Planets, Stars, Galaxies, and the Universe

Formation and Evolution of the Solar System


Additional reading from

The amount of information and detail in the past pages is significant, but in the interests of time leaves out a great amount of additional information we know about the objects in the Solar System. However, if we think specifically about the patterns in the data about the objects, a few things do stand out:

  • The largest objects are confined to a narrow plane that matches well with the equator of the Sun;
  • The large objects nearest to the Sun, are smaller, denser, have fewer satellites, and are more closely spaced than the ones further from the Sun;
  • The largest objects in the Solar System orbit the Sun in a counterclockwise direction, and most rotate counterclockwise around their axis;
  • The smaller objects in the Solar System are often irregularly shaped and found primarily in two regions; the Asteroid Belt and the Kuiper Belt;

These patterns were used to create a model for how the Solar System may have formed as part of the process of our Sun's formation as described in our discussion of star formation in Lesson 5.  The model we have used for years to describe the formation of the Solar System is often referred to as "Solar Nebula Theory", although some sources (e.g., refer to it as the condensation model.  The main idea is that after the collapsing gas cloud that formed the Sun created a flattened protoplanetary disk, planets, moons, asteroids, comets, and other objects condensed out of the disk.  

Because all of the objects in our Solar System likely condensed out of a rotating, flattened disk, the objects maintained the angular momentum of that disk, and therefore they orbit and rotate around the Sun in the same direction as the original disk. 

As the Sun was forming, the disk was first heated up, and then it began to cool.  The regions nearest to the Sun cooled more slowly than the outer part of the disk, creating distinct zones where different materials were available to form objects.  Heavy elements and molecules were able to condense and solidify in all parts of the disk.  Beyond an "ice line" several AU from the Sun, it was cold enough that different materials could form ices (e.g., water ice, methane ice, and others).  Closest to the Sun, solid worlds were able to accrete together, while beyond the ice line, lower density objects accreted because there were less dense raw materials available. 

Smaller objects formed as single objects in the disk.  The largest "protoplanets" (like the one that formed Jupiter) were able to form mini-disks around themselves, and they formed planet-sized moons in the disk around themselves.  Collisions were common all throughout the disk, though, and some collisions were able to form moons around the smaller planets or to reverse the rotation direction of the new, solid planets. 

Not all areas of the Solar System went through this process to completion, leaving behind remnants of the planet formation process.  These small, irregular bodies became the asteroids, comets, and Kuiper Belt objects we studied previously.

This was the state of our model for Solar System formation for many years.  However, as we have tried to explain some of the peculiarities in our own system as well as in other planetary systems (which we will see briefly in Lab 4 and Lesson 12), we have refined Solar Nebula Theory.  In particular, it appears that "migration" is an important process.  During the planet formation process, interactions between the planets and the disk can cause them to spiral inward towards the forming star.  Gravitational interactions between the planets can also cause them to move outward.  For example, there is a particular model referred to as the "Nice Model" (after the French city) that shows that Saturn was able to pull Jupiter outward.  During this interaction, Neptune moved outward and swapped places with Uranus.  Some of the small bodies of the Solar System were trapped into certain orbits (e.g., Trojan asteroids that lead and trail Jupiter in its orbit or Pluto, which is trapped into a 3:2 resonant orbit with Neptune), while many others were ejected from the Solar System.  As Neptune moved outward, it flung many small bodies into the inner Solar System, where they impacted the planets during a period called the "Late Heavy Bombardment".  There is a nice, short video segment on this from the BBC.  The Nice Model is not the final word on the formation and evolution of the Solar System, but we think a combination of Solar Nebula theory and migration of the planets is how our planets formed.  In the intervening 4.5 billion years, the bodies in the Solar System have continued to evolve into the worlds we see today.