How did planetesimals combine to form planets?

Planetesimals formed planets through a process of gravitational accretion, acting as the fundamental building blocks of the solar system.1

The process unfolded in several stages:

• Initial Growth: Following the collapse of the solar nebula, small particles in the disk's midplane collided and grew into grains, pebbles, and eventually planetesimals ranging from a few to several hundred kilometers in size.21
• Gravitational Coalescence: As these bodies grew, their increasing mass enhanced their gravitational fields, accelerating the capture of surrounding material. These planetesimals combined to form larger [protoplanets].1
• Differentiation by Location: The composition of these forming planets depended on their distance from the Sun. In the inner solar system, high temperatures prevented volatile substances from freezing, resulting in small, rocky terrestrial planets. Beyond the snow line, abundant ices allowed protoplanetary cores to grow massive enough to capture light gases like hydrogen and helium, forming the giant planets.21
• Final Accretion: The final stages involved violent collisions between "planetary embryos"—bodies the size of the Moon or Mars—which shaped the current characteristics of the planets.3