
Facts to be Explained
Collapse of the Solar Nebula
Condensation & Accretion
Planet Formation & Evolution
Missing Details
A Scenario
We shall conclude with a look forward to future explorations of other solar systems - the explanation of which will
be the ultimate test of our theories about solar system formation and evolution.
Theories under Development
Our understanding of the origin and evolution of the solar system is still very limited. Ideas about how the solar
system formed are still not thoroughly tested - there is no single theory that explains it all. Progress is made by
cycling through the scientific process of hypothesis, prediction, measurement, theory, hypothesis,...
How do we explore the solar system's early history? We cannot go back in time. It is a bit like trying to build a 1000
piece jig-saw from the 5 pieces you find lying under the table, after the cat has chewed them. It is not quite so bad if
we insist that the solar system evolved according to the laws of physics and chemistry--this limits the set of all
imaginable histories. but there is still a lot of guess work to be tested before we can really talk about a real theory of
solar system formation.
(1) (a) Most would agree that there is only one accurate description of the real solar system and that the real solar
system followed one path of evolution. The question is whether we've figured it out or not. In the absence of
complete knowledge about the past, can there be more than one theory that could be correct? How are multiple
theories reduced to the one theory that explains reality?
(b)In the future we will able to explore planetary systems around other stars (it is not a question of "if" but
"when"). If we find that the planetary systems are very different from our solar system, does
this necessarily mean that our theories about the formation of solar systems are wrong? Explain.
Facts that any Theory of Solar System Formation Should Explain
At the beginning of Chapter 8 there is a list of facts about the solar system that a correct theory needs to explain.
Here is an alternative set of facts:
1.
2.
4.5 billion years old. The oldest age recorded in the solar system is just over 4.5 billion years.
Prograde rotation. All planets move around the Sun in the same direction that the Sun rotates and close to
the equatorial plane of the Sun.
3. Angular momentum. Although the Sun has 99.9% of the mass in the solar system, the planets have 99.7%
of the system's angular momentum.
4. Terrestrial vs Giant Planets. The inner planets are smaller and denser than the outer planets, and are made
of silicates and metals. In contrast, the outer planets are dominated by hydrogen (close to cosmic
composition) and have many satellites that are rich in water ice and other volatiles.
5. Asteroids. The asteroids have compositions intermediate between the rock & metal rich inner planets and
the volatile-rich outer solar system, and are located between the orbits of Mars and Jupiter.
6. Meteorites. The oldest and most primitive meteorites contain grains of compounds that are expected to
have formed in a cooling cloud of cosmic abundance at temperatures of a few hundred degrees.
7. Comets. Comets, like the surface of some outer planet satellites, appear to be composed primarily of water
ice, with significant quantities of trapped or frozen gases like carbon dioxide, plus silicate dust.
8. Volatiles. Volatile compounds (such as water) must have reached the inner planets in spite of the fact that
the bulk composition of these bodies suggests formation at temperatures too high for volatiles to form solid
grains.
9. Retrograde planets. Despite the general regularity of planetary orbital and spin motion, Venus, Uranus
and Pluto all spin in a retrograde direction.
10. Regular satellites. All of the giant planets have systems of regular satellites orbiting in their equatorial
planes, rather like miniature versions of the solar system.
11. Irregular satellites. Except Uranus, the giant planets have one or more irregular satellites (which have
orbits that are either retrograde or have high inclinations and/or eccentricities).
12. Galilean satellites. The Galilean satellites of Jupiter exhibit a decrease in density with increasing distance
from Jupiter
(2) (a) Check that the list above is basically the same as the one in Chapter 8. What is added?
(b) Each person is entitled to pick their own set of facts that they would like a theory to explain - why is Pluto
such a misfit and stuck out at the edge of the solar system?. Here is perhaps a more useful catagorization of facts:
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Large bodies in the solar system have orderly motions
Planets fall into two main catagories
Swarms of asteroids and comets populate the solar system
There are several notable exceptions to these general trends.
