Revised nebular hypothesis
Also called: Schmidt-Weizsacker hypothesis · Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Class 11, Ch 2 "The Origin and Evolution of the Earth"
Meaning
The revised nebular hypothesis is the 1950 update of the old nebular idea. Otto Schmidt (Russia) and Carl Weizsacker (Germany) put it forward. They said the Sun was surrounded by a solar nebula, a cloud of mostly hydrogen and helium with some dust. The particles rubbed and crashed into each other and formed a flat, disc-shaped cloud. The planets grew from this disc by accretion, which means small bodies sticking together into bigger ones.
Example
The inner planets (Mercury, Venus, Earth and Mars) are small and rocky. The outer planets (Jupiter and Saturn) are huge balls of gas. Both groups grew from different parts of the same disc around the Sun.
Don't confuse with
- Nebular hypothesis (Kant and Laplace, 1796): this was the earlier, simpler version. It did not explain how particles in a disc collide and build up into planets.
- Tidal hypothesis (Jeans and Jeffreys, 1919): this idea depended on a passing star. It failed because a passing star cannot explain the planets' angular momentum, meaning their spin and orbital motion.