Basically what the title asks: is it possible to write non-canon works in a fictional universe you've created? In my example, I've created a fictional universe set on a non-Earth planet, however I have written a novel in which the characters on the non-Earth planet have traveled to Earth. Is it possible to make said novel non-canon, as in it has no bearing on the actual plotlines of the original novel(s)? Furthermore, is it possible for an author of significant works to publish a story or book that is non-canon to the other works they've created?
Yes, it is indeed possible, especially because it is your own creation.
Whatever you do to your characters, and whether you decree it canon or not, is up to you as an author. Such a non-canon story would count as a "side story" or some sort of bonus. If someone else wrote that non-canon piece, people would call it "fanfiction." You're basically just writing a fanfiction of your own work. This works well especially because you know your characters better than anyone else.
Just be sure to specify it as non-canon to not confuse your readers and keep them on track with your actual storyline.
All the best!
Some science fiction writers have done things similar in various series.
A.E. Van Vogt wrote several stories about the exploration voyage of a space ship called the Space Beagle. He put them together into a book called The Voyage of the Space Beagle in 1950. Anew protagonist was added and the stories were rewritten. For example, Coeurl from "Black Destroyer" has a somewhat different shape in the novel and needs a different substance to survive. In "Black Destroyer" Coeurl is supposed to have been a native of the fallen civilization on his planet, while in the novel he is speculated to be a member of an artifical species created by the civiliation.
So are the individual stories in magazines and anthologies canon with The Voyage of the Space Beagle, or is the novel sort of an alternate universe version of the stories?
James Blish did the same with some of his stories.
His 1942 story "Sunken Universe" was heavily revised and published again as "Surface Tension" in 1952. Are the two versions of the story canon with each other or in alternate universe. Blish wrote other stories in the "Pantropy" series and they were republished, probably with more rewrites, as The Seedling Stars (1956).
Another famous Blish series was his "Okie" stories of New York City and Mayor John Amalfi, which were put together in a novel Earthman, Come Home. Blish wrote a number of other stories and novels in that series which were all published together as Cities in Flight (1970). And I am not certain whether the original short stories and their later novel form are totally canonical with each other.
I note that my paperback copy of Earthman, Come Home omitted the first story, "OKie", which I later read. I noticed that several paragraphs of exposition from "Okie" were added to the beginning of "Brindlestiff", the first story in the abridged edition.
Another famous Blish story was "A Case of Conscience" which was expanded into the novel A Case of Conscience. It is usually considered to be part of a series After Such Knowledge with the historical novel Doctor Mirabilis and the fantasy novels Black Easter and The Day After Judgement.
And I can think of at least a couple of other series by Blish. But Blish often used technology such as the standing wave space drive or the Dirac transmitter, and characters such as the great scientist Dolph Haertel, in different stories. So it is hard to tell which of Blish's stories and series are canonical with each other and whch are not.
And when Blish wrote adaptations of Star Trek episodes, he mentioned some elements from his own stories in them, Thus leading to the question of how canonical Blish's Star Trek adaptations are with those other Blish stories on one hand, and with the Star Trek shows on the other hand.
Late in his life, Isaac Asimov decided to unite most of his most famous stories including the "Robot" short storie & novels, the "Galactic Empire" novels, and the "Foundation" series, into one larger super series, writing new novels to tie them together.
But Asimov's classic story "Blind Alley" is set in "The Galactic Empire", but it has never been proved, as far as I know, whether "Blind Alley" is part of Asimov's main series or is a completely separate story in a fictional unvierse of its own.
J.R.R. Tolkien published The Hobbit and The Lord of the Rings in his own lifetime. Each is totally canon with itself, and they are mostly, but not entirely, consistent and canonical with each other. Tolkien worked on a project to rewrite The Hobbit to be more consistent with The Lord of the Rings but abandoned it.
So does the present version of The Hobbit happen in the same fictional universe as The Lord of the Rings or in some sort of alternate universe?
Of course Tolkien's greatest work was his stories of the First Age of Middle-earth. Tolkien wrote and rewrote them over and over again during his life, trying to get them ready to be published. After his death, they were published, finished or not. in fact, the History of Middle-earth includes volume after volume of unfinished stories, and first, second, and third drafts of published stories, unpublished essays by Tolkien about Middle-earth, etc. And there are also The Letters of J.R.R. Tolkien(1981) in which he discusses many aspects of Middle-earth.
So there are literally hundreds of more or less complete versions of various stories, and also essays, and also letters discussing Middle-Earth. So the question of which of them are canonical with The Lord of the Rings, which are canonical with The Hobbit, which are canonical with the Silmarillion, etc., and which are canonical with eothers, is incredibly complicated.
So it seems to me that if a writer writes a story in their fictional universe and explictly states whether or not it is in the same continuity as, or an alternate universe to, other storis set in that universe, they will be clear enough. And they will be much kinder to their readers, and cause them much less confusion, than some writers have been to me.
Certainly it is possible, it’s your universe, you can do whatever you.
In the http://www.faithhunter.net/wp/books/jane-yellowrock-series/easy-pickings/, crossover novella Jane is given an item that is somewhat referenced in a later book in her series. Making it semi-canonical (while readers of this crossover will recognize it, it could easily be reconed into something else).
Lots of popular series have such stories and references in them. They can add a bit to the universe without pulling it off track.
Larry Niven created Down in Flames", in which he destroyes his own "Know Space" universe. It is intentionally consistent with previously published works in that setting, but thoroughly inconsistent with Ringworld in the version published in N-Space (see the ISFDB liating but made partly consistent with Ringworld in the online version linked above.
David Weber wrote and posted "How the Safehold Series Won't End" which he described as 'AS joke post" in which he solved the plot problems of one series by bringing in fleets from another, quite incompatible series.
Other authors have done similar "self-fan-fiction". Those happen to be two I know of and can link to.
The barrier to making anything "non-canon" would seem to be your idea of what "non-canon" means, and how that might matter.
Of course it's possible - easy, even - to write "non-canon" works in your own or anyone else's universe, so long as your use of "non-canon" matches everyone else's… which would be more likely if you'd said "Non canonical"…
Plotlines might matter if you could explain how. Until then, plotlines are tiny details which take place within worlds or universes. If you're suggesting plotlines change or in any way influence the background against which they're set, how does that happen?
How could it not be possible for any author to publish anything that "non-canon"?