PoyntingRobertsondrag

Poynting–Robertson drag, often abbreviated PR drag, is a non-gravitational force acting on small dust particles orbiting a luminous body, such as the Sun. The drag arises from the interaction with incoming photons: a moving grain absorbs solar photons and re-emits the energy in its own frame. Because emission is isotropic in the grain’s frame but the grain is moving, the net momentum carried away partially opposes the grain’s orbital motion. The result is a small drag force that slowly removes orbital energy and angular momentum, causing the particle to spiral inward toward the star.

PR drag is distinct from radiation pressure, which acts radially outward. The drag component depends on the grain’s velocity, the finite speed of light, and the radiation field. It is strongest for small grains near the star and weakens for larger grains or at greater distances. The effect scales with the star’s luminosity and the grain’s size and density.

In the Solar System, PR drag helps shape the zodiacal dust cloud and plays a role in debris-disk evolution around other stars. For micron-sized grains at 1 AU, the inward migration occurs on timescales of roughly 10^4–10^5 years, with larger grains lasting longer and distant grains taking proportionally longer. It is typically included in dynamical models of dust rings and planetary systems.

The phenomenon is named after John Henry Poynting and Howard P. Robertson, who described the drag effect in the context of solar radiation pressure.