Its name is the Sun

The name of the star that Earth orbits is "the Sun". In particular, its name is not "Sol".

Don't take my word for it, of course. Just look it up in the dictionary.

But they call it "Sol" in a science fiction story I read

That's fiction. If the Sun is called "Sol" in a work of fiction, that just means that's its name within the setting of the story. In the novel 1984, England is called "Airstrip One", but this in no way means that this is really England's name.

Fiction authors are generally well aware of this distinction. The Sun is called "Sol" in Isaac Asimov's novel Foundation, but when not writing fiction, Asimov always referred to it as "the Sun". See, for instance, his textbook The Sun.

But the English language doesn't work like that. There's no such thing as official words for anything. Whatever word English speakers use to describe something is its name.

No, that doesn't apply to proper names. Specific people, places, and things do have official names, which can't be changed by common consensus. If we all started calling you "Orville", would that mean that your name is "Orville"? Of course not.

Also, even if that were the case, nobody actually calls it "Sol" outside of fiction, except science fiction fans in informal settings. In formal nonfiction publications that have undergone editorial review, it's called "the Sun". Some examples:

Astronomers may call it "the Sun" when they're talking to the public, but they call it "Sol" in professional journal articles.

No they do not: example 1, example 2, example 3.

Who has the authority to decide the official name of the Sun?

The International Astronomical Union, an internationally recognized group of professional astronomers. And they call it "the Sun" exclusively (example).

But "sol" is simply the Latin/Spanish/Portuguese word for "sun"

True but that doesn't matter, since I'm talking about the English name for the Sun. Other languages have other names. The Portuguese word for Norway is "Noruega", but that doesn't mean the actual name of Norway is "Noruega" when you're speaking English.

But "solar" means "related to the Sun"

True but that doesn't matter. "Natal" means related to birth, but the word is still "birth", not "nat".

I don't care what's correct. I like how "Sol" sounds, and I'm going to use it, as an affectation

Fine. I don't care. Just don't tell anyone else that its name isn't "the Sun".

See also