In this instance, capitalization depends upon which definition of the word you intend. When you’re talking about the planet we live on, capitalize the word. Here’s why: Used in this capacity, Earth is a proper noun. It names a specific place. Proper nouns should be capitalized.

However, there are other uses of the word that don’t require capitalization. It’s used to describe land:

She stepped from the wooden deck and the rocky earth was cool on the soles of her feet.

While she’s on our planet Earth, the meaning of the word here refers not to the planet itself, but to the soil or dirt on the ground and, as a result, should not be capitalized.

Earth is also sometimes used as an intensive and should not be capitalized:

Gloria wanted to know what on earth was going on.