They are using the word star here (星) and not planet (惑星) while they share the same kanji, they are written different, dunno why they are doing it like that because it makes no sense.
It's not the word "star", it's the generic word for "celestial body" (eg planets, stars, comets, black hole) that doesn't exists in English and always gets defaulted to "star" by MTLs or translators unfamiliar with the word. It's been an issue in jp-to-en translation since forever.
As an example, Junji Ito's 地獄星レミナ is TLd as "HellSTAR Remina" despite Remina being a planet.
In other languages that actually have that word, eg "astre" in French, we get the generic word instead and deduce from context if it's planet, star, or something else. Eg, "Rémina, la planète de l'enfer" (hell PLANET) is the french title.