Was there someone who decided, 'this is Earth now'? Was it called something else before? Do we have no idea?
No source I've checked seem to state exactly how the word was derived, but we know the word "earth" is at least 1000 years old. The name "Earth" was inspired by two languages: Old English and German.
Source: NASA
Well, "this" was called the earth before we knew it was a planet. We used the term to separate the world around us from the heavens. In that sense, it descends from Proto-Germanic ("ertho") - a really old language, starting 500BC according to wikipedia.
We only recognized earth as a planet much later and that wasn't done by English speaking people, so "earth" would have been translated from other languages. That's why many languages use an equivalent of "dirt" to name the planet.
See also:
(https://www.reddit.com/r/AskHistorians/comments/15qbxv/why_is_the_earth_called_earth/)