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

arXiv:2606.30016 · astro-ph.EP · Submitted 2026-06-29 · Read on arXiv

Georgina Dransfield, Antoine C. Petit, Amaury H. M. J. Triaud, Tristan Guillot, François-Xavier Schmider, Lyu Abe, Abdelkrim Agabi, Khalid Barkaoui, Thomas A. Baycroft, Philippe Bendjoya, Rafael Brahm, Karen A. Collins, Billy Edwards, Phil Evans, Alix V. Freckelton, Nolan Grieves, Steve B. Howell, Franco Mallia, Djamel Mekarnia, Angelica Psaridi, Daniel Sebastian, Keivan G. Stassun, Chris Stockdale, Amalie Stokholm, Olga Suarez, Thiam-Guan Tan, Mathilde Timmermans, Cristilyn N. Watkins, Carl Ziegler, Abderahmane Soubkiou, François Bouchy, Marion Cointepas, Vincent Deloupy, Maximilian N. Günther, Michaël Gillon, Giovanni Isopi, Emmanuel Jehin, Jon M. Jenkins, Andrés Jordán, Martin B. Nielsen, Sara Seager, Avi Shporer, Julia V. Seidel, Michal Steiner, Trifon Trifonov, Joseph D. Twicken, Joshua N. Winn, Aldo Zapparata

astro-ph.EP

Submitted: 2026-06-29

Comments: 22 pages, 18 figures, Published in MNRAS

Journal ref: Monthly Notices of the Royal Astronomical Society, Volume 549, Issue 4, July 2026, stag864

DOI: 10.1093/mnras/stag864

Code: https://github.com/christopherburke/TESS-ExoClass

License: http://creativecommons.org/licenses/by/4.0/

The gist: Gas giant planets with periods 20 < P < 300 days orbiting Sun-like stars are a relatively uncommon outcome of planetary formation, and key questions about the nature and formation of this

Terminology

Abstract

Gas giant planets with periods 20 < P < 300 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 plus or minus0.033 R Jup planet on a 139.29931-0.00012+0.00011 day orbit, and TOI-791 c, a 1.155 plus or minus0.040 R Jup planet on a 232.01570-0.00071+0.00067 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 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 b=0.038 plus or minus0.008 g cm-3 and rho c=0.047 plus or minus0.006 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.

Sources

Related papers