1. Any theory about the origin of the solar system must explain why all of the planets’ orbits lie more or less in a plane and all of the planets orbit the sun in the same direction. It must also explain the differences in _________________ between the terrestrial planets and the gas giants.

2. The Nebular Theory
What is the nebular theory?
The nebular theory states that the solar system formed from a rotating cloud of dust and gas.

3. The Nebular Theory Solar nebula
The _____________ of the solar nebula, a large cloud of dust and gas, led to a disk-shaped ___________ disk, from which the sun and planets formed. Construct in your own words how planets are formed.
Solar nebula
The sun forms at the center of a protoplanetary disk.
Planetesimals form.
Protoplanets form.

4. The Nebular Theory
Formation of the Protoplanetary Disk
As the solar nebula rotated faster, it began to _________________.
Over about 100,000 years, a large disk-shaped cloud of dust and gas called a protoplanetary disk formed.
The disk was _________________ in the center and thinner toward the edges.

5. The Nebular Theory
Planetesimals and Protoplanets
The _________________also explains how the planets could have formed.
Weak surface forces held dust grains together, forming loose balls of dust. As these balls of dust collided, they grew larger and larger.

6. The Nebular Theory
Planetesimals were asteroid-like bodies that eventually combined to _________________.
_________________ grew by _________________, the process of adding mass by colliding with other planetesimals.
Assess this process and does it happen here on earth?

7. Composition and Size of the Planets
How does the nebular theory explain the composition and size of the planets?
The terrestrial planets are relatively small and rocky. In part, that is because the inner solar system was too hot during their formation for ice-forming compounds to condense.

8. Composition and Size of the Planets
The gas giants are large and have low densities because the outer solar system was cool enough for ice-forming compounds to condense.

9. Composition and Size of the Planets
The _________________took this photo of what are thought to be _________________ disks around several young stars.

10. According to the nebular theory,
Assessment Questions
According to the nebular theory,
the sun will eventually die out and become a solar nebula.
the solar system began as a solar nebula.
the solar system evolved from a protoplanetary disk to a solar nebula.
solar nebulas were asteroid-like bodies that combined to form planets.

11. Assessment Questions
Which of the following is not evidence used to support the nebular theory?
The planets’ orbits lie generally in a single plane.
The planets all revolve around the sun in the same direction.
The terrestrial planets are relatively small and rocky.
All of the planets have the same composition.