Using “earth”

In nontechnical contexts the word “earth,” in the sense of our planet, is usually lowercased when preceded by the or in such idioms as “down to earth” or “move heaven and earth.” When used as the proper name of our planet, especially in context with other planets, it is capitalized, and “the” is usually omitted. (chicago 16, 8.139, p.443)

Some still believe the earth is flat.
The gender accorded to the moon, the sun, and the earth varies in different mythologies.
Where on earth have you been?
The astronauts have returned successfully to Earth.
Does Mars, like Earth, have an atmosphere?