For our purposes, "prison planet" may be defined as follows:
"An entire planet, or at least the vast majority of its surface area, has been classified as a good place to dump convicted criminals so that they will no longer be making any trouble for the zillions of law-abiding citizens who are located far, far away, on other worlds."
I've seen this concept used again and again, but I have no idea which clever writer first came up with the basic idea that someday, when a spacefaring civilization has multiple Earth-like planets within its grasp, it may decide it is cost-effective to turn one world into a huge prison colony and thereby keep all the criminal misfits there, conveniently out of sight and out of mind.
There are variations, of course. For instance, some "prison planets" are treated as the larger equivalent of modern prisons, where you can actually serve out the number of years the judge sentenced you to, and then you are offered a lift back home to "rejoin society" (if you're still alive, of course). In other cases, being sentenced to a prison planet is expected to be a one-way trip -- if convicted, you are landed there via shuttle, but you are never supposed to leave the planet's surface again. (Although a clever protagonist can usually find a way.)
Sometimes the prisoners are essentially treated as slave labor, expected to work hard in the mines (or do other difficult and/or dangerous tasks) if they want to be housed and fed . . . but on other occasions, the authorities just turn you loose in the woods and then let you fend for yourself, sink or swim!
And sometimes the "prison warden and his guards" (or whatever those officials are called) maintain a secure base there on the planet -- inside a walled city, or on one remote island, for instance -- while in other cases they don't bother; they either live up in orbit (on a space station or a moon, perhaps), or else it turns out that no "representatives of government" bother to reside anywhere in that solar system! Transport ships just drop off fresh loads of prisoners at regular intervals and then head back home. (That last approach makes a bit more sense if no other spacefaring civilizations are known to exist, anywhere in the galaxy, and thus nobody back at headquarters is terribly worried about alien conquerors discovering, and then interrogating or recruiting, the prisoner population.)
All of the above variations can qualify as "prison planets" for my purposes. For that matter, since I'm asking where the concept began, I could even accept an old story which just had a few lines of dialogue referring to the existence of a prison planet, even if no scenes were actually set on its surface.
On the other hand, I did come up with some homebrewed rules about what I don't see as real contenders for the proud distinction of "this was the first prison planet to be mentioned in a science fiction story."
What I Don't Consider Eligible:
Historically, fiction has been full of stories about people who accidentally get stuck on some remote island, with no way to sail back home to "civilization" any time soon, nor to communicate their plight to people who might rescue them. Classic examples have included Robinson Crusoe, Swiss Family Robinson, and Gilligan's Island.
But none of those characters had been dumped on those islands by government order after being convicted of felonies back in their homelands; they just suffered from sheer bad luck. So if you happen to remember any science fiction examples which used very similar premises to get the plot rolling, but had the "castaways" stranded on an unexplored planet light-years away from here, that would likewise not qualify as "this planet was being used as a prison."
The difference between being a "Castaway" and being "Marooned" is that someone did, in fact, make a conscious decision to leave you all alone, presumably until you died, on some obscure little island. And they followed through on that decision! But the people making that decision were not a duly constituted court of law representing a national (or global, or interstellar) government.
I first became aware of the concept of "marooned" when, as a kid, I read Robert Louis Stevenson's Treasure Island. Ben Gunn had been marooned by his fellow pirates years earlier, and was very grateful to have a chance to hitch a ride back home. A similar event is part of the backstory of Captain Jack Sparrow, as established in dialogue in Pirates of the Caribbean: The Curse of the Black Pearl.
I'm sure there have been parallel situations in science fiction which involved someone deliberately leaving a man on a wilderness planet, with no plans to come back for him any time soon (if ever!) -- but I'm not interested in those unless this sentence was pronounced upon him by something I would call "a proper court of law representing the government of a large civilization." (Nine people on an exploration vessel, voting to dump the tenth guy on the next world they discover which has a breathable atmosphere, does not qualify as "a large civilization and a proper court of law.")
3. Extreme Isolationists.
In other words, the first generation of pioneers were people who chose to land on this planet to make new homes for themselves, and they (or their descendants, a few generations later) also chose to stay as isolated as possible from outside influences. Ideally, one hundred percent isolated. One way to do this is to ensure that nobody back on Earth has a clue where this ship full of colonists was headed for!
In this instance, the profound lack of immigration and emigration, to and from the planet's surface, is not because some faraway "interstellar government" is forcing this isolation upon the local residents. On the contrary, it's something the local residents are enthusiastically working to maintain no matter how anyone else feels about it.
(I have read several science fiction stories in which the main character is basically a spy assigned to infiltrate such a world in an attempt to find out just what the heck is really going on down there in this very isolationist culture.)
I've seen such concepts as: "This inhabited world has been placed 'Off Limits' to all space travel from other places. A military blockade has been established in orbit to enforce this edict." The quarantine might be because the world in question had been overcome by some incurable, fatal, and highly contagious disease . . . or by fast-replicating nanotech which no one could control . . . or the world might be inhabited by people with frightening psychic abilities . . . or the "quarantine" might be a way of punishing the people of that planet for something outrageous which their government had done (such as sterilizing another planet in a recent war) . . . but none of the above would be the same thing as "the first generation of human settlers on this planet were transported here because they'd previously been convicted of serious crimes -- and now it is a prison planet for them and their descendants."
5. Lost Colony.
Another popular trope in science fiction is to have someone in a starship stumble across a long-lost colony, settled by human pioneers centuries earlier, which everyone else has subsequently lost track of! For instance, if a nuclear war devastated Earth, it might be that no one was left who had accurate records of just where a certain colony ship had been headed for when it left Sol System.
But just because a planetary population has been "cut off from contact with everybody else" for a long time is not the same thing as "they are convicted felons on a prison planet."
6. "The Prime Directive Says We Can't Interfere!"
Yes, I swiped that phrase from Star Trek. But I've seen the idea used elsewhere. A human survey vessel finds a pre-spaceflight culture inhabiting a planet . . . studies it from orbit (possibly not even landing to take samples) . . . and, as a matter of government policy, the world is left strictly alone to "find its own destiny" (or whatever catchphrases the more advanced culture prefers to use). The "primitive world" in question may also be a Lost Colony of the same species, or it may be inhabited by an indigenous alien race, but either way, nobody is treating this world as a "prison."
So, with those limits in mind, what do you think was the first published science fiction story to clearly present the concept of "they turned a planet into a prison planet"? Who wrote it and when was it published?
Note: It doesn't have to be a piece of prose fiction. If you think "a prison planet" was first mentioned in a newspaper comic strip, or an old movie, or a science fiction story told in some other medium of expression, then please say so!