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Chapter 4: Earth as a Planet

Difficulty Level: At Grade Created by: CK-12
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Is Earth a planet?

Yes! No one doubts that Earth is a planet, even though it’s a lot different from the other planets in our solar system, like enormous Jupiter or scorching-hot Venus. The most different planet of all, though, was the planet that is no longer a planet, Pluto. Pluto is now categorized as a dwarf planet, more akin to Ceres and Makemake than Earth or Uranus. Changing Pluto’s status forced scientists to confront the issue of what a planet is. We will look at some of the characteristics that make Earth a bona fide planet.

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Chapter Summary


A planet must (1) orbit a star, (2) have enough mass to be nearly spherical, and (3) have cleared the area around its orbit of smaller objects. Earth is and does all these things! The planet rotates on its axis, so that one half is always facing the Sun and another half is always facing away from the Sun. This rotation creates the day-night cycle. Earth's axis of rotation is tilted relative to its plane of orbit, which creates the seasons. Like other planets, Earth also revolves around the Sun. Earth's trip takes a 365 day year. Earth has a magnetic field, due to convection in its liquid metal outer core. Besides an iron and nickel metal core, the planet has a mantle made of dense rock and a crust made of lighter rock, mostly mafic rock makes up the seafloor and a variety of rocks that have a less dense composition overall make up the crust. The crust and uppermost mantle make up the brittle lithosphere, which rides on the ductile asthenosphere, which is made up of the upper mantle below the lithosphere.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Jan 21, 2016
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