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

Question

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

Answer 1

“Planet Nine” is a hypothetical planet proposed to exist by legit astronomers Mike Brown and Konstantin Batygin to explain the way the orbits of certain transneptunian objects appear to be arranged. You can see conferences and presentations of them on YouTube, they’re very good at stating their case. However, until they find it (and they’re searching for it, but it’s not an easy task at all), it’s just a hypothesis. Nothing’s changed in the last year with regards to their search.

“Planet X” was a planet proposed to exist by astronomer Percival Lowell at the beginning of the 20th century, to explain certain perturbations believed to be observed in the orbit of Neptune. The search for Planet X eventually led to the discovery of Pluto, but none of Lowell’s predictions turned out to be true (basically, Pluto isn’t nearly massive enough to perturb Neptune’s orbit). The “perturbations” in the orbit of Neptune turned out to be just innacurate observations from back in the day. When the Voyager spacecraft flew by Neptune and we got more accurate data, the “perturbations” vanished. So it’s safe to say that Planet X never existed and that the discovery of Pluto was just a lucky coincidence.

Any news in he media (and specially if it’s not a specialized and reputable source, like, say “Sky & Telescope” or “Nature”) regarding this subjects is probably just click-baity material you shouldn’t waste your time on.