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Terrestrial Planets: Definition & Characteristics

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  • 0:00 Planetary Basics
  • 1:03 Terrestrial Planets
  • 2:09 Terrestrial Planets in…
  • 4:02 Lesson Summary
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Lesson Transcript
Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

In this lesson, you will learn what a terrestrial planet is, what the characteristics of terrestrial planets are, and a little bit about each of the terrestrial planets in our own solar system.

Planetary Basics

Planets are formed during the formation of a star. As the nebula collapses under its own gravity, the dust and debris begin to spin. Over time, they flatten into a protoplanetary disk, the birthplace of planets. Different types of planets form in different areas of the disk. There are basically two types of planets: rocky terrestrials and gas giants. The terrestrial planets form in the area of the inner disk, which is closer to the star and heavy with dust and rocks. The gas giants, on the other hand, form in the exterior area of the disk, which is further from the star and contains condensing gas and ice.

The official definition of a planet was set forth in 2006 by the International Astronomical Union, or IAU. A celestial body is a planet if:

  • It orbits a star
  • Is large enough that its gravity pulls it into the shape of a sphere
  • It has cleared other objects in its orbital neighborhood or path

Terrestrial Planets

A terrestrial planet is one with a heavy metal core, a rocky mantle, and a solid surface. It also must meet the three planetary criteria as set forth by the IAU. Terrestrial planets share many common characteristics. This is because they all form from similar bits of metal and rock in inner protoplanetary disks. As this debris collided, it stuck together, forming a celestial tumbleweed. When it became large enough, its own gravity was able to pull it into a spherical shape.

The core of this planetary sphere contains heavy elements, typically iron. The middle mantle layer will consist of rock, and there will be a thin crust layer on the surface. Terrestrial surfaces often have varied terrain that has been shaped by asteroid impacts, earthquakes, volcanoes, and/or weather. In comparison to the gas giants, the terrestrial planets are all relatively small. In addition, they have a defined surface, but do not have rings. They also do not have numerous moons, as are commonly found around the gas giants.

Terrestrial Planets in Our Solar System

In our solar system, there are eight planets. Half of these are terrestrial planets that have both very similar and very different characteristics. In order of proximity to the sun, they are Mercury, Venus, Earth, and Mars.

Mercury is the smallest planet and is only about the size of our moon. It has very little atmosphere, which causes it to be struck quite often by asteroids. A day on Mercury lasts for 58 Earth days, and a year lasts for 88 Earth days. The lack of an atmosphere combined with the long day leads to dramatic temperature shifts.

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