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protoplanet(proh -toh-plan-it) An evolving planet in the process of accretion, together with any of its satellites forming by accretion at the same time in the vicinity of the planet. The dust cocoon around some T Tauri stars has the attributes of a protoplanetary system. In the one typical case, HL Tauri, it forms a flattened disk, radius 160 AU, containing about one Earth mass of dust; allowing for dust condensed into planetesimals and the system's gas content, it could condense into several planets. Infrared observations from the satellite IRAS show dust around over 40 slightly older stars, including Vega, indicating either a protoplanetary system or a swarm of comets. See also Beta Pictoris.
A precursor of one of the giant planets, which is believed to have formed, along with its satellites, from a minisolar nebula in a manner similar to that of the formation of the sun and planets.