(also: planetary rotor)
Compact weight segment of an automatic watch, which can be well integrated into the movement because of its smaller diameter. So the full case height can be exploited. The other competing principle is called the central rotor.
One challenge for micro-rotor movements is the limited amount of power they can generate. The typical solution is to use a more complex bi-directional winding system along with a heavier rotor. Even so, micro-rotor movements are notorious for not fully winding the mainspring without quite a lot of motion.
The micro-rotor movement was pioneered at Büren and championed by Chief Design Engineer Hans Kocher there. He opted for the term, "planetary", to describe his micro-rotor movement, since the small rotor reminded him of a planetary gear. At the same time, Universal Geneve was developing a similar technology and trademarked the term "microrotor" to describe it. The Büren calibre was released as the "Super Slender", even though it was about the same height as competing movements at 4.15 mm. Universal was initially unable to beat this, releasing their 4.2 mm Cal. 1-69 around the same time. Piaget was another early micro-rotor pioneer, beating both with their 2.3 mm thick Cal. 12P (with a solid gold weight segment) in 1959.
The second generation of micro-rotor movements began in 1962 with but Büren Intramatic line. It was just 3.15 mm thick, and the race was on as Universal Geneve launched their 2-handed Cal. 2-66 at just 2.5 mm.
Another shining moment for the micro-rotor came in 1969 when one of three groups in the race for the world's first automatic chronograph leveraged the technology. The Chronomatic used a Büren Cal. 1281 ebauche to achieve a reasonable size to go to market.
Patek Philippe launched their own micro-rotor movement, Cal. 240, in 1977. In the decades since, this has become their leading high-end movement and powers many of their most desirable watch models. Piaget remains committed to the design as well, using it in many of their ultra-thin movements, including their Cal. 12P, the thinnest automatic movement ever at 2.3 mm, and the Piaget Altiplano 900P. Chopard is another champion, working with Michael Parmigiani and Philippe Dufour to develop a double barrel thin calibre of their own, L.U.C 96.