Is it a plane ? No, it's a... uhh... orbital anisotropy, or something

We haven't heard from satellite planes from a while, so let's re-open that old wound and fill it with hornets. 

My stance on satellite planes calcified quite some time ago. Claims that satellite galaxies orbit their parents in narrow planes are, in my view, mainly due to an over-reliance on numerical estimates of statistical significance and an apparent disdain for actually looking at the bloody data. Most such structures don't look like anything very much at all to me, let alone the insurmountable challenge to the standard models of cosmology they're supposed to be. See the previous link for a much more in-depth and angry examination of this.

The major exception is the plane around our own Milky Way galaxy. This one is visually unmistakable, and regardless of whether such planes are indeed common or not, this surely deserves an explanation. Of course, it's possible our own galaxy just happens to be unusual, but this isn't very satisfying.

This paper takes another look and says there's nothing remarkable about this at all.

Frankly, I am not sure I either agree with or fully understand their arguments here. They examine the plane only using the 11 brightest "classical" satellites, noting that the anisotropy is dominated by just one or two objects. Move them around a bit and actually the "plane" becomes nothing very remarkable.

Hmm... maybe. It would have been nice to see an example, but more fundamentally, they don't seem to consider that basically all the more recent satellite galaxies also follow the plane. I think it's unfair for people to add other features like stellar streams as independent evidence of the plane (because if the galaxies are in a plane, then all features resulting from their interactions must be there too), but ignoring the other galaxies seems strange. To be fair, this seems motivated by previous studies which also limited themselves to this subset.

Their other claims feel on firmer (or at least clearer) ground. Earlier studies couldn't reproduce the planes, they say, because of numerical limitations in their simulations. Apparently some satellite galaxies are disrupted through purely numerical effects (though I am not sure what exactly), but they are able to account for this and reproduce systems more closely resembling the Milky Way.

What seems to me their strongest argument is new data from Gaia for proper motions of the galaxies. If the plane was indeed long-lived and stable, as claimed, we'd expect there to be little velocity dispersion perpendicular to its axis of rotation : in fact, they say, the dispersion both parallel and perpendicular is about the same. That's not at all consistent with rotation, and indicates the plane is just a chance alignment. So while people have generally concentrated on what fraction of galaxies in simulations show planes at any one moment, it might be interesting to consider what fraction ever show a plane at any point in their history.

All this is nice, but I can't help looking at the structure and still thinking, "really ? that's normal, is it ?" Just as it would take something dramatic to convince me of the counter-claim that "planes are everywhere !", so it's going to take me a bit more than this present work to convince me that "the Milky Way's plane is nothing special."