Two main theories exist for how comets are born. In both cases, ‘pebbles’ start assembling from debris in the solar nebula, reaching sizes of about 1 cm.
Then, according to the collisional rubble pile theory (left column), large objects such as the trans-Neptunian objects (TNOs) formed rapidly, within the first one million year of the solar nebula, aided by turbulent gas streams and gravity that rapidly accelerated their growth to sizes of up to 400 km. These objects also underwent internal heating caused by the decay of radioactive substances, which resulted in their dense, low-porosity structure, and kept growing over the following 400 million years, some of them even reaching sizes of Pluto or Triton-sized objects. In this scenario, comets form from fragments created in collisions between TNOs in the outer Solar System, and therefore are relatively young.
According to the primordial rubble pile theory (right column), instead, comets took a different path. After the rapid initial growth phase of the TNOs, leftover grains and ‘pebbles’ of icy material in the cold, outer parts of the solar nebula started to come together at low speed, undergoing a gradual growth with no thermal processing to their interior and yielding comets roughly 5 km in size by the time gas has disappeared from the solar nebula. The larger TNOs played a further role in the evolution of comets: by ‘stirring’ the cometary orbits, additional material was accreted at somewhat higher speed over the next 25 million years, forming the outer layers of the comets. The stirring also made it possible for the few kilometre-sized objects in size to bump gently into each other, leading to the bi-lobed nature of some observed comets.
In the second hypothesis, comets are ancient objects made out of debris left over from the main planet-building phase and which contain preserved remnants of the early solar nebula materials. Evidence collected by Rosetta strongly favours the primordial rubble pile hypothesis, namely that comets were built up slowly through low-speed accumulation of material into the shapes observed today.
Full story: How comets are born