Protoplanet

A protoplanet, often called a planetary embryo, is a massive celestial body in the early stages of planetary formation. These objects originate within a protoplanetary disk, which is a swirling collection of gas and dust surrounding a young star. Protoplanets are essential because they serve as the primary building blocks for mature planetary systems. As these bodies accumulate more mass, they eventually achieve hydrostatic equilibrium. This means their own gravity pulls them into a spherical shape. Once they reach this stage, they are classified as dwarf planets. If a dwarf planet grows large enough to dominate its own orbital path, it becomes a full planet.

Most protoplanets form through a process known as runaway growth. This process begins with planetesimals, which are solid objects made of dust and rock. These planetesimals can range in size from just a few meters to hundreds of kilometers in diameter. As these objects orbit within the protoplanetary disk, their gravity begins to perturb, or disturb, the orbits of nearby objects. This leads to frequent collisions where the objects coalesce, or stick together, to form larger bodies. This growth often occurs in distinct generations. First, collisions between small embryos create a second generation of larger embryos. These then collide to form a third generation of even larger bodies. Eventually, only a few massive embryos remain to complete the assembly of the final planets.

Another way for protoplanets to form is through disk fragmentation. This is also known as gravitational instability. In this scenario, the protoplanetary disk itself breaks apart under its own gravity. This specific pathway is thought to be more common for giant planets that exist on very wide orbits far from their host stars. While runaway growth relies on the slow collision of solid chunks, disk fragmentation involves the direct collapse of gas and dust within the disk.

As a protoplanet grows, it undergoes a significant internal change called planetary differentiation. This happens because the growing body becomes incredibly hot. The heat comes from three main sources: the energy of constant impacts, intense gravitational pressure, and the presence of radioactive elements. Early protoplanets contained more radioactive elements than they do today, but this amount has decreased over time due to radioactive decay. When these zones melt, the materials inside separate based on their weight. Heavier elements sink toward the center to form a core, while lighter elements rise to create the outer layers. We see evidence of this differentiation in the composition of certain meteorites.

In our own Solar System, we can find remnants of these early stages. Scientists believe that hundreds of planetary embryos once existed, with masses between 10^22 and 10^23 kg and diameters of a few thousand kilometers. In the inner Solar System, the asteroids Ceres, Pallas, and Vesta have survived relatively intact. Other objects show signs of much more violent histories. The asteroid Psyche may be the surviving core of a protoplanet that had its rocky outer layers stripped away by a hit-and-run collision. The Moon may have also formed from a massive event. The giant impact hypothesis suggests that a protoplanet named Theia collided with the early Earth, creating the Moon from the debris.

Astronomers also search for protoplanets orbiting other stars, known as exoplanets. Detecting them is difficult because they are often hidden within thick, gas-rich disks. Scientists look for specific clues, such as rings, gaps, spirals, or shadows in the disk, which might be caused by a protoplanet's gravity. A new method involves using gas velocity maps to look for "kinks" in how gas moves. For example, the protoplanet HD 97048 b was detected using these disk kinematics. Other candidates, like AB Aur b, are being studied by the Hubble Space Telescope and the Subaru Telescope. AB Aur b is particularly interesting because it is a massive gas giant with an orbit three times further from its star than Neptune is from our Sun.

Studying protoplanets helps us connect the dots between small dust particles and massive planetary systems. By observing objects like PDS 70 b or HD 169142 b, researchers can see how mass and orbital periods develop. Even when detections are disputed or refuted, such as the case with LkCa 15 b, these observations push the boundaries of science. Every discovery provides more data on how gravity, heat, and collisions work together to build the worlds we see in the universe.