Look at the list of 12 facts above and organize them into these 4 main catagories.
(c) Do you see anything that is left out from either list? The evolution of life, perhaps?
Collapse of the Solar Nebula
We have to start somewhere--the formation of the Sun seems a good place. Theories of star formation are based on
observing millions of stars of different ages. We start with a nebula of gas and dust.
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Nebula = noun = "cloud" (plural = nebulae)
Nebular = adjective = "cloud-like"
(So, this section could have been called "The Collapse of the Nebular Solar Nebula").
If we look up at the constellation Orion, in a region near his "belt" there is a cloud illuminated by neighboring stars this is the Orion Nebula.
Links:
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Orion Nebula Learn about some Hubble Space Telescope images that
were taken in this region and aboutproplyds, objects which might be
young solar systems that we are catching during formation.
Figures 8.4 and 8.8b show clouds of interstellar dusk and gas that look dark because they block light from the stars
behind. Looking at these dark clouds with infrared light we see that the dust is warm. Spectral studies of the Orion
nebula show that there are complex molecules, including hydrocarbons. If a blob of cloud is dense enough, its own
gravity causes it to collapse onto itself. The solar system is thought to have collapsed from a cloud that was initially
about a million times larger than the current solar system.
As the cloud contracts, it spins faster and faster, conserving angular momentum (see p 141) - just like a skater
retracting his/her arms. The cloud contracts to form a disk with a large, dense blob in the center the protosun (sketched in Figure 8.6). If the initial nebula started out with a lot of angular momentum, it collapses
into more than one protosun - 80% of all systems are believed to have multiple stars orbiting each other.
We are going to completely avoid the complexities of star formation - that comes in the follow-on course to this
"Stars and Galaxies". Suffice it to say that when the pressures and densities of hydrogen in the center of the
collapsed nebula become great enough, nuclear fusion starts at the center of the new star, converting hydrogen to
helium and releasing lots of heat. Just as our Sun began to do 4.5 billion years ago - and continues to do, to the great
pleasure of us here on Earth.
Surrounding the protosun (or protosuns) a disk of dust and gas extends for 100 AU or so. This is sketched in Figure
8.6). This is the solar nebula In Figure8.8 is an image of such a disk of dust and gas around the recently-formed star
Beta Pictoris.
(3) What determines in which direction the collapsing nebula spins?* Our solar system has a prefered sense of
rotation that is anti-clockwise looking down from the north (as if you were looking at it from the star Polaris). Is it
just as likely that our solar system could have the opposite rotation?
*Think of water in a bathtub: before you pull out the plug you can stir up the water in different ways - large scale
clockwise motions, large scale anti-clockwise motions, small scale turbulent motions - but when you pull the plug it
either goes clockwise or anti-clockwise, depending on what was the dominant motion. Even if you leave the bath tub
to settle for several hours before you pull the plug, there are residual small scale eddies that start the flow going in
one direction or the other. (NO - it is the same in both north and south hemispheres - really - the effect of the Earth's
rotation is negligible compared with the original motions in the bathtub water).
Condensation and Accretion
We have already formed the Sun. Now, let's make the planets. the remaining dust and gas collapsed to a disk. The
diagram below is a sketch of the disk before the planets formed. Let's now think about what happened to the solar
nebula after the sun formed in the center, heating up the dusk and gases. Since the source of heat is greatest at the
center of the disk, close to the sun and where the cloud is the densest, the disk will be hotter near the center and
cooler farther away. The temperatures in the early solar nebula dropped rapidly from temperatures of 1000s K inside
1 AU to few 100s K farther out. The diagram below shows how the temperature decreased with radial distance from
the proto-sun in the solar nebula.
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Refractory materials have very high melting and condensation temperatures - they tend to be solids except
at very high temperatures - e.g. metals and silicates (rocks)
Volatile materials have very low melting and condensation temperatures - they tend to be gases (or maybe
liquids) unless the temperature drops to very low temperatures (e.g. ices of water, ammonia, carbon
dioxide, nitrogen..)
