"This planet is small."
Translation:כוכב הלכת הזה הוא קטן.
פלנטה should be accepted too. It is wildly common to call it that way in Israel
Without any context, there’s no way to know if כוכב means “star” or “planet”. כוכב הלכת means only “planet”, so there’s no misunderstanding what is being said.
I'm just guessing, but I think since the root of לכת is "to go"-- לכת is "going"-- that it distinguishes a planet from a star because the planets change their position in the sky with respect to constellations, so it's like stars don't move but planets are "going"