A Ground-based Transit Observation of the Long-period Extremely Low-density Planet HIP 41378 f

We present a ground-based transit detection of HIP 41378 f, a long-period (P = 542 days), extremely low-density (0.09 ± 0.02 g cm⁻³) giant exoplanet in a dynamically complex system. Using photometry from Tierras, TRAPPIST-North, and multiple Las Cumbres Observatory Global Telescope sites, we constrain the transit center time to T_C,6 = 2460438.891 ± 0.052 BJD TDB. This marks only the second ground-based detection of HIP 41378 f, currently the longest-period and longest-duration transiting exoplanet observed from the ground. We use this new detection, along with a recently published transit time from Rossiter–McLaughlin observations, to update the transit timing variation (TTV) solution for HIP 41378 f. We predict the next two transits will occur at T_C,7 = 2460980.793_−0.129^+0.098 BJD TDB (2025 November 1) and T_C,8 = 2461522.653_−0.238^+0.213 BJD TDB (2027 April 27). Incorporating new TESS Sector 88 data, we also rule out the 101 days orbital period alias for HIP 41378 d, and find that the remaining viable solutions are centered on the 278, 371, and 1113 days aliases. The latter two imply dynamical configurations that challenge the canonical view of planet e as the dominant perturber of planet f. Our results suggest that HIP 41378 d may instead play the leading role in shaping the TTV of HIP 41378 f.