Poynting-Robertson effect

The Poynting-Robertson effect (or Poynting-Robertson drag) is an effect on dust grains orbiting a star that causes dust grains to lose angular momentum so their orbits grow smaller, which can end with the dust grains falling into the star. Photons from a star hit the grain at a very small angle from a line directly from the center of the star, due to the grain's orbital motion. This slows down the grain and it loses orbital speed, causing it to move toward the star. The effect is analogous to air resistance, e.g., to a car. If the air is moving at right angles to the car's motion, the number of molecules that get in the way of the car is basically the same as for still air. In the same manner, photons from the star end up in the way of the orbiting grain. While the photons are also pushing the grain outward, that force component remains essentially constant no matter what the orbital speed and merely contributes to the forces defining the grain's orbit, and a continually slowing orbital speed still results in a shrinking orbit. The effect depends upon the manner in which the dust scatters light as well as its mass and surface area. Dust at 1 AU from the Sun, for example, experiences enough such drag to spiral into the Sun on the order of 10,000 years. The effect may be significant in circumstellar disks.