Solar Nebular Hypothesis: Definition & Explanation

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  • 0:01 Solar Nebular Hypothesis
  • 0:20 A Cloud of Gas
  • 1:04 Sun Formation
  • 1:44 Planet & Asteroid Formation
  • 2:54 Lesson Summary
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Lesson Transcript
Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

The solar nebula theory describes how our solar system formed from a nebula about 4.5 billion years ago. This lesson will cover the aspects of the theory. Afterwards, you will have a chance to complete a quiz to test your knowledge.

The Solar Nebular Hypothesis

The solar nebular hypothesis describes the formation of our solar system from a nebula cloud made from a collection of dust and gas. It is believed that the sun, planets, moons, and asteroids were formed around the same time around 4.5 billion years ago from a nebula.

A Cloud of Gas

It's believed that before our solar system was formed 4.5 billion years ago, a nebula, which is an interstellar cloud of gas and dust, was present in our location. As gravity does with everything, it began to condense the gas into varying regions of density. The denser regions began to grow into clumps of matter, which, over the course of time, would be the seeds for the formation of our sun, planets, and moons.

As gravity condensed the gas, rotation of the gas increased, spreading the gas cloud into a rotating disk that would form the plane of the solar system as we know it today. Evidence of this can be seen because all of the planets revolve around the sun in the same plane and direction.

Sun Formation

The center of the disk of spinning gas experienced the least amount of centripetal force, which allowed a majority of mass from the nebula cloud to be attracted to the center by the force of gravity. In fact, 99.85% of all the mass in the solar system is in the sun.

As gravity compacted the material in the sun, mostly hydrogen gas, pressure began to increase and heat the gas. About 4.5 billion years ago, the mass reached a critical point, and the hydrogen at the center was under so much pressure that it fused with another hydrogen atom, creating helium. This fusion was the birth of our star, the sun.

Planet and Asteroid Formation

Planetesimals, which were early planets, began to grow in size from the clumps of matter and slam into each other as they revolved around the early forming sun. The theory of how our moon was formed lines up with this. It's believed that during the early formation of our solar system, a young Earth and an object about the size of Mars collided, sending debris into space and reforming within Earth's gravity and forming the moon.

The image here shows how a planet may have formed in an early forming solar system.

planet formation

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