Observing a 542-day transiting giant with large TTVs: The 2025 transit of HIP 41378 f and new constraints on the outer system
Pietro Leonardi, Alexandre Santerne, Luca Borsato, Salomé Grouffal, Christopher R. Mann, Giampaolo Piotto, Karen A. Collins, Patrick Tamburo, Yugo Kawai, Denise C. Stephens, Juliana García-Mejía, Edward M. Bryant, Krzysztof Sz. Zielinski, Daniel Bayliss, David Charbonneau, Jerome P. de Leon, Gareb Fernández-Rodríguez, Akihiko Fukui, Eric G. Hintz, Keith Horne, Keisuke Isogai, James S. Jenkins, Norio Narita, Ramotholo Sefako, Avi Shporer, Richard P. Schwarz, Gregor Srdoc, Peter Wheatley, Jamie B. Williams, Jorge Fernández Fernández
astro-ph.EP
Submitted: 2026-06-22
Comments: 14 pages, 10 figures. Accepted for publication in Astronomy & Astrophysics. This manuscript is complementary to the paper by Grouffal et al. (2026, A&A), entitled "A decade of monitoring the HIP 41378 planetary system: masses and orbital periods of the six planets and an additional planet candidate."
DOI: 10.1051/0004-6361/202660305
Code: https://github.com/jasonfrowe/neossat
License: http://creativecommons.org/licenses/by/4.0/
The gist: Characterizing long-period transiting exoplanets is inherently challenging due to the rarity and long duration of transit events.
Terminology
Abstract
Characterizing long-period transiting exoplanets is inherently challenging due to the rarity and long duration of transit events. Yet, these systems provide unique insights into planetary formation, migration, the detection of exomoons, and primordial atmospheres by occupying a sparsely populated region of the exoplanet parameter space. The complexity increases further for long-period planets near mean-motion resonances, where transit timing variations (TTVs) can reach amplitudes of several hours to days. We present a coordinated space- and ground-based observing campaign, using photometry from NEOSSat, multiple LCOGT sites, MuSCAT, MuSCAT3, Tierras and NGTS, to capture the 19-hour transit of the long-period giant exoplanet HIP 41378 f (P = 542 d, R = 9.5 R) on 31 October 2025. Our transit analysis constrains the time of inferior conjunction to T C = 2460980.888 plus or minus 0.029 BJD TDB, occurring about 7 hours earlier than predicted from its linear ephemeris. This significant offset is consistent with the previously reported TTVs of HIP 41378 f, making it the longest-period exoplanet known to exhibit measurable TTVs. By combining this new precise measurement to the transit timings of the two outer planets in the system (HIP 41378 d and HIP 41378 e), we perform a dynamical modeling of the system, using the N-body integrator TRADES, refine the ephemeris of HIP 41378 f, and predict future transit events for all three outer transiting planets.
Sources
- A Ground-Based Transit Observation of the Long-Period Extremely Low-Density Planet HIP 41378 f
- A decade of monitoring the HIP 41378's planetary system
- An extremely low-density and temperate giant exoplanet
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