eli5: Is the possible existence of a 9th planet actually back on the table now? If so, what did we find out?

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I’ve heard about this a few times recently, at first I chalked it up to the old, but viral “Planet X or Planet 9 discovered” hoax that’s been made on multiple occasions, but there’s a variety of recent articles pointing to the possibility. I am aware that such news brings a lot of trafic (so money) to news outlets, so I was wondering if this was simple sensationalist news or something we’re actually proposing again?

I definitely remember learning about how unlikely it was for a **real** planet beyond the orbit of Neptune to exist. (Fairly) Modern calculations and observations were also pointing towards it not even being a possibility.

So what changed in the last year? What have we learnt, observed, discovered, calculated, etc.? Is this misinformation or an actual old scientific hypothesis that’s being reinvestigated again in light of recent and new data?

(Possibly used the wrong flair, but it kinda made sense.)

In: Physics