"There are billions of us on earth."
Translation:Nous sommes des milliards sur terre.
From what I can see, it is simply the way it is expressed unless words such as "the" or "this" are specified.
Read the third entry here: http://www.larousse.fr/dictionnaires/anglais-francais/earth/577549 & here: http://www.larousse.fr/dictionnaires/anglais-francais/earth/577549
It's just the way "on earth" is expressed.
When "Terre" is used as a proper noun it should be capitalized, as it should be here Duo! There is also the phrase "ciel et terre" which suggests the older sense. We wouldn't, of course, capitalize Sky which is silly, though it might be closer to Heaven. Basically I was taught we should distinguish Earth the planet as we know it in the Space Age from earth the physical/material/experiential realm as it used to be thought of. It's a rule more observed in its breaking, unfortunately . . .