I don't know what the problem was with the teacher @yung.carl.jung had. With my teacher, it might have been that she was irritated that I used science and space exploration themes in as much of my writing as possible (I was a new Trek fan at the time and was reading as many astronomy books as I could get my hands on... and saw no reason not to include such things in my English assignments as long as the actual requirements of the assignment were met).I've also noticed that some of my English teachers had oddly small vocabularies.
Probably because they hardly used English outside of school. The internet wasn't a thing and they couldn't spend their free time arguing about politics with some native speakers.
But that's still no excuse for not knowing that Earth is a planet that is just one of several others (all of which have names that start with a capital letter).