The word “earth” is treated as a proper noun only when it refers to the celestial body (e.g., “The Earth is the third planet from the sun”).
“Earth” is typically treated as a common noun (i.e., lowercase) when it denotes the surface of the planet, the sum of its inhabitants, the realm of the living, or the dry land as opposed to water or sky.
The earth has been nicknamed “the Blue Planet.”
The Earth has been nicknamed “the Blue Planet.”
You’re my favorite person on Earth.
You’re my favorite person on earth.