History & Evolution of the Solar System

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

The solar system you see today didn't look like this millions of years ago. This lesson discusses the history and evolution of the solar system by focusing on Late Heavy Bombardment, planet migration and solar output.

The Solar System

From the point of elementary school, you began learning about the solar system. You became familiar with the nine planets that were deemed components of the system. And you likely felt comfortable that you knew quite a bit of information about the solar system.

Then, all of a sudden, you learned that Pluto is no longer classified as a planet. What happened? As scientists learn more about the formation and evolution of the solar system, they're better able to determine things like what constitutes a planet.

Formation of the Solar System

The solar system hasn't always been arranged the way it is today. There have been changes over billions of years, and changes will continue for billions more years. During the earliest stages of development of the solar system, the planets were located very close together. Then, due to gravitational pull and collisions in space, they began to spread out and create their own orbits around the sun.

Depiction of the current organization of the solar system
Diagram of the Solar System

Let's look at a few events that led to the current organization of the solar system.

Late Heavy Bombardment

Many planetary scientists believe that the Late Heavy Bombardment, also called the lunar cataclysm, occurred around 4 billion years ago. Abbreviated as LHB, the Late Heavy Bombardment was a period of time when there were many asteroids colliding with the four inner planets. You might recall that the four inner planets - Mercury, Venus, Earth and Mars - are referred to as the terrestrial planets. Exactly what caused the LHB has and continues to be up for debate.

One hypothesis is that there was a fifth inner planet, referred to as Planet V. It's believed that this planet became unstable and, as a result, left its orbit and entered the asteroid belt. This caused asteroids to enter Earth's orbit, and as a result, asteroids started colliding with other planets.

Another hypothesis is that the LHB happened because Uranus and Neptune formed so much later than the other planets. Their late formation could have caused asteroids to begin moving and colliding with other planets. This hypothesis is not widely accepted and has pretty much been disproven.

The most likely, and believed, cause of the LHB is that it occurred due to the migration of the outer larger planets. The slow movement of the larger planets in different directions caused the gravitational pull they have on each other to change. With this change, the planets began to migrate faster, causing the outer solar system to become unstable. As it became unstable, the gravitational push and pull on each other began to affect debris in the asteroid belt, sending asteroids into the inner solar system to collide with the terrestrial planets.

Planetary Migration

As mentioned earlier, the planets were moving around as they formed. The inner planets didn't exhibit much migration and are pretty much in the same location they've been in since formation. The outer planets are a different story. They migrated quite a bit and affected each other's migration in the process.

The most popular and widely accepted model for planetary migration has the planets forming and migrating with Jupiter and Saturn as the driving source for migration. Jupiter and Saturn formed in proximity to each other and began to have resonance, or a gravitational effect on each other. This resonance caused Jupiter to move closer to the sun and Saturn to move further away from the sun. During this same time, Uranus and Neptune formed close to Jupiter and Saturn. As the resonance between Jupiter and Saturn increased, this pushed Uranus and Neptune further away from the sun.

Solar Output

You're probably aware that the center of our solar system is the sun. The sun emits radiation, which affects the temperature on the planets. The further planets are from the sun, the less radiation and, subsequently, heat they receive. The opposite is also true. The closer a planet is to the sun, the more radiation and heat it receives. So as the planets migrated, the amount of radiation from the sun that they received changed as they moved further away from the sun.

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