> This seems more plausible than an undiscovered big planet.

Both are plausible, both are intriguing. To determine what in fact happened there's no way around looking up and searching until we exhaust the possibilities. Kudos for Terry Phan and his team for putting in the work, regardless of what hypothesis it ends strenghtening.

> This seems more plausible than an undiscovered big planet.

Why?

From the first link:

> The Solar System planets accumulated from a disk of dust and gas that once orbited the Sun. Therefore, the planets move close to their common plane on near-circular orbits. About 3,000 small objects have been observed to orbit the Sun beyond Neptune (rp > 35 au); surprisingly, most move on eccentric and inclined orbits. Therefore, some force must have lifted these trans-Neptunian objects (TNOs) from the disk where they formed and altered their orbits markedly.

I feel there is a strong bias towards objects that are only discoverable because of their highly eccentric orbit

just thinking the same thing "something big out there", and that has ,ha!, huge implications, like is there any model for failed giant planets orbiting as diffuse blobs of stuff, way ,way out there, or several less big blobs that like up and give a good tug once in a while, rings with lumps in the oort cloud? we know there are chunks out there big enough to ruin your whole planet, but that are essentialy invisible from here, so the mass could be there, and could be more organised than we realise, but still realy tricky to see.

No, it's evidence which supports both hypotheses, without ruling out either. More evidence is needed.

This linked paper is a textbook chapter; it's intense.

40 pages of two-column astrophysical journal survey and analysis.

I can see why it must be required reading for a discussion of TNO dynamics.

Wow.

"Planet X" was used before Pluto was discovered. The X didn't mean 10. In fact, Pluto was thought to be Planet X before they realised it was much, much smaller than that.

I can settle for Planet 9 From Outer Space.

Planet X is Eris.

I'll give the website some credit for being perfectly readable and painless to view with javascript disabled.

Sub pixel resolution is possible by observing the space over time

Basically all observations of solar system objects besides the big planets are going to be unresolved point sources. IE: We don't have telescopes which can resolve most objects into an image. An example, Hubble famously had to image a ton to get a grainy, ~12 pixel approximation of Pluto. We can do a lot with measurements of points, even reconstruct 3d models if we have enough data.

Resolving power is related to the PSF (Point Spread Function) size. The PSF is the image on your detector if you have an infinitely small point source. A quick google search says that DECam has a PSF of at least an arcsecond (atmosphere is probably causing issues for that). Which means anything smaller than an arcsecond is going to be unresolvable.

However, you still want pixels smaller than your PSF, since PSFs are typically gaussian-ish, having a bunch of measurements within the gaussian allows you to estimate the center accurately. This is vital for Astrometry (the measurement of position).

I think that's about right, but the real purpose isn't resolving spatial details on the planet (which you correctly found isn't possible), but to find the planet at all—to distinguish a point of light out of background noise. A fine angular resolution is still helpful, for SNR reasons: smaller pixels contain less noise, wheras the signal pixel contains the same amount of signal!

If you jump into it you restart the universe.

I agree. I'm also not sure why this isn't the leading theory of what's causing the gravitational weirdness, yet still manages to evade our detection.

(Disclaimer: I know nothing)

I mean how else would the aliens have gotten here?

I'm not sure it's even the largest. Eris and Sedna are pretty close. But that's not really relevant here.