Ch. 28.3 Formation of the Solar System

The solar system includes the sun and all bodies revolving around the sun. Nebular theory—the entire solar system formed at the same time. Sun and planets condensed out of the same spinning nebula.

Formation of the Sun Solar nebula—the cloud of dust and gas that developed into our solar system. A shock wave hitting the nebula caused it to start contracting 4 or 5 billion years ago. The sun formed in its center. 99% of the nebula’s matter became the sun.

Formation of the Planets Planets formed in the outer regions of the nebula, from small bodies called planetesimals. They joined together through collisions to form protoplanets. Protoplanets condensed into existing planets and moons.

The four inner planets contain high percentage of heavy elements like iron. They couldn’t accumulate lighter gasses because of their weak gravity and the stripping action of the solar wind.

The outer planets could maintain their lighter gases, due to greater gravity and further distance from the sun. They are known as gas giants.

Formation of the Earth Newly-formed earth was very hot due to… Retained heat from planetesimals collisions. Heat from compression of outer layers on inner layers. Radioactive decay heat.

The Solid Earth Heavier elements (mostly iron) flowed to the center of the hot molten earth. Lighter, less dense elements forced to outer layers. Eventual development of three distinct layers…a dense iron/nickel core, a thick rock layer called the mantle, and a thin solid crust of less-dense materials.

The Atmosphere As the developing earth accumulated mass and increased gravity, it attracted a first atmosphere of hydrogen and helium from the surrounding nebula. This was lost due to weak gravity and the solar wind. A second atmosphere of mostly CO2 and water vapor came from the earth’s interior due to volcanic eruptions (outgassing).

Sunlight converted ammonia gas into nitrogen, and when green plants appeared, photosynthesis caused oxygen to slowly increase. An ozone layer developed and served as a UV shield.

The Oceans When the earth cooled enough, water vapor began to condense (3 to 3.5 billion years ago). Rainfall filled up the ocean basins. Ocean water absorbed much CO2. By 1.5 billion years ago, the atmosphere was similar to today’s.