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# Long description

The figure has three parts: In part (a), a cross-section of a protoplanetary disc is shown. The cross-section resembles a triangle, which is broader on the right and tapers towards the left. The leftward arrow at the left end is labelled ‘to star’. A thin green layer is shown bounding the top and bottom surfaces of the disc. The interior of the disc is a blue region having two groups of black dots of varying sizes. From each of the black dots near the top surface, a downward arrow is shown. From each of the black dots near the bottom surface, an upward arrow is shown. This diagram is labelled ‘coagulation and vertical settling’. Part (b) shows the same cross-section of a protoplanetary disc. Now, the blue region is empty. A series of short leftward arrows is shown in a horizontal line passing through the middle of the disc. These arrows are labelled ‘radial drift’. The middle portion of the disc is labelled ‘possible pile-up of solids in inner disc’. A rectangular section of the middle portion of the blue region is magnified. In the magnified image, two thin green bands are shown between thick blue bands. These thin bands are labelled ‘gravitational collapse of streaming instability over-densities’. In part (c), the same cross-section is shown. Now the blue region is filled with black dots. The dots nearer to the green bands at the top and at the bottom are smaller in size, while the dots lying in the middle portion of the blue region are larger in size. The smaller dots are labelled ‘remaining solids’. This diagram is labelled ‘formation of planetesimals with a range of initial masses’.