What is the Hypothesis of Protoplanets?


The hypothesis of protoplanets is a scientific theory that explains the formation of planets in our solar system. It suggests that planets are formed from the accumulation of dust and gas in a protoplanetary disk, which is a rotating disk of material that surrounds a young star.

1. Formation of Protoplanetary Disks

When a star is born, it is surrounded by a large cloud of gas and dust called a molecular cloud. As gravity causes this cloud to collapse, it starts spinning faster due to the conservation of angular momentum. The material in the cloud begins to flatten into a disk shape, known as a protoplanetary disk.

1.1 Accretion of Dust and Gas

The protoplanetary disk contains dust particles, ice, and gas. As the disk evolves, the dust particles start to collide and stick together, forming larger and larger objects. This process is known as accretion. The larger objects, called planetesimals, can range in size from a few meters to several kilometers.

1.1.1 Gravitational Instability

In some cases, the protoplanetary disk can become gravitationally unstable due to local perturbations. This instability can lead to the formation of clumps or spiral arms within the disk. These clumps can continue to grow through further accretion, eventually forming protoplanets. Role of Gas in Accretion

The gas present in the protoplanetary disk plays a crucial role in the accretion process. It helps to dampen the relative velocities of dust particles, allowing them to stick together more effectively. Additionally, gas drag can cause the dust particles to migrate towards the center of the disk, where they can continue to accumulate and form protoplanets.

2. Planetesimal Formation

As the protoplanetary disk evolves, the planetesimals continue to grow in size through collisions and accretion. These planetesimals can reach sizes of several kilometers and become the building blocks for future planets.

2.1 Growth through Collisions

Collisions between planetesimals can result in the formation of larger bodies, known as planetary embryos. These planetary embryos continue to grow by accreting more planetesimals, eventually becoming protoplanets.

2.1.1 Role of Gravity

Gravity plays a significant role in the growth of planetary embryos. As the mass of an embryo increases, its gravitational pull becomes stronger, allowing it to attract more planetesimals and grow even further. Differentiation of Protoplanets

As protoplanets grow larger, their interiors can differentiate into distinct layers. This process occurs due to variations in density and temperature, causing heavier materials to sink towards the core while lighter materials rise to the surface.

Astronomy – Ch. 8: Origin of the Solar System (11 of 19) The Protoplanets – 1

3. Protoplanet Evolution

Once protoplanets form, they continue to evolve through various processes, including further accretion, collisions, and gravitational interactions with other bodies in the protoplanetary disk.

3.1 Clearing the Orbit

As protoplanets grow, their gravitational influence can clear their orbits of smaller objects, such as planetesimals and dust particles. This process, known as orbital clearing, allows the protoplanets to establish their dominance in the region and prevent further growth of other bodies.

3.1.1 Formation of Planetary Gaps

During the clearing process, the protoplanets create gaps or cavities in the protoplanetary disk. These gaps can be observed in the form of dark rings or regions of reduced density, indicating the presence of protoplanets. Migration of Protoplanets

Protoplanets can also undergo migration, where they move from their original formation location to a different orbit within the protoplanetary disk. This migration can occur due to gravitational interactions with other protoplanets or the residual gas in the disk.

4. Planet Formation

Over time, protoplanets can continue to grow and evolve into fully formed planets. This process involves the accretion of additional material, as well as the clearing of their orbits from remaining debris in the protoplanetary disk.

4.1 Final Stages of Accretion

As protoplanets grow larger, their gravity becomes stronger, allowing them to attract more material and increase their size. Eventually, they reach a point where their gravitational pull is sufficient to capture gas from the protoplanetary disk, leading to the formation of a gaseous envelope around the growing planet.

4.1.1 Differentiation and Planetary Layers

Once a planet reaches a certain size, it can undergo further differentiation, similar to the earlier stages of protoplanet differentiation. This process leads to the formation of distinct layers, such as a solid core, a mantle, and an atmosphere. Terrestrial vs. Gas Giant Planets

The final composition and structure of a planet depend on its location within the protoplanetary disk. Protoplanets closer to the star tend to have a higher concentration of heavy elements, leading to the formation of terrestrial planets. Protoplanets farther from the star can accumulate more gas, resulting in the formation of gas giant planets.


The hypothesis of protoplanets provides a comprehensive explanation for the formation and evolution of planets in our solar system. Through the accumulation of dust and gas in protoplanetary disks, the growth of planetesimals, and the subsequent development of protoplanets, the diverse range of planetary bodies we observe today is the result of a complex and fascinating process that has shaped our cosmic neighborhood.

Rate article
Add a comment