5.3 Formation of the Solar System
The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula.
The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity at the center increased, and the cloud started to spin because of its angular momentum. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin.
Much of the cloud’s mass migrated to its center, but the rest of the material flattened out in an enormous disk. The disk contained hydrogen and helium, along with heavier elements and even simple organic molecules.
As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the pressure in the center of the disk was high enough, nuclear fusion within our star began, and the blazing star stopped the disk from collapsing further.
Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud, and small pieces of dust started clumping together to create ever bigger clumps of matter. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted more massive particles, such as rock and metal, and lighter particles remained further out in the disk. Eventually, the planetesimals formed protoplanets, which grew to become the planets and moons that we find in our solar system today.
The gravitational sorting of material with the inner planets, Mercury, Venus, Earth, and Mars, dense rock and metal formed. The outer planets, Jupiter, Saturn, Uranus, and Neptune, condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where it is frigid, these materials formed solid particles.
The nebular hypothesis was designed to explain some of the essential features of the solar system:
- Orbits of the planets lie in nearly the same plane with the Sun at the center
- Planets revolve in the same direction
- Planets mostly rotate in the same direction
- Axes of rotation of the planets are mostly nearly perpendicular to the orbital plane
- Oldest moon rocks are 4.5 billion years
Attribution
“2.3 The Solar System” from Physical Geography and Natural Disasters by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution Non-Commerical Share-Alike 4.0 International License, except where otherwise noted.
