Protoplanetary disks — the gas-and-dust structures around young stars where planets form — have been modeled as essentially two-dimensional objects. Thin, flat, dominated by Keplerian rotation. Vertical motions exist in theory but were assumed to be weak perturbations: gentle meridional circulation, small deviations from the midplane. The standard picture is a pancake with ripples.
The exoALMA survey (arXiv:2603.12861, March 2026) measures vertical gas flows in 14 protoplanetary disks using molecular line observations from ALMA. Vertical flows are not perturbations. They are ubiquitous, present in every disk observed, and in two systems — MWC 758 and CQ Tau — the vertical velocities reach 500 meters per second. That is nearly the local sound speed. The gas is not rippling. It is storming.
The vertical flows trace spiral patterns in at least two disks, suggesting they are driven by gravitational perturbations from embedded planets or by gravitational instabilities in the disk itself. The spiral structure means the vertical motion is organized, not turbulent — coherent upwellings and downwellings that span large fractions of the disk.
The implications for planet formation are direct. Dust settling — the process by which solid particles drift to the midplane and accumulate into planetesimals — assumes a quiescent vertical environment. Vertical flows at the sound speed can loft dust particles back into the upper layers of the disk, delaying or disrupting the settling process. The 2D models that predict where and when planets form are missing a dynamical component that operates at the same velocity scale as the processes they model.
The structural lesson: a system's dimensionality is not a property of the system but of the observation. Disks looked 2D because the observations that constrained them — continuum emission, integrated spectral lines — averaged over the vertical direction. ALMA's ability to resolve velocity structure along the line of sight reveals the third dimension. The pancake was always a storm; we were measuring it with a rolling pin.