"It is not a star but a planet."
Translation:O bir yıldız değildir ama bir gezegendir.
You can shorten this sentence down to: "Yıldız değil ama bir gezegen." The "-dir" at the end of a noun-sentence is mostly omitted if it is used to give a "certainty" to the sentence. It also can give a probability to a noun or verb sentence. In this situation, it is not omitted most of the time. "Şimdiye kadar eve gelmiştir." -->"(I suppose) she/he should have come home until now." Or "Annem evdedir." --> "(I suppose) my mother is at home." You can tell them from each other depending on the context.