How Do Protoplanets Grow?

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  • 0:01 What Is a Protoplanet?
  • 1:55 The Terrestrial Planets
  • 3:07 The Jovian Planets
  • 4:55 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will teach you about protoplanets, how they formed, and most importantly, how their formation resulted in the planets we see today. We'll also cover outgassing, gravitational collapse, heat of formation, differentiation, and more!

What Is a Protoplanet?

The biggest thing I ever feared when riding a bumper car was having a head-on collision with another one. That's because it would give me a pretty serious jolt, sometimes even knock me a bit out of the seat! Instead, I liked to side-swipe people, especially from behind when they didn't know any better! That way, it was still fun but not as jarring.

If planetesimals, objects that combined to form protoplanets, were to collide head on in the early solar nebula, they would shatter to the point where they would be unable to grow into a larger size, a protoplanet. A protoplanet is a massive object that will eventually become a planet. Instead, these planetesimals moved in the same direction in the plane of the solar nebula and sort of side-swiped each other at lower speeds. Such slightly more gentle collisions would then combine, instead of blow apart, planetesimals into protoplanets.

How Did Protoplanets Grow?

Initially, protoplanets were about the size and mass of our moon and weren't large enough to have a gravitational field strong enough to capture much, if any, gas. Instead, they grew by capturing solid material, like metals and rocks. At first, all of this coalescing material remained in a solid form, even if it was close to the protosun, an early stage in the formation of our sun.

However, as the protoplanets grew, they were heated through two different ways, radioactive decay and heat of formation. Heat of formation refers to the heat that is generated by way of infalling matter during the formation of a planet. The heat of formation, as well as heat generated from radioactive decay of elements like uranium, in the forming planet would have initially melted the planet and allowed for something known as differentiation to occur. Differentiation refers to the settling and separation of material based on density.

The Terrestrial Planets

Basically, because the terrestrial planets began their life as at least partially molten rocky spheres, material within this molten sphere could move about freely, as if through water. This means that denser stuff, like iron and nickel, would sink to the center of the planet, while lighter (less dense) silicon minerals would float to the top and form the crust. This helps explain why rocky planets have dense iron cores.

You can even demonstrate this process for yourself by filling up a glass of water and sprinkling some dirt into it. Some particles of dirt, the denser stuff, will sink to the bottom, while the lighter stuff will settle at the top.

Anyway, the materials that make up the terrestrial planets were pretty scarce in the solar nebula, and thus these planets ended up having a relatively small mass and a relatively weak ability for gravitational attraction. This meant that gases, like hydrogen and helium, in the solar nebula could not be captured to form the atmosphere on such planets. As a result, atmospheres on the terrestrial planets developed by way of outgassing, the release of gases trapped in the interior of a planet, through things like volcanic events.

The Jovian Planets

As for the gas giants, the jovian planets (Jupiter, Saturn, Uranus, and Neptune), these outer planets also formed by accretion of planetesimals into protoplanets. However, there is one big difference between the formation of these planets when compared to the terrestrial ones. Because the outer reaches of the solar nebula had lower temperatures, different kinds of ice could survive here in addition to rocky material. The elements that made up this ice were much more abundant than the stuff that made the terrestrial planets.

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