New tools for studying planarity in galaxy satellite systems: Milky Way satellite planes are consistent with ΛCDM

We introduce a new concept - termed 'planarity' - which aims to quantify planar structure in galaxy satellite systems without recourse to the number or thickness of planes. We use positions and velocities from the Gaia EDR3 to measure planarity in Milky Way (MW) satellites and the extent to which planes within the MW system are kinematically supported. We show that the position vectors of the MW satellites exhibit strong planarity but the velocity vectors do not, and that kinematic coherence cannot, therefore, be confirmed from current observational data. We then apply our methodology to NEWHORIZON, a high-resolution cosmological simulation, to compare satellite planarity in MW-like galaxies in a Lambda cold dark matter ($\rm {\Lambda CDM}$)-based model to that in the MW satellite data. We demonstrate that kinematically supported planes are common in the simulation and that the observed planarity of MW satellites is not in tension with the standard $\rm {\Lambda CDM}$ paradigm.