This is an image of the Earth.
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NASA
The Cooling of Earth
Planets cool off for many different reasons.
When Earth formed about 4 Billion Years ago, it became relatively large and warm. Earth also contains many warming elements such as the radioactive minerals, so it has not cooled as rapidly as some other planets.
Earth has had a long a complicated cooling history. An understanding of the Earth's history might begin with the record of geologic time.
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