Besides, this is a hypothesis of a planet explaining observations, a real thing.

In that parable, we do know enough of what the missing “dog” looks like and where it’s at. That’s why astronomers have the idea that they can search for it (not too faint, not “all sky” search space).

Or, are you proposing a single event perhaps?

That wouldn’t work, the detached objects are in resonance so need a periodic perturber – an orbiting planet.

That – using science – is of course what they are doing.

“Several possible origins for Planet Nine have been examined including its ejection from the neighborhood of the known giant planets, capture from another star, and in situ formation.” [ https://en.wikipedia.org/wiki/Planet_Nine#Origin ]

If you read further in that article your idea is not listed. Either they missed it, it has been rejected by the many papers studying this – there needs to be a capture mechanism – or it has simply not been published. If you quantify your model you could get it published – you know, as in doing science.

The “passing stars” are a likely scenario when the systems are still close together in their origin cluster before they disperse as Sun and its siblings must have done. The typical distances are parts of light years or an order of magnitude less than typical for the Milky Way disk.

The scenario fit Planet Nine well, since our scattering happened early [ https://www.sciencemag.org/news/2020/01/cataclysmic-bashing-giant-planets-occurred-early-our-solar-systems-history ].

“The hunt is on for more observations that can parse what happened during those first 100 million years, whether from asteroid samples, clusters of primordial asteroid families, or craters on the Moon and Mars. “Now, the question is, was it a few million years after or 80 million years?” Morbidelli says. “Honestly we don’t know.””

Best we stop seeking reasons to look aimlessly and start following the evidence where it leads. Y’know, like that scientific method we keep hearing about?!