Kavli Affiliate: Sara Seager
| Summary:
Gas giant planets with periods $20~<~P~<~300~rm days$ orbiting Sun-like stars are a relatively uncommon outcome of planetary formation, and key questions about the nature and formation of this sub-population remain unanswered. Theoretical models for the location of their formation (in- or ex-situ) and for their subsequent migration predict different outcomes in terms of planet masses and eccentricities, indicating that observations have a key role to play in disentangling their histories. In this work we present the discovery and confirmation of a pair of long-period Jupiter-sized planets transiting an F7 star: TOI-791 b is a $0.993pm0.033rm~R_Jup$ planet on a $139.29931_-0.00012^+0.00011~rm day$ orbit, and TOI-791 c, a $1.155pm0.040rm ~R_Jup$ planet on a $232.01570_-0.00071^+0.00067~rm day$ orbit. The two planets are within 0.07% of a second-order 5:3 period commensurability leading to transit timing variations (TTVs) of up to 50 minutes. We confirm their planetary nature using ground-based photometry, including multiple full detections of the $>11~rm hr$ transits of both TOI-791 b and c from Antarctica with ASTEP, making these the longest-duration transits ever observed in their entirety from the ground. Our detailed analysis of the TTV signal allows us to measure dynamical masses for both planets, which yield densities of $ρ_rm b=0.038pm0.008 rm ~g~cm^-3$ and $ρ_rm c=0.047pm0.006 rm ~g~cm^-3$, indicating that TOI-791~b and c are two of the lowest density giant planets ever detected. While these measurements are robust, further follow-up is needed to fully characterise the TTV signal and the architecture of the system.
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