The Theoretical Ninth Planet
As far as fact is concerned, Pluto is a dwarf planet, not a planet proper - a distinction which I think matters not at all from a storytelling standpoint.
But some Astronomers cautiously theorize that they have found what seems to be the evidence of one more planet orbiting our sun. This planet would have to be 10 times larger than Earth at least, coming 30 billion kilometers from the sun at its closest approach, and would have begun its life among the gas giants. Its orbit would take about 20,000 years to complete, and it would have to be fairly cool to have avoided detection, so not a hot giant like Jupiter or Saturn.
Whether it truly exists or not is of little importance to us, for it is currently being taken seriously, and, I argue, that allows it inclusion into the mythologized solar family.
"But Dylan," you say, "Even if they found the ninth planet, it would be a recent discovery and therefore have no place in the OSS."
And you're right. The OSS exists exclusively in the science of the past. But need it be that way? After all, it is not chronology that makes the OSS great, it is the spirit of it. I would argue that the three most important aspects of OSS fiction are the planet's individual characters, the presence of alien life and the relative ease of space travel. There is no reason why we cannot keep those things and incorporate new discoveries where they suit us.
Such thinking allows for stories about Martians building canals to Valles Marineris or a lost city in the volcanoes of Io. By simply ignoring the discoveries which inhibit the genre, we can use modern astronomy to strengthen the characters of the worlds, not destroy them. The OSS can live on. By adapting the aspects of reality we like to suit our needs, we can keep it alive.
And so that brings me to my point. The OSS authors didn't create everything themselves, they drew from the established characters of the planets, based on the scientific theories of the day. If what we know about planet nine were known back then, what would have been its character? What would they have called it? What might life have been like there, and what sort of stories would they tell?