A Ground-Based Transit Observation of the Long-Period Extremely Low-Density Planet HIP 41378 f

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We present a ground-based transit detection of HIP 41378 f, a long-period ($P = 542$ days), extremely low-density ($0.09 \pm 0.02$ g cm$^{-3}$) giant exoplanet in a dynamically complex system. Using photometry from Tierras, TRAPPIST-North, and multiple LCOGT sites, we constrain the transit center time to $T_{C,6} = 2460438.891 \pm 0.052$ 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 TTV solution for HIP 41378 f. We predict the next two transits will occur at $T_{C,7} = 2460980.793^{+0.098}_{-0.129}$ BJD TDB (2025 November 1) and $T_{C,8} = 2461522.653^{+0.213}_{-0.238}$ BJD TDB (2027 April 27). Incorporating new TESS Sector 88 data, we also rule out the 101-day orbital period alias for HIP 41378 d, and find that the remaining viable solutions are centered on the 278, 371, and 1113-day 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.

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