Formation of Planetesimals

A protoplanetary disk is a rotating disk of gas surrounding a newly formed star. The disk also includes many micron-sized dust grains. Planets form from the accumulation of these dust grains. In the process of planet formation, kilometer-sized solid objects form, which are called planetesimals. One of the theories to explain the process of planetesimal formation from dust grains is gravitational instability. Here we introduce numerical simulations of planetesimal formation based on this theory.

In order to investigate the detailed structure during planetesimal formation, we extracted a small patch of the disk and performed a numerical simulation in it. In this movie, we depict dust as white fluffy particles. One particle corresponds to a dust aggregate whose size is larger than a centimeter.
Due to the gravitational force from the central star and the frictional forces between the gas and dust, dust aggregates gradually settle to the midplane of the disk, forming a thin layer of dust. As the dust layer becomes narrower, the dust density increases and the mutual gravitational attraction between the dust aggregates becomes stronger.
After a short time, periodic stripe patterns appeared. Such patterns form when the dust layer becomes sufficiently compressed. Because of the mutual gravity between dust particles, dense dust regions attract the surrounding dust particles. Thus, such regions become denser and denser and a stripe pattern forms. This phenomenon is called gravitational instability.
One year has elapsed since the last scene. In that time, we find very dense agglomerates in the stripes. Next we will look closely at one of the dense agglomerates.
Looking closely, we find that a number of dust particles have agglomerated into an object known as a planetesimal. Mutual gravity and collisions among particles are effective in the dense parts of the stripe pattern, facilitating planetesimal formation.
Another year has elapsed since the last scene. The planetesimals absorb the surrounding dust particles. Now, the stripe pattern dissolves and the planetesimals become large.
Because many planetesimals form, they collide with each other frequently. The colliding planetesimals merge into larger ones. The initial size of agglomerated planetesimals is about 1 — 10 km. After a large number of collisions between planetesimals, they grow to protoplanets.

Details of Numerical Simulation

PurposePlanetesimal formation process by gravitational instability
Calculation ModelN-body simulation with mutual gravity and inelastic collisions
Number of Particles: ~ 150,000 particles
Time ScaleAbout 5 years
Spatial Scale~ 10,000 km
ResearchersShugo Michikoshi(now at the University of Tsukuba; Doshisha University and NAOJ at the time of the simulations)
ReferenceMichikoshi, S., Inutsuka, S.-I, Kokubo, E., and Furuya, I., 2007, ApJ, 657, 521

Details of Visualization

In this simulation, it is assumed that the left (front) boundary wraps around to connect to the right (back) boundary and vice versa (This assumption is known as periodic boundary conditions.) To make the movie, we made copies of the simulated region results and arrayed them over a wide area.
To represent a fluffy appearance, we applied a smoke-like texture map to polygon planes.
Since a 360-degree stereoscopic version of this movie has been created, it is also possible to view it with a head-mounted display (HMD).

YouTube for VR

Web Browsers: All directions can be seen by dragging the mouse over the YouTube screen.
Smartphones and Tablets: Using YouTube App, all directions can be seen by facing your device in your chosen direction. With VR views like Google Cardboard, you can enjoy the full effect of stereoscopic vision in all directions.
※Please note that there are some cases which this video cannot be watched depending on environments.


  • Simulation: Shugo Michikoshi, Eiichiro Kokubo
  • Visualization: Hirotaka Nakayama, Yayoi Narazaki
  • Four-Dimensional Digital Universe Project, NAOJ

360p (.mov, zip file : 298.3 MB) (.wmv, zip file : 48.1 MB)

720p (.mov, zip file : 828.6 MB) (.wmv, zip file : 192 MB)

1080p (.mov, zip file : 1.65 GB) (.wmv, zip file : 216.6 MB)

Side-by-Side Stereovision, 1080p (.mov, zip file : 1.89 GB) (.wmv, zip file : 210.7 MB)

Panoramic Video for VR (Stereovision with Side-by-Side, .mp4, zip file : 2.22 GB) (Stereovision with Top-and-Bottom,.mp4, zip file : 2.11 GB) (Non-Stereo, .wmv, zip file : 2.21 GB)

Other file formats

  • 1080p Stereovision (files for right and left eyes, .wmv, .mov)
  • 4096x4096 Stereo/Non-Stereo Dome Master (.mp4, .png sequence files)
If you want to use these file formats, please contact us.


  • 2016.7 Version 1.0 was released.