Click here to see a table of volatiles in the solar system.
As the cloud cooled (due to thermal radiation - infrared emission), the gas temperatures dropped below the
condensation temperatures of metals in the inner solar system, silicates (rock) near Earth, water ice out near Jupiter
and other volatiles (ammonia, methane, carbon dioxide, nitrogen) farther out. So there tends to be refractory
materials closer in to the Sun and more volatiles farther from the Sun. Figure 15.6 shows a plot of temperature vs.
distance from the Sun in the early solar nebula - and the distances at which different materials begin to condense out.
So, the condensation of refractory materials leads to rocky/metallic terrestrial planets - why are there gas giants?
Temperature is one factor controlling the amount of different materials, the other factor is abundance. The original
nebula is generally believed to have the same composition as the Sun - which is pretty typical of most of the
universe - so it is called cosmic abundance. Here is the table of cosmic abundances of the main elements.
We want to make simple compounds that will condense to a solid. The easiest thing to combine with the most
common element, hydrogen (The next abundant, Helium, is a "noble" gas - it rarely combines with anything - neon
and argon are noble too). Oxygen and hydrogen make water. The next likely candidates are ammonia (NH 3) and
methane (CH4) which are volatiles - freezing out at low temperatures. Water is by far the most abundant simple
compound.
In the solar nebula the temperature dropped below 0° C (273 K) somewhere between 3 and 4 AU - this distance is
sometimes called the "snowline" - beyond which water condensed and clumped into snowballs, eventually
coalescing into many planetesimals. With the large volumes of the outer solar system occupied by snowballs, these
accumulated into LARGE planets - large enough to hold in hydrogen. Since hydrogen is so abundance, these
became GIANT planets.
(4) How does this idea of condensation of different materials according to temperature in the solar nebula and to
cosmic abundances lead to just two types of planets - terrestrial and giant - rather than a continuous spectrum or
4-5 different kinds of planets?
Planet Formation and Evolution
As the nebula cool and materials began to condense and clump into chunks, chunks of rock/metal in the inner solar
system and chunks of ice in the outer solar system. These chunks of material that eventually coalesced to form the
planets are called planetestimals - this link shows small planetesimals forming a thin disk and orbiting the new Sun.
Why a disk? The reason is the same as the reason that Saturn's rings form a disk - particles that are NOT in regular,
circular, equatorial orbits will collide and will either break up or be forced to conform to a regular orbit. This
process acts both to confine material to a thin disk (what we now call the eccliptic) as well as causing the orbits of
the surviving objects to be regular circles that are spaced apart, so that there are no more collisions. This is
illustrated in Figure 8.7.
The initial process whereby clumps of solid material begins to stick together is really not understood at all. But we
know that as clumps get bigger they can graviationally attract more material and grow - "snowballing" to bigger
objects, protoplanets. Quite quickly (in less than 100 million years - that's short compared to the 4.5 billion year age
of the solar system) the collision and coalescence leads to a few large objects that orbit in roughly circular orbits,
with a fair amount of junk in between.
At some point all of the gas that was left in the solar nebula was blown away, probably when the Sun went through a
phase of strong out-flowing wind (which is observed in newly-formed stars similar to the Sun).
The accretion process - planetesimals colliding to form planets - heated up the planet (think of rocks and ice blocks
crashing into the planet - heat is generated in the collision). As the solid materials were heated up they became liquid
- the denser liquids fell to the center of the planet. This differentiation (core formation) further heated the planet.
This heating happened to all planets - but the bigger the planet the more heat that was generated.
Slowly the planet begins to loose heat - by conduction, convection, eruption and radiation - the smaller the planet,
the quicker it lost heat. On the smaller, terrestrial planets a crust of solid rock formed on the surface. The largest
planets - the gas giants - still retain much of their primordial heat of formation.
For the first billion years there was still a considerable amount of chunks of rock and ice flying around the solar
system - material that had not accreted into a planet. Until about 3.8 billion years ago collisions were rife.
