A Super-Earth caught in a trap

Abstract

This paper is an extension of the work done by Pierens & Nelson (2008) in which they have investigated the behaviour of a two-planet system embedded in a protoplanetary disc. They have put a Jupiter mass gas giant on the internal orbit and a lower mass planet on the external one. We consider here a similar problem taking into account a gas giant with masses in the range of 0.5 to 1 Jupiter mass and a Super-Earth as the outermost planet. By changing disc parameters and planet masses we have succeeded in getting the convergent migration which allows for the possibility of their resonant locking. However, in the case in which the gas giant has the mass of Jupiter, before any mean motion first order commensurability could be achieved, the Super-Earth is caught in a trap when it is very close to the edge of the gap opened by the giant planet. This confirms the result obtained by Pierens & Nelson (2008) in their simulations. Additionally, we have found that, in a very thin disc, an apsidal resonance is observed in the system if the Super-Earth is captured in the trap. Moreover, the eccentricity of the small planet remains low, while that of the gas giant increases slightly due to the imbalance between Lindblad and corotational resonances. We have also studied analogous systems in which the gas giant is allowed to take Sub-Jupiter masses. In this case, after performing an extensive survey over all possible parameters, we have succeeded in getting the 1:2 mean motion resonant configuration only in a disc with low aspect ratio and low surface density. However, the resonance is maintained just for few thousand orbits. Thus, we conclude that for typical protoplanetary discs the mean motion commensurabilities are rare if the Super-Earth is located on the external orbit relative to the gas giant. (abridged)Comment: 10 pages, 11 figures, accepted for publication in MNRA