As promised last week, my review on the Planes of Satellite Galaxies Problem is now out on the arXiv: arxiv.org/abs/1802.02579 “The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions" 
Some time ago I was invited by MPLA to write a brief review on the Planes of Satellite Galaxies issue. Since there wasn't any review on the topic out there yet, and because people had been asking me frequently for a good introduction to the topic, I happily accepted.
Instead of repeating the review's content here, let me just give you a summary of what you can find in it. I was asked to write a "brief review" of typically 15 pages. What I came up with is not quite as brief, but could well have been three times as long.
My intention was to provide an introduction into the topic, by focussing on three aspects: The observational evidence for satellite planes, how they compare to cosmological simulations, and what origins/solutions have been suggested. This splits the review into three parts.
In part 1, I discuss the Vast Polar Structures (VPOS) of the Milky Way, the Great Planes of Andromeda (GPoA), and the "new" Centaurus A Satellite Plane (CASP), as well as some other possibly related structures in the Local Group and beyond. 
Part 2 focusses on how rare analogs to the observed satellite planes are in cosmological simulations, which is what makes this a problem. The plots for the VPOS, GPoA & CASP show a measure of flattening on the horizontal and a measure of kinematic coherence on the vertical axis. 
Part 3 discusses suggested solutions … and why don't work (yet?). This includes ideas compatible with the standard-cosmological model (accretion of satellites along filaments or as part of compact group; hydrodynamical simulations are discussed as well) … 
… but also the maybe more "exotic" idea that the satellites were formed as tidal dwarf galaxies in a galaxy collision, why this might be inconsistent with some constraints, and how one can try to find a way out (e.g. via MOND).
So, all in all I hope some people will find this interesting, and the plots potentially useful to illustrate the problem in talks, even though the review only scratches the surface of this – nowadays pretty active – research topic.
Many interesting papers on the topic unfortunately didn't make it into this text, so I strongly encourage everyone to look into the original literature for more detailed information and further references.
(Side note: when I started to work on the weird polar alignment of the Milky Way satellite galaxies as a PhD student eight years ago, I would not have dreamt that the topic would become this prominent. Feels a bit surreal to look back at how things have evolved over that time.)
