A protoplanetary disk (aka preplanetary disk, either abbreviated PP disk or PPD) is a circumstellar disk, consisting of dust and gas orbiting a young star or protostar such as a T-Tauri star. Radii can be as much as 1000 AU. Such disks are thought to provide the material for planet formation. The disks often develop a flared torus shape due to a combination of heat and radiation pressure from the central star. They can last several million years, evolving through accretion, outflows, photoevaporation, and/or condensation into larger bodies, small to large (planetesimals or planets). Though it is millions of years, this is short enough that it has been considered as a possible limiting factor in the type of planets emerging, e.g., how large a gas planet can grow, since the gas portion of the disk disappears.
The term proplyd is used for observed protoplanetary disks, visible for being illuminated, typically by the star's ultraviolet radiation, resulting in photodissociation, ionization, and velocity dispersion.
Protoplanetary disks are also detected through an infrared excess: disk material such as dust heated by the protostar produces black-body radiation at a lower temperature range. Some have been observed in silhouette due to glowing nebulae behind them, e.g., the Orion Disks. Direct imaging of such disks have been carried out by the Hubble Space Telescope, Atacama Large Millimeter Array, and Subaru Telescope.
The dust in a protoplanetary disk necessarily varies in size if solid objects are to form. The dust inherited from the star-forming region is presumed to be up to micron sized. Some information about the location of different sizes can be gleaned from resolving it at different wavelengths: grains tend to emit thermal radiation most efficiently at a wavelength on the order of their diameter. Disks generally show fewer larger grains further out which matches the notion that the effects of radiation pressure on dust grains are significant.