Protoplanet

A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior.

Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant planets.

A planetesimal is an object formed from dust, rock, and other materials, measuring from meters to hundreds of kilometers in size.

According to the Chamberlin–Moulton planetesimal hypothesis and the theories of Viktor Safronov, a protoplanetary disk of materials such as gas and dust would orbit a star early in the formation of a planetary system.

Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets.

[citation needed] In February 2013 astronomers made the first direct observation of a candidate protoplanet forming in a disk of gas and dust around a distant star, HD 100546.

AB Aur b is among the largest exoplanets identified, and has a distant orbit, three times as far as Neptune is from the Earth's sun.

[16] Rings, gaps, spirals, dust concentrations and shadows in protoplanetary disks could be caused by protoplanets.