[adrian_b]

As I have already written in another comment, only if we would see changes in the orbits of the known bodies, then that would be evidence for a currently existing outer planet.

The parent article contains several sentences like "The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction."

All those sentences do not support the existence of an outer planet now, they only demonstrate that at some moment in the past there was a big body in that direction.

The papers that I have linked report the results for the simulation of the close passage of a star in the past, which match pretty well what we see now in the outer Solar System.

Such close encounters between stars are known to happen from time to time, because superposed on the general rotation around the galaxy center all stars have random own motions, so the distances between them are changing all the time and even collisions are possible.

[jajko]

We detect planets elsewhere by either them passing in front of the star or star wobbling IIRC. How come we can't detect this hypothetical big outer body by Sun wobbling a bit? We are pretty close to see minute changes. If its there it must have some effect, no?

[kristianc]

My understanding is that radial velocity detection only works when you’re watching the entire system from afar. Since Earth is part of the solar system, we’re inside the moving frame. We can’t measure the Sun’s wobble relative to the solar system barycentre without comparing it to some external fixed reference.

[ddahlen]

2 big reasons, first is that wobbles which we normally observe require that the star move enough to be detected on a shorter time scale. IE: if the orbit takes 100 years and we look twice in 5 years, the planet will have only moved 5% of an orbit and the wobble will be near 0. Second is the less mass and further the planet is away, the less noticeable the wobble. Something at 500 au is going to produce no measurable wobble in our lifetime.

[bagels]

There are other ways besides seeing changes in orbits to confirm the existence of a body. Venus, Mars, Jupiter, and Saturn are easily seen with the eye, for instance.

Planet 9 might be confirmed with infrared surveys as a post from last week discussed or some other method.

[adrian_b]

You are right, I was only replying to the parent article, where the incorrect argument was stated, that the orbits pointing to an external attractor mean that it exists now in that direction.

There may be one or more big planets at great distances from the Sun, but not for the reason stated in the parent article, which is better explained by an ancient star flyby.