It's unlikely that any sizeable novel will remain entirely within one genre. Love, horror, fantasy and thriller are all elements that may help the plot along. Usually there will be an overriding theme that defines what genre a story will be, and as Toby Frost points out, the fact its set on another planet with aliens (and also spaceships) clearly puts it into the category of science fiction.
There are however other stories that feature elements of science fiction whose overriding genre arguably lies elsewhere. Some will say that Avatar or Star Wars are fantasy, or science fantasy, and dystopian tales such as 1984 an Farenheit 451, which in many ways are political novels and feature few (if any) technological advancements.
Mary Shelley didn't write Frankenstein as a work of science fiction (even if the genre had existed back then), she wrote it as a horror story. George Orwell did not write 1984 as a work of science fiction; just as with Animal Farm he wrote it as a cautionary tale of what may happen if we continued along the same political and social path.
But science fiction is such a huge umbrella that takes in many stories and aspects that do not sit comfortably elsewhere. Which is why it is one of the most diverse and entertaining of all genres out there. Long may it continue to be so.