a planet of a few Earth masses could have formed at such large separations, where coagulation and planetesimal growth timescales are significantly longer. Alternative scenarios such as the one suggested by [@Pearce2015doublering] to explain HD 107146’s gap, could avoid these issues. In that scenario a broad gap is produced by secular interactions between a planetesimal disc and a similar mass planet on an eccentric orbit, which formed closer in and was scattered out by an additional massive planet. That scenario also predicts that the planet’s orbit should become nearly circular and the planet would be located at the inner edge of the disc at the current epoch. The model also predicts the presence of asymmetries in the disc such as spiral features that would be detectable in deeper ALMA observations.

A second alternative scenario was proposed by [@Golimowski2011] to explain the double-ring structure around HD 92945 seen in scattered light. As shown by [@Wyatt2003], planet migration can trap planetesimals in mean motion resonances; resulting in overdensities that are stationary in the reference frame co-rotating with the planet. Small dust released from these trapped planetesimals can exit the resonances due to radiation pressure, forming a double-ring structure that could be observable 