Toddlers in Resonance I: low masses and eccentricities of the 17 Myr giant planets HIP 67522 bc from transit timing variations

Hannah R. James; Madyson G. Barber; Andrew W. Mann; Pa Chia Thao; Adam L. Kraus; Benjamin M. Tofflemire; Andrew Vanderburg; Lyu Abe; Tristan Guillot; Olga Suarez; Djamel Mekarnia; Amaury H. M. J. Triaud; Vincent Deloupy; Mayuko Mori; Jerome P. de Leon; Norio Narita; Akihiko Fukui; John H. Livingston; George Zhou; Camille M. L. Guenée; Juliette Becker; Ana Isabel Lopez Murillo; Karen A. Collins; Richard P. Schwarz; Chris Stockdale; Thiam-Guan Tan; Keith Horne; Leah J. Boff; Mackenna L. Wood; Nidia Morrell; Luke G. Bouma

arXiv · 2609.35979

Toddlers in Resonance I: low masses and eccentricities of the 17 Myr giant planets HIP 67522 bc from transit timing variations

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Abstract

Infant ($<$100\,Myr) transiting planet systems provide glimpses into the processes that shape the observed mature planet population. Masses for these planets are powerful data points for testing theories of planet formation and evolution. Because young systems tend to be in mean-motion resonances (MMR) and show higher rates of transit timing variations (TTVs), we can take advantage of deviations in the expected versus observed transit timings due to gravitational interactions between the planets to determine the planetary masses and orbital eccentricities. Here we report the TTV analysis of HIP\,67522, a 17\,Myr two-planet system near a 2:1 orbital resonance. The analysis is based on 55 transit observations (50 epochs) of planet b and 29 transit observations (22 epochs) of planet c, taken across 11 facilities. Assuming circular orbits, we determine masses of $M_b=25.0^{+7.6}_{-7.8}M_\oplus$ and $M_c=11.2\pm1.4M_\oplus$. Allowing for eccentric orbits, we find $M_b=23.1^{+15.4}_{-12.4}M_\oplus$ and $M_c=11.8^{+3.0}_{-2.3}M_\oplus$, and low eccentricities ($<0.1$) for both planets. Adopting a Gaussian prior on $M_b$ from the JWST transmission spectrum tightens the eccentric-solution mass for planet c to $9.7^{+1.9}_{-1.6}M_\oplus$, which is our recommended solution. The low masses and large radii are consistent with a contracting atmosphere planet formation model, such as Gas Dwarf formation. Given the challenge of radial-velocity masses and the high rate of TTVs seen in young systems, TTV-masses will be essential for testing theories of the formation and evolution of super-Earths and sub-Neptunes and determining the conditions that create the most common types of planets in the Galaxy.

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Hannah R. James, Madyson G. Barber, Andrew W. Mann, Pa Chia Thao, Adam L. Kraus, Benjamin M. Tofflemire, Andrew Vanderburg, Lyu Abe, Tristan Guillot, Olga Suarez, Djamel Mekarnia, Amaury H. M. J. Triaud, Vincent Deloupy, Mayuko Mori, Jerome P. de Leon, Norio Narita, Akihiko Fukui, John H. Livingston, George Zhou, Camille M. L. Guenée, Juliette Becker, Ana Isabel Lopez Murillo, Karen A. Collins, Richard P. Schwarz, Chris Stockdale, Thiam-Guan Tan, Keith Horne, Leah J. Boff, Mackenna L. Wood, Nidia Morrell, Luke G. Bouma. 2026-09-28. Toddlers in Resonance I: low masses and eccentricities of the 17 Myr giant planets HIP 67522 bc from transit timing variations. https://arxiv.org/abs/2609.35979

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