Small solid body from 5 to 10 km in diameter and a mass of about thousand billion tons.

A planétésimal is formed within a Nébuleuse of remains and usually enters the formation process of a Planet.

As soon as a body reaches a kilometric size, it gravitationally attracts dust in a perimeter which exceeds its diameter.

With the attack of this stage, it is at the same time gravitational for what surrounds and resistant in term of trail.

The planétésimal thus formed has a future, that it is like small body (Astéroïde or Comet) or like planet.

Simple: Planetesimal