A First Look at Solar System Formation

In the introduction video for Module 8, I made some quick remarks about the nebular hypothesis—the idea that the Sun and all the planets formed from the collapse of a nebula, or giant gas cloud. When I mentioned the nebular hypothesis, I told you why I didn’t want to discuss it until after we had finished exploring the planets—i.e. because, as with any other hypothesis, the nebular hypothesis is based on observations (along with the known laws of physics), and we had yet to discuss a lot of those observations. Now that we have made our way through the planets and, in the last module, into the farthest reaches of our Solar System, the stage is finally set to examine how we believe they came together as they did.

Learning Activity


The first 4:22 in the video is a meagre recap of the history of Solar System astronomy up to Newton, plus Phil Plait’s opinion on the definition of a planet, which you do not need to watch since both have already been covered in better detail earlier in this course. As you watch from 4:23 onwards, you will see right away how the scientific method is at work in the development of the nebular hypothesis; how it is by recognising trends in the data, and incorporating known principles of physics, that scientists have found answers to the following five questions:

  • Why do all the planets exist in the same plane?
  • Why are there small rocky planets closer to the Sun?
  • What two reasons explain why there are giant planets with large concentrations of light elements further out?
  • Why is there an asteroid belt between Mars and Jupiter, and why is there an even greater abundance of icy bodies past Neptune, rather than a planet in each location?
  • Is our Solar System unique, or are there similar planetary systems throughout the Universe?

As you explore the answers to these questions, look for key terms such as planetesimals and protostars, as you are introduced to the concept of conservation of angular momentum and the role it is thought to have played in the formation of our Solar System.