(5) Go back to our table of 12 facts we need to explain. How are we doing at this point? Which aspects of the
solar system have we explained?
Age of the Solar System
Here we are happily talking about the solar system being 4.5 billion years old, but how do we KNOW that the solar
system is this old? What is the scientific evidence? The main evidence comes from radioactivity. A few elements are
unstable and are likely to "decay" - that is, emit a particle and become a different element. For example, an isotope
of potassium (potassium-40) decays to an isotope of argon (argon-40) with a half-life of 1.3 billion years. This
means that 1 kilogram of pure potassium-40 would, over 1.3 billion years, turn into 1/2 a kilogram of argon-40 and
1/2 kilogram of remaining potassium-40. Then, another 1.3 billion years later, the 1/2 kilogram of potassium-40
reduces to 1/4 kilogram and another 1/4 kilogram of argon-40. Therefore, we can find out the age of a lump of rock
by measuring the ratio of potassium-40 to argon-40 - see figure 8.17.
The oldest rocks on Earth are about 3.9 billions years old. There are not very many of such old rocks around since
the surface of the Earth has been thoroughly resurfaced. The oldest lunar rocks are about 4.4 billion years old. The
oldest rocks ever encountered are meteorites, some of which are as old as 4.6 billion years. These meteorite rocks
are thought to have formed during the early condensation of the solar nebula. The planets formed about 0.1 billion
(100 million) years later. So, the age of the Earth is probably close to about 4.5 billion years.
(6) (a) If you pick up a fresh piece of lava (having waited for it to cool down, obviously!) would you expect the
ratio of potassium-40 to argon-40 to be close to 0 or to a large amount? (Hint: look at Figure 8.17) (b) Next, think
about the old meteorites, what is the potassium-40/Argon-40 ratio in the old meteorites?
Missing Details
So we now have separate planets - terrestrial planets in the inner solar system and giant, gas planets (with regular
satellites) in the outer solar system. But there are some details that are not yet explained:
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Atmospheres of the terrestrial planets
Asteroids
Comets
Venus slow backwards spin, Pluto/Charon and Uranus tipped on their sides
Earth has life
Role of Comets: while the volume of the original nebula was huge in the outer solar system and led to large
numbers of "iceballs" being generated, they did not all accrete into planets. Many were scattered out into a spherical
cloud about 100,000 AU across - the Oort cloud. These comets were occasionally perturbed and sent into the inner
solar system. In the early phases of the solar system a much higher flux of comets than the present rate probably
brought volatile ices and gases into the inner solar system - collisions of these icy bodies with the terrestrial planets
could been the main source of the terrestrial planet atmospheres.
Role of Major impacts: long after the planets formed their remained fairly large planetesimals on eccentric orbits.
Thus, there was a chance of major impacts. The Earth's Moon is thought to be the result of a Mars-size object
impacting the Earth. Similarly, Charon is thought to have been captured in a large impact. Large impacts may have
tipped Uranus on its side and changed Venus' spin.
(7) The above are listed aspects of the solar system that may have been caused by large impacts that occured
quite late in the formation of the solar system. Given the large size of the solar system and all of the objects in it planets, moons, asteroids, comets - are these a large number of coincidences/ catastophes? Or are these 'mis-fit'
aspects just a few "wrinkles" that make our solar system unique? That is, they give our solar system its own
special character - just as each litter of labrador puppies look and behave in a predictable way but, on closer
inspection, chance has resulted in differences that make each litter different (a floppy ear here, a white patch
there, etc.;).
A Scenario for the Origin and Evolution of the Solar System
Here is a hypothesis--a scenario for the formation of the solar system. This is an active area of research--different
people are working on different parts of the story. Some are building computer models of the physics and chemistry-others are searching for clues of conditions in the early solar system by exploring the more primitive bodies-comets, meteoroids and asteroids. Others are looking for solar systems around other stars to see if there is a range of
different kinds of solar systems that can form.
