Douglas Harper's Etymology Dictionary
Wiktionary
n. An astronomical object, approximately the size of the Moon, formed from the mutual gravitational attraction of planetesimals; they are thought to collide with each other and slowly form planets
Wikipedia
A protoplanet is a large planetary embryo that originated within a protoplanetary disc 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.
In the case of the Solar System, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to Ceres and Pluto with masses of about 10 to 10 kg and were a few thousand kilometers in diameter. Over the course of hundreds of millions of years, they collided with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually, only a handful of embryos were left, which collided to complete the assembly of the planets proper.
Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center, whereas lighter elements rose to the surface. Such a process is known as planetary differentiation. The composition of some meteorites show that differentiation took place in some asteroids.
According to the giant impact hypothesis the Moon formed from a colossal impact of a hypothetical protoplanet called Theia with Earth, early in the Solar System's history.
In the inner Solar System, the three protoplanets to survive more-or-less intact are the asteroids 1 Ceres, 2 Pallas, and 4 Vesta. 16 Psyche is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet. The asteroid 21 Lutetia also has characteristics that resemble a protoplanet. Kuiper-belt dwarf planets have also been referred to as protoplanets. Because iron meteorites have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the asteroid belt that since have been disrupted and that are the source of these meteorites.
In February 2013 astronomers made the first direct observation of a protoplanet forming in a disk of gas and dust around a distant star.
Usage examples of "protoplanet".
Using these, I assume that the nucleus forming the original protoplanet had an orbit of cometary eccentricity, which was not completely rounded out by collisions during the process of sweeping up nearly all the raw material in the vicinity of its sun.
It had started existence as part of a glacierlike ice moon of the protoplanet Uranus, shattering, melting, and recrystallizing in the primeval eons of relentless bombardment.
To her, the tones and texture of the black-cratered chips of the destroyed protoplanet seemed familiar.
A good solid nugget of hard nickel-iron, torn from the fused and fractionated core of some lost protoplanet in the system's stormy origin.
The early protoplanets and comets all passed through this great magnetism as they circled round and round the newborn sun.