A SixPlanet Resonant Chain in K2138?
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Tuesday 9th

A SixPlanet Resonant Chain in K2138?
The K2138 system hosts six planets and presents an interesting case study due to its distinctive dynamical structure. Its five inner planets are near a chain of 3/2 twobody meanmotion resonances, while the outermost body (planet g) is significantly detached, having a meanmotion ratio of n_f /n_g ∼ 3.3 with its closest neighbor. We show that the orbit of m_g is actually consistent with the firstorder threeplanet resonance (3PMMR) characterized by the relation 2 n_e − 4 n_f + 3 n_g = 0 and is the first time a pure firstorder 3PMMR is found in a multiplanet system and tied to its current dynamical structure. Adequate values for the masses allow to trace the dynamical history of the system from an initial capture in a 6planet chain (with n_f /n_g in a 3/1 resonance), up to its current configuration due to tidal interactions over the age of the star. The increase in resonance offset with semimajor axis, as well as its large value for n_f /n_g can be explained by the slopes of the pure threeplanet resonances in the meanmotion ratio plane. The triplets slide outward over these curves when the innermost pair is pulled apart by tidal effects, in a pantographlike manner. The capture into the 3PMMR is found to be surprisingly robust given similar masses for m_g and m_f, and it is possible that the same effect may also be found in other compact planetary systems.