  We report on the discovery of a unidirectional continuous orbital motion of a
single pair of nanoparticles which occurs spontaneously in room-temperature air
and can be manipulated by light. By varying the relative position of two
nanoparticles, we demonstrate a phase transition between two Brownian particles
and a pair of co-orbiting particles. The orbital motion is sensitive to air
pressure and is vanishing at low pressure, suggesting that the orbital motion
is supported by air. Our results pave the way for manipulating nanoscale
objects on the basis of their cooperative dynamics.
