Protoplanet

Space has baby planets.

They start as small rocks. These rocks crash into each other. This helps them grow much larger.

As they grow, they get very hot. They melt inside. The heavy parts sink to the middle. The light parts stay on top.

Some baby planets stay small. We call these dwarf planets. Others grow into big planets.

We can still see some of these today. Vesta is one example.

It is fun to learn about space!

Space has baby planets called protoplanets. They start in a disk of gas and dust. This disk orbits a young star. Small rocks called planetesimals form in the disk. These rocks crash into each other. This is called runaway growth. It helps the rocks get much bigger.

As they grow, protoplanets get very hot. This heat comes from crashing rocks and radioactivity. This is when parts of the protoplanet melt. Heavier parts sink to the center. Lighter parts rise to the surface. This way of sorting parts is called differentiation.

Some protoplanets become dwarf planets. Others grow into full planets. We can still see some survivors today. Vesta is one example in our asteroid belt.

Scientists also look for protoplanets around other stars. They use tools like the Hubble Space Telescope. They look for shapes in disks of gas. These shapes might show a new planet forming. It is hard to be sure. Some things look like planets but are not. We are still learning how they work.

A protoplanet is a growing body in space. It starts inside a protoplanetary disk of gas and dust. These objects are also called planetary embryos. They are the building blocks of real planets. As they grow, they can become dwarf planets. If they grow large enough to rule their orbit, they become full planets.

Protoplanets grow through a way called runaway growth. It begins with small chunks called planetesimals. These chunks can be meters or hundreds of kilometers wide. Gravity pulls these objects toward each other. They crash and stick together to make bigger things. This process can happen in generations. First, small embryos collide to make a second generation. Then, those larger embryos collide to make a third generation. Eventually, only a few huge embryos remain to finish the planets.

As they grow, protoplanets get very hot. This heat comes from crashing rocks and radioactive elements. This heat causes parts of the protoplanet to melt. A process called differentiation then happens. In these melted zones, heavy elements sink to the center. Lighter elements rise to the surface. This is how a planet gets a heavy core and a light crust.

We can see survivors of this process in our own Solar System. The asteroids Ceres, Pallas, and Vesta are survivors. They are found in the inner Solar System. Other objects like Psyche might be the remains of a protoplanet that lost its outer layers. Some scientists think the Moon formed from a giant hit. A protoplanet named Theia may have crashed into the Earth. This is called the giant impact hypothesis.

Astronomers also look for protoplanets around other stars. They use tools like the Hubble Space Telescope. They look for shapes like rings or gaps in disks of gas. In 2012, they saw a candidate near the star LkCa 15. In 2013, they saw a candidate near HD 100546. Another large one is AB Aur b near the star AB Aurigae. It orbits three times as far as Neptune does from our Sun. Finding these young worlds helps us understand how space works.

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.