Skyhop Shuiguo
That definition doesn't exclude stories on different planets about different races in some ways. Aliens are still a question of what if, and people on different planets is what if, too.
Yes, but neither is a what if about <i>our</I> world, which is the definition you originally gave. Furthermore, fantasy can <i>also</I> pose what ifs about our world, as in urban fantasy and magic realism, and it can posit alien lifeforms on alien planets. They may not be called "aliens" nor "planets," but that's semantics, not a fundamental difference in intent.
Skyhop Shuiguo
We can't fly to any other planet and have people living on it yet, not even in this solar system. The universe that is used in science fiction stories is the same universe, more often than not.
Yes, and? It could as easily be argued that the universe used in fantasy stories is the same universe. It's the worlds that are different. Sometimes that matters, sometimes it doesn't, in both sf <i>and</i> fantasy.
Skyhop Shuiguo
They still answer questions, for example "what if there was life on other planets?" or "what would life be like on other planets?" Straight fantasy doesn't do this.
Exactly what is your definition of "straight fantasy?" Do you have a corollary definition for "straight sf?" Fantasy asks questions about other lives on other worlds. It asks questions about <i>this</i> life on <I>this</i> world. As I said before, the key difference is that the power that makes the "what if" possible in sf is science, and the power that makes the "what if" possible in fantasy is magic. The questions themselves remain the same. What if we could fly? What if we were immortal? What if we could bring people back to life? What if we could duplicate ourselves in some fashion (cloning or golems or doppelganger)? And, of course, the <I>big</i> question that so much fantasy and sf both address: <I>Should</I> we do a thing simply because we <I>can</I> do a thing? And what are the consequences of doing it?
Skyhop Shuiguo
There is a difference in the writing styles, I agree. Dune is a great example of this. It is pure science fiction, there are no elements of fantasy. But it's written like a fantasy novel.
It's not a difference in style, it's a difference in language, in semantics. Yes, there are styles that are "traditional" to both sf and fantasy, but none of those styles are essential to the genre itself. What is essential is the genre vocabulary, wizards versus scientists versus warp drive versus dragons. Often, it's not only the questions that overlap, but the concepts. They're simply expressed differently.