Quite often in science fiction we read an Alien character say:

"They're going to the Terran system".

"That's a Terran ship."

"They're from the planet Terra".

I get that Terra is the Latin name for Earth. I'm trying to work out how it became a science fiction idiom for referring to things from Earth, or things near the Earth. (Our solar system being the Terran system.) It feels like a jump that the Aliens would know Latin.

My question is: What is the origin of the word "terrans" when referring to humanoids and other earth-related things?