Protoplanetary disks are imaged in dust continuum emission, which reveals rings, gaps, and spirals — structures that may be carved by forming planets. But dust structures can also arise from other mechanisms: snowlines, magnetically driven instabilities, or pressure bumps. Distinguishing a gap carved by a planet from a gap carved by physics is the central challenge.
Izquierdo et al. (arXiv:2603.13157) demonstrate tomographic detection of embedded planets using molecular line observations with ALMA. Instead of looking at dust patterns, they analyze the velocity field of the gas — how fast the gas moves at different positions and heights in the disk. A planet perturbs the gas velocity in a characteristic pattern: deviations from Keplerian rotation that depend on the planet's mass and position, visible as velocity kinks and line broadening in the molecular emission.
The tomography works because different molecular lines trace different heights in the disk. CO isotopologues (¹²CO, ¹³CO, C¹⁸O) become optically thick at different altitudes, providing a vertical stack of velocity measurements. A planet embedded in the midplane produces perturbations that propagate upward through the disk atmosphere, and the pattern of perturbation versus height constrains the planet's location and mass.
The sensitivity is remarkable: planets more massive than 0.1% of the stellar mass (roughly a Neptune for a solar-mass star) produce detectable velocity signatures with only a few hours of ALMA integration at moderate angular resolution. Applied to HD 135344B and MWC 758 — two disks with known spiral structures — the technique identifies potential embedded planetary systems. The velocity field sees what the dust continuum merely suggests.