Big Picture Goals for this Chapter on the Formation of the Solar System


1.  Have a good mental “movie” of the collapse of the solar nebula, and an understanding of what physical processes were important during the collapse.


2.  Have a good mental “movie” of the process of building planets through accretion.


3.  Understand how the solar nebula theory accounts for the 4 challenges of our solar system.

Check out a summary of this process.

Formation of the Solar System


Here’s a quick overview of the layout of the solar system:


Motion of the planets in their orbits:

Aside from planets, there are also:


asteroids littered about, but primarily in the asteroid belt between Mars and Jupiter


comets, which mostly reside in the Kuiper Belt (just beyond the orbit of Pluto, at about 40 AU) and the Oort cloud (probably at about 100 AU), which has not yet been observed.



?? Consider this overview of the solar system.   How would you describe the general properties of the solar system?


Any theory which explains the formation of the solar system must at least account for 4 main challenges:


Large bodies in the solar system have orderly motion. 

All planets and most satellites have nearly circular orbits, in the same direction, and nearly in the same plane.  The Sun and most of the planets rotate in the same directions as well.

Planets fall into two main catelgories.

Small, rocky terrestrial planets near the Sun and large, hydrogen-rich jovian planets farther out.  The jovian planets have many moons and rings made of rock and ice.

Swarms of asteroids and comets populate the solar system.

Asteroids are concentrated in the asteroid belt, and comets populate the regions known as the Kuiper belt and the Oort cloud.

There are several important exceptions to these trends.

Planets with unusual tilts, very large moons, or moons with unusual orbits.

The Solar Nebula Hypothesis


Based on these observations, astronomers think that the best model for how the solar system formed is from the collapse of an interstellar gas cloud.  There were other ideas about the formation of the solar system, but they didn’t fit these four important characteristics. 


There is other evidence that we are on the right track.  We now see protoplanetary disks around other stars.





What were the properties of the cloud to begin with?


·      Large and diffuse, slowly rotating – it had some angular momentum.

This property of the cloud primarily accounts for the motions of the planets. Why? Think about momentum and energy…

·      Composed primarily of hydrogen and helium, but there must have been some heavier elements, including metals, since we find them in the terrestrial planets for example.

This property of the cloud will dictate how the planets ended up in two main types.  Think about phases of matter…


Elements heavier than lithium are formed only in stars! 


?? Consider the planetary masses listed in Table E.1.  Can you make a prediction about how much of the gas cloud was heavy elements compared to hydrogen and helium by making some simple assumptions?


Evolution of the Solar Nebula


The collapse is triggered by an increase in density, and driven by gravity.

 What do we mean by “collapse”? 


During the collapse…


1.  The cloud’s rotation rate increases, due to conservation of angular momentum.


2.  The cloud heats up, as compressing the gas cause the particles to speed up, increasing the temperature of the gas and dust particles.


3.  The disk of gas and dust flattens, as collisions between the particles of the cloud to lose energy in the direction perpendicular to the cloud’s rotation.




So we can now account for one of the four challenges:  the orderly nature of the orbits of planets in the solar system is due to conservation of energy and angular momentum during the collapse of the gas cloud from which they formed. 


The direction of rotation is dictated by the angular momentum of the cloud. 


The inclination of the planets is due to the flatness of the nebular disk after collapse.



Building Planets




The nebula heats up during the collapse.  The densest, hottest part of the nebula is at the center.  As a result of this, all material very near the protosun existed in a gaseous state.  As you move outward, the nebula is cooler.   At different radii, the temperature is low enough for certain materials to condense.


?? Why are there two types of planets, terrestrial and Jovian?


A.  The force of gravity due to the massive Sun draws the heavier, dense material of the terrestrial planets closer.

B.  Initial orbits of the terrestrial planets bring them closer to the Sun where they fall into smaller orbits.

C.  Near the Sun, only heavy elements and rocky material can condense from the solar nebula.

D.  Jovian planets form first and draw much of the gaseous material to them via gravity, leaving only the heavier elements and rocky material behind.



So beyond the frost line, which lies between the orbits of Mars and Jupiter, temperatures had dropped enough for ices such as water, ammonia and methane to condense.  Notice that these ices are hydrogen rich, since there was plenty of hydrogen to go around out there.


??  Which of the following pictures best describes the distribution of material in the solar system?



How to grow planetessimals:


·      Initially small particles of gas and dust were able to stick together via their electrostatic attraction.


·      As they grew larger, their gravity began to be strong enough to attract particles as well, and their growth accelerated.


·      Once large enough, gravity pulls the planetessimal into a spherical shape.


·      Once a planetessimal reaches a certain size (around 1 km) this process really takes off and it begins to gravitationally dominate everything around it.


We can now account for the second important challenge of explaining our solar system, the division of planets into two basic types.


Rocky, metallic material of the terrestrial planets could condense nearer to the Sun than the ices.   Hydrogen and helium gas remained gaseous throughout the solar system.


Forming the Jovian planets through nebular capture


Once accretion finished building the seeds of the Jovian planets, their large masses meant that their gravity was strong enough to accumulate large amounts of the remaining nebular gases – i.e.  the force of gravity of the planet was stronger than that from the Sun at that point, so that the gas went from orbiting the sun to orbiting the planet.


This process proceeded in basically the same way as the nebular collapse which formed the solar system, forming similar disks of material around the Jovian planets.  Some of the material contributed to the planet, and some to satellite systems through accretion.


The Solar Wind blows out the Remaining Nebular Gases


The solar wind is composed of charged particles from the Sun’s hot (millions of degrees K) corona which carry the Sun’s magnetic field. 


We see evidence (T Tauri stars) that this solar wind is very strong in young stars.  Radiation pressure from the young sun is also important – photons have momentum. 


These effects work to clear out the remaining gases, before they cool enough for ices to condense in the inner solar system.


The last two challenges

Once we understand the process of accretion, the solutions to the last two challenges follow naturally.


Asteroids and comets are leftover planetessimals of terrestrial and Jovian planets.  The nebular theory predicts that their compositions should be quite different, which they are:  asteroids are mostly rocky with very small amounts of ices, comets are “dirty snowballs”.


The early solar system must have been full of planetessimals, so that there was a period of heavy bombardment during which impacts were very common.  We have direct evidence that some of these impacts involved large bodies, which may have led to the exceptional situations in our solar system (e.g. the tipping over of Uranus, the formation of Earth’s large moon).


Age of the Solar System


How do we know the age of our solar system? 


We use a technique called radioactive dating.


We can apply this to many different samples:


·      Earth rocks

·      Moon rocks

·      Meteorites


Some meteorites have not changed since they were formed via accretion, and provide the most reliable age of the solar system,  4.6 billion years.


Compared to the Universe (10-15 billion years), that is not very old.

Summary of steps for building a Solar System ala the nebular theory:

1.  Collapse of the nebula and formation of the protoplanetary disk and protosun.

2.  Condensation of planetessimals.

3.  Accretion of planetessimals to form planet seeds.

4.  Formation of Jovian planets through nebular capture.

5.  The solar wind of young Sun clears away the remaining gas.