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 our cosmology, 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:

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. 4.5 billion years old. The oldest age recorded in the solar system is just over 4.5 billion years.
  2. 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:

  • 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.

(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.
  • Orion Nebula Learn about some Hubble Space Telescope images that were taken in this region and about proplyds, 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.

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 (NH3) 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:

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.

(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:

Model answers to the comprehension questions.