Eighth Edition
by Harold L. Levin

Chapter 8 - page 2

The Earliest Earth: 2,100,000,000 Years of the Archean Eon

Formation of the Solar System - The Solar Nebula Hypothesis

Schematic view of the Solar System.

Lines of evidence that must be considered for any hypothesis on the origin of the Solar System:

1. Planets revolve around sun in same direction - counterclockwise (CCW).
2. Planets lie roughly within sun's equatorial plane (plane of sun's rotation).
3. Solar System is disk-like in shape.
4. Planets rotate CCW on their axes, except for:
1. Venus - slowly clockwise
2. Uranus - on its side
3. Pluto - on its side
5. Moons go CCW around planets (with a few exceptions).
6. Distribution of planet densities and compositions is related to their distance from sun.
1. Inner, terrestrial planets have high density
2. Outer, jovian planets have low density
7. Age - Moon rocks and meteorites are as old as 4.6 billion years.

Solar Nebula Hypothesis or Nebular Hypothesis:

1. Cold cloud of gas and dust contracts, rotates, and flattens into a disk-like shape.
2. Roughly 90% of mass becomes concentrated in the center, due to gravitational attraction.
3. Turbulence in cloud caused matter to collect in certain locations.
4. Clumps of matter begin to form in the disk.
5. Accretion of matter (gas and dust) around clumps by gravitational attraction. Clumps develop into protoplanets.
6. Solar nebula cloud condenses, shrinks, and becomes heated by gravitational compression to form Sun.
7. Ultimately hydrogen (H) atoms begin to fuse to form helium (He) atoms, releasing energy (heat and light). The Sun "ignites."
8. The Sun's solar wind drives lighter elements outward, causing observed distribution of masses and densities in the Solar System.
9. Planets nearest Sun lose large amounts of lighter elements (H, He), leaving them with smaller sizes and masses, but greater densities than the outer planets. Inner planets are dominated by rock and metal.
10. Outer planets retain light elements such as H and He around inner cores of rock and metal. Outer planets have large sizes and masses, but low densities.

Illustration of solar nebula hypothesis for the origin of the Solar System.

How long ago was the Solar System born?

Based on radiometric dates of moon rocks and meteorites, the Solar System is about 4.6 billion years old.