A resonant pair of warm giant planets revealed by TESS

We present the discovery of a pair of transiting giant planets using four sectors of TESS photometry. TOI-216 is a 0.87M_⊙ dwarf orbited by two transiters with radii of 8.2 and 11.3R_⊙, and periods of 17.01 and 34.57 d, respectively. Anticorrelated TTVs are clearly evident indicating that the transiters orbit the same star and interact via a near 2:1 mean motion resonance. By fitting the TTVs with a dynamical model, we infer masses of 30⁺²⁰_-14 and 200⁺¹⁷⁰_-100M_⊙, establishing that the objects are planetary in nature and have likely sub- Kronian and Kronian densities. TOI-216 lies close to the southern ecliptic pole and thus will be observed by TESS throughout the first year, providing an opportunity for continuous dynamical monitoring and considerable refinement of the dynamical masses presented here. TOI-216 closely resembles Kepler-9 in architecture, and we hypothesize that in such systems these Saturn analogues failed to fully open a gap and thus migrated far deeper into the system before becoming trapped into resonance, which would imply that future detections of new analogues may also have sub-Jupiter masses.