Well, as all the other planets have ancient god’s names, where did our own name come from?
Earth. It’s what you’re standing on. Get it?
I believe this is simply the correct answer. The English word earth comes from Old English eorthe, from Germanic *ertho, from the Proto-Indo-European root **er-*, meaning ‘earth, ground.’ That’s all. I mean, this is one very fundamental fact of human existence, to always have earth under our feet.
To the Greeks, the earth was a goddess named Gaia. To the Romans, a goddess named Tellus. To the ancient Germans, a goddess named Nerthus.
Remember that to the ancients the earth was not a planet. It was the ground that we stand on. The womb of the Mother from which life emerges and to which it returns. Not one of those little lights in the sky, those are obviously something really different.
It comes from the Quenya word “arda”. 
And in the Norse pantheon she was Jord (also Scandinavian for Earth and earth), and the mother of Thor.
Borquon VII was already taken.
As a side question, when was it agreed by everyone to call it Earth, as opposed to Gaia, Terra, or something else?
I thought that a Latin name ending in “-us” was male? e.g. - “Julius” -> male; “Julia” -> female.
Not everybody calls it Earth, only anglophones do.
There are some fourth-declension feminine nouns that end in “-us”, such as “manus” and “quercus”.
I looked in Cassell’s Latin Dictionary:
tellus -uris, f. the earth.
Personif. Tellus, the Earth, as the all-nourishing goddess, Cic., Hor., Liv.
The genitive form telluris is cited after the headword because it is the key to identifying the declension. I did not find this declension identified in A Student’s Latin Grammar, but the dictionary says it’s feminine.