Planetary mixing device (1) comprising a central spindle (2), a first (5) and a second mixing section (6) arranged in succession and comprising a respective inner chamber and a respective plurality of planetary spindles arranged about, and intermeshed with, the central spindle (2), and a stop ring (10) interposed between the first and second mixing sections (5, 6) to maintain in position the planetary spindles of the first and second mixing sections (5,6), wherein the stop ring (10) comprises an adjustment device (20) comprising one or more elements (22) housed in the stop ring and movable in order to vary a free cross section (21) as a function of the position of the elements (22).