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 size and composition 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
The generally accepted explanation for the formation of the solar system is the nebular theory.
A large, thin cloud of dust and gas like the one that eventually formed our solar system is called a solar nebula.
According to the nebular theory, the solar nebula formed from the material expelled by previous stars.

4. The Nebular Theory
The contraction of the solar nebula, a large cloud of dust and gas, led to a disk-shaped protoplanetary disk, from which the sun and planets formed.
Solar nebula
The sun forms at the center of a protoplanetary disk.
Planetesimals form.
Protoplanets form.

5. The Nebular Theory
It is likely that a shock wave from the explosion of a nearby star caused the solar nebula to collapse.
As the cloud contracted, it began to spin faster.
The gravitational attraction between particles became stronger as the solar nebula shrank.

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

7. The Nebular Theory
Nearly all of the mass of the solar nebula became concentrated near the center.
The density at the center of the disk increased until the temperature reached a few million degrees.
About 10 million years after the formation of the solar nebula, nuclear reactions began and the sun was born.

8. The Nebular Theory
Planetesimals and Protoplanets
The nebular theory 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.

9. The Nebular Theory
Planetesimals were asteroid-like bodies that eventually combined to form planets.
Planetesimals grew by accretion, the process of adding mass by colliding with other planetesimals.

10. The Nebular Theory
At about a kilometer in diameter, planetesimals began to exert a significant gravitational attraction on nearby objects.
The planetesimals attracted more material, causing them to grow even faster.
The result was the accretion of planetesimals into a much smaller number of moon-sized protoplanets.

11. The Nebular Theory
Protoplanets joined to form the current planets in a series of immense collisions.
The nebular theory provides an explanation for the current motions of the sun, planets, and most moons–nearly all revolve in the direction that the protoplanetary disk was spinning.

12. 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.

13. 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.

14. Composition and Size of the Planets
At low pressures, such as those found in space, cooling materials generally condense directly from a gas into a solid.
When the solar system formed, temperatures near the sun were very high. Rocky materials condensed, but water and other ice-forming materials did not.

15. Composition and Size of the Planets
Ice-forming materials could condense in the outer solar system because it was much colder.
As the outer planets grew, their gravity increased, and they were able to capture hydrogen and helium gas.

16. Composition and Size of the Planets
Astronomers have been able to observe protoplanetary disks around distant newborn stars.
Astronomers have also detected more than a hundred planets in orbit around distant stars.
Evidence of planets forming around other stars, plus the results of computer simulations, provides support for the nebular theory.

17. Composition and Size of the Planets
The Hubble Space Telescope took this photo of what are thought to be protoplanetary disks around several young stars.

18. 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.

19. 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. ANS: B

20. 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.

21. 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. ANS: D