2019MNRAS.485.4601P

We validate the planetary origin of the KOI-1599 transit time variations (TTVs) with statistical and dynamical tests. We re-analysed KEPLER Q1-Q17 light-curves of the star, and we independently derived the TTVs. They appear as strongly anticorrelated, suggestive of two mutually interacting planets. We found similar radii of the candidates, 1.9 ± 0.2R_⊕ for the inner KOI-1599.02, and 1.9 ± 0.3R_⊕ for the outer KOI-1599.01. The standard MCMC TTV analysis constrains the planet masses safely below the dynamical instability limit of ≃3 M_Jup. The best-fitting MCMC model yields (9.0±0.3) M_⊕, and (4.6±0.3) M_⊕, for the inner and the outer planet, respectively. The planets are trapped in 3:2 mean motion resonance (MMR) with anti-aligned apsides (Δp = 180°) at low-eccentric (e ≃ 0.01) orbits. However, we found that the TTV mass determination depends on eccentricity priors with the dispersion in the (0.01,0.05) range. They permit a second family of TTV models with smaller masses of ≃7 M_⊕, and ≃3.6 M_⊕, respectively, exhibiting two modes of Δp = 0°, 180° librations. The 3:2 MMR is dynamically robust and persists for both modes. In order to resolve the mass duality, we re-analysed the TTV data with a quasi-analytic model of resonant TTV signals. This model favours the smaller masses. We also reproduced this model by simulating the migration capture of the system into the 3:2 MMR.