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The solar system formed 4.5 billion years ago (based on the oldest rocks and estimated age of the sun as a
star)
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A clump of interstellar material (hydrogen, helium, molecules and dust) collapses under self-gravity to for a
protosun and a surrounding disk.
As the nebula collapses it
o Heats up - eventually nuclear reactions are initiated in the sun
o Spins up - conserving angular momentum
The sun must have lost most of its angular momentum (see pages 223-4) - 2 possible ways:
o A period of strong solar wind
o Interaction of the sun's magnetic field with ionized material in the disk, braking the sun's rotation
There was a strong temperature gradient in the disk nebula:
o Close to the sun everything remained vaporized except refractory materials
o Outside 3-4 AU ice grains condensed
The outer disk collapsed from a larger volume, providing
o More material
o More planetesimals
As the nebula cooled
o Close in - only refractory materials condensed => terrestrial planets
o Farther out more and more volatile material condensed => ices
Planetesimals collide and accrete to form planets:- in the outer solar system more material => larger planets
which were able to graviationally bind lighter gases (hydrogen and helium)
Period of strong solar wind blew away remaining gas in the nebula disk
The objects that did not accrete to form large enough bodies to differentiate retain the chemical
composition of the original c ondensed solar nebula. Thus we believe the oldest meteorites (and comets,
shoul d we ever get to sample them) hold clues about the early phases of solar system evolution.
Jupiter kept perturbing the orbits of the planetesimals inside Jupiter - the planetesimals kept crashing into
each other at too high speed to coalesce - they broke up instead. These objects formed the asteroid belt.
Conditions in the proto-Jupiter nebula mimics the solar system: satellites closer to Jupiter are rocky while
farther satellites have more ice (and lower density).
Some of the remnant planetesimals were captured by the giant planets to form the irregular satellites.
Comets from the outer solar system bring volatiles in to the terrestrial planets (=> atmospheres of terrestrial
planets)
(8) (a) Writing out a "scenario" - printing it in nice type - can make it seem "real". Yet, much of this is just
guesswork. We have an idea that something must have caused a particular feature (such as the initial coalescance
of condensed grains) but we really have no real idea how this happened. Because the planets have evolved
considerably since they formed, they are not likely to be the places where we are going to find clues about the
early solar system. If not the planets themselves, where else are we going to find clues about the early solar
system and how it formed?
(b) We have completely ignored the issue of life. At what point in the above scenario could life have begun to
successfully evolve? The issues of how and where life evolved are perhaps the most challanging and exciting
questions to answer.
Planets Around Other Stars
In the past 5 years astronomers have discovered a dozen or so planets around other stars. This exploration is
happening at a furious pace - we are realizing that there are indeed solar systems other than ours. Thus, we are ready
to test if the ideas we developed about our solar system can be applied elsewhere - can we apply the above scenario
to other planetary systems? How does it need to be modified for different conditions?
The figure above shows our solar system at the top - this sets the hoizontal scale (in AU). Below our solar system
are 9 different stars that are orbited by a planet. The name of the star is given in red in the middle of the diagram.
The planet is shown in brown or green at its proper location from its parent star and the mass of the planet is given
in Jupiter-masses. So, the first system below our solar system is the system in Ursa Major (that's the big dipper!) and
the planet is at about 2.2 AU from the star and has a mass of about 2.4 times the mass of Jupiter.
Detection of a planet is made by measuring the minute wobble the planet's gravity causes to the star that is orbited.
At the moment, we can only measure the wobble caused by large planets that are close to the parent star. This means
that the planetary systems detected so far seem rather different from ours (look at the masses of the planets and their
location in AU in the diagram above). To detect terrestrial planets (or jovian planets farther from the star) we will
need much more sensitive instruments - probably located in space.
Links about Planets that have been Discovered around Other
Stars:
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The Nine Planets - brief summary plus more links.
A comprehensive essay on extrasolar planets
Latest summary from one of the main discoverers discussion of HOW extrasolar planets are found.
A table of properties of discovered objects.