The Poynting–Robertson effect, also known as Poynting–Robertson drag, named after John Henry Poynting and Howard P. Robertson, is a process by which solar radiation causes a dust grain orbiting a star to lose angular momentum relative to its orbit around the star. This is related to radiation pressure tangential to the grain's motion. This causes dust that is small enough to be affected by this drag, but too large to be blown away from the star by radiation pressure, to spiral slowly into the star. In the case of the Solar System, this can be thought of as affecting dust grains from to in diameter. Larger dust is likely to collide with another object long before such drag can have an effect. Poynting initially gave a description of the effect in 1903 based on the luminiferous aether theory, which was superseded by the theories of relativity in 1905–1915. In 1937 Robertson described the effect in terms of general relativity.
History
Robertson considered dust motion in a beam of radiation emanating from a point source. A. W. Guess later considered the problem for a spherical source of radiation and found that for particles far from the source the resultant forces are in agreement with those concluded by Poynting.
Relation to other forces
The Poynting–Robertson effect is more pronounced for smaller objects. Gravitational force varies with mass, which is , while the power it receives and radiates varies with surface area. So for large objects the effect is negligible. The effect is also stronger closer to the sun. Gravity varies as whereas the Poynting–Robertson force varies as, so the effect also gets relatively stronger as the object approaches the Sun. This tends to reduce the eccentricity of the object's orbit in addition to dragging it in. In addition, as the size of the particle increases, the surface temperature is no longer approximately constant, and the radiation pressure is no longer isotropic in the particle's reference frame. If the particle rotates slowly, the radiation pressure may contribute to the change in angular momentum, either positively or negatively. Radiation pressure affects the effective force of gravity on the particle: it is felt more strongly by smaller particles, and blows very small particles away from the Sun. It is characterized by the dimensionless dust parameter, the ratio of the force due to radiation pressure to the force of gravity on the particle: where is the Mie scattering coefficient, and is the density and is the size of the dust grain.
Impact of the effect on dust orbits
Particles with have radiation pressure at least half as strong as gravity, and will pass out of the Solar System on hyperbolic orbits if their initial velocities were Keplerian. For rocky dust particles, this corresponds to a diameter of less than 1 μm. Particles with may spiral inwards or outwards depending on their size and initial velocity vector; they tend to stay in eccentric orbits. Particles with take around 10,000 years to spiral into the sun from a circular orbit at 1 AU. In this regime, inspiraling time and particle diameter are both roughly. Note that, if the initial grain velocity was not Keplerian, then circular or any confined orbit is possible for. It has been theorized that the slowing down of the sun's outer layer may be caused by a similar effect.
