The Global 2:1 Mean-motion Resonance in HD 110067 is Not Vacant!

Abstract

Extensive modeling of observations obtained by two space telescopes and 14 ground-based observatories revealed the presence of at least six closely packed planets orbiting HD 110067. Their orbital periods form a nearly perfect resonant chain, in which adjacent periods are in a geometric sequence with a ratio of r = 3/2 (planets b to e) that switches to r = 4/3 (planets e to g). The latter part of the sequence is based on single transits of planets f and g and their periods are model-dependent, as only first-order mean-motion resonances (MMRs) were so far investigated; and there is no physical justification for this assumption besides the simplicity imprinted on to the resonant chain itself. Furthermore, the global 2:1 MMR is apparently occupied, whereas this orbit is vacant in all other multibody (extra) solar systems. We summarize the physical arguments for/against the above r = 4/3 period ratios in anticipation of future observations to resolve the issue.