Planetary system formation. Illustration of a planetesimals colliding in a protoplanetary disc around a young star. Protoplanetary discs are compressed circles, spirals or ellipses of gas and dust that form around protostars in the early stages of their development. Dust within a disc coalesces into planetesimals, which with the addition of more material may form planets. Collision of the planetesimals can lead to the formation of moons.