This planetary system is most likely in a 2:1 mean motion resonance. The inner planet orbits the star exactly twice during one orbit of the outer planet. This is a very stable configuration. Mean motion resonances help us to understand the formation of multiplanteray systems. The existense of this resonance is a strong indication that interactions with the protoplanetary disk migrated the planets to their current positions. A detailed analysis of the HD 128311 system shows that there also had to be non-adiabatic effects during the migration phase such as a third planet (Sandor and Kley 2006) or turbulence in the protoplanetary disk (Rein and Papaloizou 2009). Link to scientific paper Your name Note: Your commit will be vetted before it is merged into the public version. It is crucial that you include a link to a scientific publication. For more information on the meaning of the different datafields see the readme file in the Open Exoplanet Catalogue repository on github.