"Earth" can be both a common noun and a proper noun, depending on the context.

Common Noun:

* When referring to the planet itself, it is a common noun.

* Example: "The Earth revolves around the sun."

Proper Noun:

* When referring specifically to our planet, it is a proper noun, and should be capitalized.

* Example: "Life on Earth is unique."

Therefore, the noun "earth" can be both a common and a proper noun, depending on the context and meaning.