  We investigate a repulsion mechanism between two low-mass planets migrating
in a protoplanetary disk, for which the relative migration switches from
convergent to divergent. This mechanism invokes density waves emitted by one
planet transferring angular momentum to the coorbital region of the other and
then directly to it through the horseshoe drag. We formulate simple analytical
estimates, which indicate when the repulsion mechanism is effective. One
condition for a planet to be repelled is that it forms a partial gap in the
disk and another is that this should contain enough material to support angular
momentum exchange with it. Using two-dimensional hydrodynamical simulations we
obtain divergent migration of two super-Earths embedded in a protoplanetary
disk because of repulsion between them and verify these conditions. To
investigate the importance of resonant interaction we study the migration of
planet pairs near first-order commensurabilities. It appears that proximity to
resonance is significant but not essential. In this context we find repulsion
still occurs when the gravitational interaction between the planets is removed
sugesting the importance of angular momentum transfer through waves excited by
another planet. This may occur through the scattering of coorbital material
(the horseshoe drag), or material orbiting close by. Our results indicate that
if conditions favor the repulsion between two planets described above, we
expect to observe planet pairs with their period ratios greater, often only
slightly greater, than resonant values or possibly rarity of commensurability.
