This Wiktionary article mentions that the term for “planet” is 行星 and that it was “[c]oined by Alexander Wylie and Li Shanlan in 1859,” from characters meaning “travel” and “star,” which makes sense as planets are, to the eye, “traveling stars”—their position changes (slowly) from one night to the next.
I find odd, though, that—if we take the article at face value—there would not have been any word/expression for “planet” before these authors’ work, or even before European contact. Chinese astronomers of long ago saw and reported on their observations of planets—there is, for example, a report of Gan De in the fourth century BCE having observed Jupiter—so they must have used a common word to describe them all, and not just each one by its proper name.
My question is: Was there an earlier word/term/expression that just got “replaced” by 行星? What was it? And how would it translate to English?