National Astronomical Observatory of Japan



Formation of Planetesimals


When a star like the Sun is born, a gas disk called a “protoplanetary disk” is formed around the star. The protoplanetary disk contains solid particles (dust) on the order of a micron, and planets are formed through the accretion of this dust. The celestial objects produced in this process which are about several kilometers in size are called “planetesimals.” There is a theory that planetesimals are formed though the dust’s gravity. We visualized a planet formation process numerical simulation based on this theory.

Dust forms Stripe Patterns: Gravitational Instability

Dust concentrates in a thin layer due to the influence of the gravity from the main star, collisions between dust clumps, and the friction between the gas and dust which constituent the disk. As the layer becomes thinner, the more dense dust clumps together, so the effects of the dust’s gravity increase. The parts where the dust is more concentrated than the surroundings draw in the nearby dust. That’s why the stripe patterns form in the disk. This phenomenon is called “gravitational instability.” In these striped areas, dust concentration is accelerated; the dust becomes clumps which create celestial bodies known as planetesimals. As those planetesimals collect the surrounding dust, the disk’s stripe pattern disappears with time. In the calculation, the planetesimals grew in size to about 10 kilometers. After this, the planetesimals collide repeatedly with each other and evolve into planets.

Virtual Reality (VR) Video

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Video Data

Computer UsedGRAPE-DR
Time ScaleAbout 5 years in total
Spatial ScaleAbout 10,000 kilometers
Calculations byShugo Michikoshi (now at Kyoto Women's University; Doshisha University and NAOJ at the time of the simulations)
CreditsShugo Michikoshi; Eiichiro Kokubo; Hirotaka Nakayama; Yayoi Narazaki; Four-Dimensional Digital Universe Project, NAOJ

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