Context. Binary Solar System objects are common, ranging from satellite systems with very large mass ratios, M1/M2, to those with mass ratios approaching unity. One well-known example of a binary is the Pluto-Charon system. With Charon being 'only' eight times less massive than Pluto, the question arises (as in the case of many other systems) as to why the mass ratio is still close to unity. There is much evidence that (binary) planet(esimal) formation happened early, when the protoplanetary gas disk was still present. It is likely that (at least some of) these binaries evolved together, as a result of pebble accretion. Pebble accretion is a new key paradigm in planetary formation and it is believed to play a major role in many aspects of the formation of planetary systems, from the radial transport of material to the rapid growth of planetary embryos throughout the system.

Aims. Here, we focus on the question of how the mass arriving in the gravitational influence zone of the binary during pebble accretion is distributed over the binary components for a given initial mass ratio. We also consider whether accretion over time leads to equal-mass binaries (converging mass ratio) or to a dominant primary mass with a small moon (diverging mass ratio).

Methods. We numerically integrated two-dimensional (2D) pebble trajectories in the same typical fashion as for a single mass that is subject to pebble accretion. We tracked the efficiency of accretion for the two separate binary components, compared to a single body with the same mass. These numerical simulations were done for a range of binary mass ratios, mutual separations, Stokes numbers, and two orbital distances, 2.5 and 39 au.

Results. We find that in the limit where pebbles start to spiral around the primary (this holds for relatively large pebbles), the pebble preferentially collides with the secondary, causing the mass ratio to converge towards unity. In our tested case, where the total binary mass is equal to that of the Pluto-Charon system, this takes place on ~Myr timescales. In this regime the total sweep-up efficiency can lower to half that of a pebble-accreting single body because pebbles that are thrown out of the system, after close encounters with the system. These timescales and sweep-up efficiency are calculated under the assumption our 2D simulations compare with the 3D reality. The results show that systems such as Pluto-Charon and other larger equal mass binaries may well have co-accreted by means of pebble accretion in the disk phase without producing binaries, with highly diverging mass ratios.