Uncovering the true periods of the young sub-Neptunes orbiting TOI-2076
Authors: Osborn H. P. , Bonfanti A. , Gandolfi D. , Hedges C. , Leleu A. , Fortier A. , Futyan D. , Gutermann P. , Maxted P. F. L. , Borsato L. , Collins K. A. , Gomes da Silva J. , Gómez Maqueo Chew Y. , Hooton M. J. , Lendl M. , Parviainen H. , Salmon S. , Schanche N. , Serrano L. M. , Sousa S. G. , Tuson A. , Ulmer-Moll S. , Van Grootel V. , Wells R. D. , Wilson T. G. , Alibert Y. , Alonso R. , Anglada G. , Asquier J. , Barrado y Navascues D. , Baumjohann W. , Beck T. , Benz W. , Biondi F. , Bonfils X. , Bouchy F. , Brandeker A. , Broeg C. , Bárczy T. , Barros S. C. C. , Cabrera J. , Charnoz S. , Collier Cameron A. , Csizmadia S. , Davies M. B. , Deleuil M. , Delrez L. , Demory B.-O. , Ehrenreich D. , Erikson A. , Fossati L. , Fridlund M. , Gillon M. , Gömez-Munoz M. A. , Güdel M. , Heng K. , Hoyer S. , Isaak K. G. , Kiss L. , Laskar J. , Lecavelier des Etangs A. , Lovis C. , Magrin D. , Malavolta L. , McCormac J. , Nascimbeni V. , Olofsson G. , Ottensamer R. , Pagano I. , Pallé E. , Peter G. , Piazza D. , Piotto G. , Pollacco D. , Queloz D. , Ragazzoni R. , Rando N. , Rauer H. , Reimers C. , Ribas I. , Demangeon O. D. S. , Smith A. M. S. , Sabin L. , Santos N. , Scandariato G. , Schroffenegger U. , Schwarz R. P. , Shporer A. , Simon A. E. , Steller M. , Szabó G. M. , Ségransan D. , Thomas N. , Udry S. , Walter I. , Walton N. 
Published: March 2022

Context. TOI-2076 is a transiting three-planet system of sub-Neptunes orbiting a bright (G = 8.9 mag), young (340 ± 80 Myr) K-type star. Although a validated planetary system, the orbits of the two outer planets were unconstrained as only two non-consecutive transits were seen in TESS photometry. This left 11 and 7 possible period aliases for each.
Aims. To reveal the true orbits of these two long-period planets, precise photometry targeted on the highest-probability period aliases is required. Long-term monitoring of transits in multi-planet systems can also help constrain planetary masses through TTV measurements.
Methods. We used the MonoTools package to determine which aliases to follow, and then performed space-based and ground-based photometric follow-up of TOI-2076 c and d with CHEOPS, SAINT-EX, and LCO telescopes.
Results. CHEOPS observations revealed a clear detection for TOI-2076 c at $P = 21.02538_{ - 0.00074}^{ + 0.00084}$ d, and allowed us to rule out three of the most likely period aliases for TOI-2076 d. Ground-based photometry further enabled us to rule out remaining aliases and confirm the P = 35.12537 ± 0.00067 d alias. These observations also improved the radius precision of all three sub-Neptunes to 2.518 ± 0.036, 3.497 ± 0.043, and 3.232 ± 0.063 R⊕. Our observations also revealed a clear anti-correlated TTV signal between planets b and c likely caused by their proximity to the 2:1 resonance, while planets c and d appear close to a 5:3 period commensurability, although model degeneracy meant we were unable to retrieve robust TTV masses. Their inflated radii, likely due to extended H-He atmospheres, combined with low insolation makes all three planets excellent candidates for future comparative transmission spectroscopy with JWST.