The use of のだ/のです at the end of a sentence or phrase functions as a signal to show that a conclusion is being made or a reason for something is being given. When used in a question too, it can be used to ask for a reason or explanation for what a person is doing. For example: どうしてパーティーに行かなかったのですか？　Why didn't you go to the party? 忙しかったのです。 Because I was busy. のだ/のです can also be shortened to んだ/んです. Example: いつきたんですか？　When did you come? (note: in this example んです is being used more for emphasis, which is also a way it can be used regularly) This is the best way I know how to explain it and how I understand it. I hope this clears it up.

A basic sentence should be 火星とよみます not 火星とよむです. 火星とよみます。⇒change ます-form to のだ／のです-form⇒火星とよむのです。 As Lee-san has explained, のだ／のです add some feeling to a sentence, appealing to a listener. In this case, with のです a talker is trying to make a listener realize what he/she doesn't. So, probably before this utterance, somebody misread 火星（かせい）and say like "ひぼし". Then, his teacher made a correction saying （ひぼしではありません。）かせいとよむのです.

Thank you both for your answers. It's clear now.