Planes of satellites, at once transient and persistent

The appearance of highly anisotropic planes of satellites around the Milky Way and other galaxies was long considered a challenge to the standard cosmological model. Recent simulations have shown such planes to be common, but they have been described as either "transient", short-lived alignments, or "persistent", long-lived structures. Here we analyse Milky Way analogue systems in the cosmological simulation TNG-50 to resolve this apparent contradiction. We show that, as the satellite populations of individual hosts rapidly change, the observed anisotropies of their satellite systems are invariably short-lived, with lifetimes of no more than a few hundred million years. However, when the progenitors of the same satellites are traced backwards, we find examples where those identified to form a plane at the present day have retained spatial coherence over several Gyr. The two ostensibly conflicting predictions for the lifetimes of satellite planes can be reconciled as two perspectives on the same phenomenon.