Planetesimal: Definition, Theory & Hypothesis

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  • 0:01 Definition
  • 0:15 Theory
  • 1:21 Planetesimals
  • 2:37 Lesson Summary
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Lesson Transcript
Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Planetesimals are small rock fragments that combined together to eventually form planets. This lesson will discuss the theory behind planet formation and the importance of planetesimals. After the lesson, there will be a quiz to test your knowledge.


A planetesimal is a rock-type object formed in the early solar system from collisions with other objects in the solar system. The collisions eventually formed larger objects that led to the formation of planets.


Beginning with a cloud of microscopic cosmic dust and gas, the formation of the early solar system began as hydrogen gas collected at the center, growing in size, starting the formation of our Sun. Away from the center, gas began to rotate around the center and spread out into a disk that would lead to the formation of planets, asteroids, moons, and other small cosmic objects.

The oldest objects in meteorites on Earth have been chemically dated back 4.56 billion years. Scientists believe this is the time frame for the formation of our solar system. The oldest-known rock found on Earth has been dated back to 4.28 billion years ago.

While it is very possible Earth and other planets were forming 4.56 billion years ago or earlier, we know planet formation for Earth was finished by at least 4.28 billion years ago. This is only one example; there is no known time frame for planet formation, and each planet may have formed at different times and different rates.


The planetesimal theory, put forth by Viktor Safronov in 1941, explains planet formation in the early solar system from accretion of small bodies, growing in size as gravity attracted more and more objects. As the small bodies orbit, their gravity is very weak and they must rely on non-gravitational forces to stay together, such as radiation pressure and the emission of thermal photons.

Growth of the small planetesimal caused the strength of its gravity to increase. Growth to approximately one kilometer in size allowed the gravity of planetesimals to attract objects to them, increasing their size to seeds for planetary formation.

Collisions were common during the formation of the solar system and planets. Planetesimals continually collided and destroyed each other, but some planetesimals were able to withstand the impacts and grow, eventually forming into planets.

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