A transit timing variation observed for the long-period extremely low-density exoplanet HIP 41378 f
Triaud, Amaury H. M. J.; Wheatley, Peter J.; Bayliss, Daniel; Tan, Thiam-Guan; Kosiarek, Molly; Moyano, Maximiliano; Günther, Maximilian N.; Eigmüller, Philipp; Fukui, Akihiko; Narita, Norio; Palle, Enric; Henderson, Beth A.; Casewell, Sarah L.; Gill, Samuel; Goad, Michael R.; Alves, Douglas R.; Anderson, David R.; Armstrong, David J.; Burleigh, Matthew R.; Jenkins, James S.; Lendl, Monika; Udry, Stéphane; Vines, Jose I.; West, Richard G.; Rackham, Benjamin V.; de Wit, Julien; Delrez, Laetitia; Ducrot, Elsa; Stockdale, Chris; Bryant, Edward M.; Pozuelos, Francisco J.; Gan, Tianjun; Demory, Brice-Olivier; Parviainen, Hannu; Gillon, Michael; Jehin, Emmanuel; Pedersen, Peter P.; Acton, Jack S.; Tilbrook, Rosanna H.; Awiphan, Supachai; Santerne, Alexandre; Burdanov, Artem; Niraula, Prajwal; Sebastian, Daniel; Cooke, Benjamin F.; Thompson, Samantha J.; Nascimbeni, Valerio; Murray, Catriona A.; Hardee, Bronwen; Odden, Caroline E.
United Kingdom, France, Italy, Belgium, United States, Chile, Thailand, Switzerland, Germany, Japan, Spain, China, Australia
Abstract
HIP 41378 f is a temperate 9.2 ± 0.1 R⊕ planet with period of 542.08 d and an extremely low density of 0.09 ± 0.02 g cm-3. It transits the bright star HIP 41378 (V = 8.93), making it an exciting target for atmospheric characterization including transmission spectroscopy. HIP 41378 was monitored photometrically between the dates of 2019 November 19 and 28. We detected a transit of HIP 41378 f with NGTS, just the third transit ever detected for this planet, which confirms the orbital period. This is also the first ground-based detection of a transit of HIP 41378 f. Additional ground-based photometry was also obtained and used to constrain the time of the transit. The transit was measured to occur 1.50 h earlier than predicted. We use an analytic transit timing variation (TTV) model to show the observed TTV can be explained by interactions between HIP 41378 e and HIP 41378 f. Using our TTV model, we predict the epochs of future transits of HIP 41378 f, with derived transit centres of TC, 4 = $2459\,355.087^{+0.031}_{-0.022}$ (2021 May) and TC, 5 = $2459\,897.078^{+0.114}_{-0.060}$ (2022 November).