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 and their orbits grow smaller, ultimately until the dust grain falls 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. (I presume the outward radiation pressure merely results in a stable orbit slower than Keplerian, yet the grain still has a specific orbital speed that balances inward and outward tendencies, and an effect that slows the grain from this speed still shifts the balance in favor of inward migration.)
The effect depends upon the manner in which the dust scatters light as well as its mass and surface area. An example is dust at 1 AU from the Sun, spiraling into the Sun in on the order of 10,000 years.
The effect may be significant in circumstellar disks.