I went to a talk at the air and space Smithsonian museum in DC where Mike Brown talked about his hypothesis that “Planet X” is out there. Hes done some brilliant math and research and can show that a planet should be there, he just has to find it.
This was years ago and I haven’t kept up with it but I know he was trying to get time on the Subaru telescope in Hawaii to try and find it.
This sounds like very similar hypothesis-es? (Hypothesii ?)
https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/
I was thinking about planet X, too! If it exists, maybe it’s a captured exoplanet. That would be pretty cool.
Wait wait wait, wasn’t it called Planet Nine?
I always heard it as Planet X. But someone else comments below it is now more common to refer to it as planet nine.
Though OPs article is suggesting there could be many such planets.
I also thought they were speculating that Planet X was huge. Though I can’t remember anymore it’s been too long since I followed this story.