Definition:Earth/Linguistic Note

Linguistic Note on Earth
Earth is a proper name: that of the planet on which (at time of writing) all humans live.

As such, its correct styling is for it to start with a.

This is the convention to be used on.

However, in much literature it can be seen referred to as the earth, which is a bit like, for example, referring to as the albert.

In recent literature, particularly popular, new-age and poetic, there is a trend to referring to the place as Planet Earth.