ASTEP confirmation of a pair of long-period Jupiter-sized planets with extremely low densities transiting TOI-791

Abstract

Gas giant planets with periods $20<P<300\mathrm{days}$ 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 $0.993\pm 0.033{\mathrm{R}}_{\mathrm{Jup}}$ planet on a $139.29931_{-0.00012}^{+0.00011}\mathrm{day}$ orbit, and TOI-791 c, a $1.155\pm 0.040{\mathrm{R}}_{\mathrm{Jup}}$ planet on a $232.01570_{-0.00071}^{+0.00067}\mathrm{day}$ 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 $>11\mathrm{hr}$ 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 ${\rho }_{\mathrm{b}}=0.038\pm 0.008\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ and ${\rho }_{\mathrm{c}}=0.047\pm 0.006\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, 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.

arXiv comment22 pages, 18 figures, Published in MNRAS

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