  Quantum mechanics has so far not been tested for mechanical objects at the
scale of the Planck mass $\sqrt{\hbar c/ G} \simeq 22\,\mu\mathrm{g}$. We
present an experiment where a 1 mm quartz micropillar resonating at 3.6 MHz
with an effective mass of 30 $\mu$g is cooled to 500 mK with a dilution
refrigerator, and further optomechanically sideband-cooled to an effective
temperature of 3 mK, corresponding to a mode thermal occupancy of 20 phonons.
This nearly 1000-fold increase in the mass of an optomechanical system with
respect to previous experiments near the quantum ground state opens new
perspectives in the exploration of the quantum/classical border.
