When Merriam-Webster indicates that a term is “capitalized” or “usually capitalized,” the MLA capitalizes the term in its publications. When Merriam-Webster indicates that a term is “often capitalized,” our practice varies. We usually lowercase sun, moon, and earth, but, following The Chicago Manual of Style, when the does not precede the name of the planet, when earth is not part of an idiomatic expression, or when other planets are mentioned, we capitalize earth:

The earth revolves around the sun.

The astronauts landed on the moon.

The space shuttle will return to Earth next year.

The four planets closest to the sun—Mercury, Venus, Earth, and Mars—compose the inner solar system.