How do Planets Form? – Step-by-Step Process of Planet Formation

Planets form through a process known as planetary formation or accretion within protoplanetary disks surrounding young stars. This process involves the accumulation of material within a rotating disk of gas and dust, eventually leading to the formation of planetesimals and, ultimately, planets. Here’s a step-by-step overview of how planets form:

Process Of Planet Formation
Process Of Planet Formation. Image: Technische Universität Braunschweig

Formation of Protoplanetary Disk:

  • Planetary formation begins in a molecular cloud, a region of space with a high concentration of gas and dust.
  • Gravitational forces cause the cloud to collapse, forming a rotating disk known as a protoplanetary disk around a young star, which is often referred to as a T Tauri star during this phase.

Dust Grain Collisions:

  • Within the protoplanetary disk, microscopic dust particles collide and stick together, forming larger particles. This process is governed by various forces, including electromagnetic forces and van der Waals forces.

Formation of Planetesimals:

  • As the dust particles continue to collide and accumulate, they form larger bodies called planetesimals. These can range from a few centimeters to kilometers in size.

Gravitational Instabilities:

  • Some planetesimals experience gravitational instabilities, causing them to attract more material and grow in size. This marks the beginning of the assembly of planetary embryos.

Protoplanet Formation:

  • The largest of these planetary embryos, called protoplanets, continue to accrete material from the surrounding disk, growing in size.
  • Protoplanets may undergo collisions and mergers with other protoplanets, leading to the formation of larger bodies.

Clearing of Orbital Debris:

  • Protoplanets with sufficient mass can gravitationally clear their orbits of smaller debris, establishing a more defined path around the star.

Terrestrial and Giant Planet Formation:

  • In the inner regions of the protoplanetary disk, where temperatures are higher, terrestrial planets (rocky planets) form through the accumulation of metal and rock.
  • In the outer regions, where temperatures are lower, gas giant planets form by capturing substantial amounts of hydrogen and helium gas.

Final Orbital Adjustments:

  • Planets undergo further orbital adjustments through interactions with the remaining gas and other planets, resulting in the final configuration of the planetary system.

Disk Dissipation:

  • Over time, the protoplanetary disk loses its gas and dust through processes like photoevaporation and stellar winds, leaving behind a mature planetary system.

This general model of planetary formation is supported by observations of protoplanetary disks around young stars and computer simulations. However, the details can vary, and different planetary systems may exhibit unique characteristics based on factors like the properties of the host star and the initial conditions of the protoplanetary disk.

Time it Takes to Form a Planet

The time it takes to form a planet can vary depending on several factors, including the characteristics of the protoplanetary disk, the size of the planet, and the specific details of the planetary formation process. In the case of our solar system, the overall planet formation process is estimated to have taken millions to tens of millions of years.

Difference between Star and Planet formation:

  • Scale: Stars, including the Sun, are significantly more massive than planets. The Sun contains more than 99% of the total mass of the solar system.
  • Energy Source: Stars, once formed, derive their energy from nuclear fusion in their cores. Planets do not undergo nuclear fusion and primarily reflect or retain the heat they receive from their host star.
  • Outcome: The outcome of star formation is a stable, long-lived celestial body that undergoes nuclear fusion. Planets are smaller bodies without ongoing nuclear fusion, and their composition varies based on distance from the star.


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