A protoplanet is a large planetary-mass object that has yet to undergo the process of planetary accretion. Protoplanets are thought to form in protoplanetary disks around young stars, and they are often the precursors to gas giant planets and brown dwarfs.
Observations of protoplanetary disks indicate that they are commonly very massive, with some estimates suggesting that they can contain as much mass as an entire star system. This high mass makes it difficult for protoplanets to form via traditional methods of planet formation (such as the core accretion model), and instead they are thought to grow via gravitational instabilities.
Once a protoplanet has formed, it must then begin accreting material from its surrounding disk in order to continue growing. The rate at which a protoplanet can accrete depends on its size; larger objects can more easily gravitationally capture smaller pieces of material. If a protoplanet is able to grow large enough, it will eventually reach critical mass and begin undergoing nuclear fusion, at which point it will become a star.
Protoplanetary disks are believed to be the birthplaces of planets, and as such they have been the subject of intense observational and theoretical study. In recent years, advances in telescope technology have allowed for the direct imaging of several young planetary systems in various stages of formation. These observations have yielded new insights into how planets may form and evolve over time.