Dynamically Selected Mass-Radius Relationship for Low Mass Exoplanets

We study the mass-radius relationship of planets in multiplanet systems as a function of their dynamical architecture. We isolate those planets whose proximity to resonance indicates that they have not undergone significant dynamical instability since formation, and therefore have not experienced a late giant impact with another planet. We compare the properties of these planets to those whose orbital architectures suggest a high probability of having experienced a giant impact. We find that planets that are likely to have experienced an impact are, on average, more massive than those that did not -- consistent with prior claims. However, we find that the Hydrogen envelope mass fractions of these planets are no smaller than those of planets that did not experience collisions. Taken together, these findings suggest that dynamical evolution and planetary collisions are an integral part of the evolution of compact planetary systems, but that they must occur early enough that collisional remnants are still able to recapture gaseous envelopes from the dissipating protoplanetary disk.