Poynting–Robertson effect

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.

In the Solar System, this affects dust grains from about 1 μm to 1 mm in diameter.

Larger dust is likely to collide with another object long before such drag can have an effect.

Robertson considered dust motion in a beam of radiation emanating from a point source.

[1] The effect can be understood in two ways, depending on the reference frame chosen.

Therefore, the absorption of this radiation leads to a force with a component against the direction of movement.

This anisotropic emission causes the photons to carry away angular momentum from the dust grain.

While the dust grain thus spirals slowly into the star, its orbital speed increases continuously.

is the radius of the dust), while the power it receives and radiates varies with surface area (

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.

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.

may spiral inwards or outwards, depending on their size and initial velocity vector; they tend to stay in eccentric orbits.

[5] 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 rotation of Sun's outer layer may be caused by a similar effect.