HD 28109 hosts a trio of transiting Neptunian planets including a near-resonant pair, confirmed by ASTEP from Antarctica

• Published in: Monthly Notices of the Royal Astronomical Society
• Main Authors: Dransfield, Georgina, Triaud, Amaury, Guillot, Tristan, Mekarnia, Djamel, Nesvorný, David, Crouzet, Nicolas, Abe, Lyu, Agabi, Karim, Buttu, Marco, Cabrera, Juan, Gandolfi, Davide, Günther, Maximilian, Rodler, Florian, Schmider, François-Xavier, Stee, Philippe, Suarez, Olga, Collins, Karen, Dévora-Pajares, Martín, Howell, Steve, Matthews, Elisabeth, Standing, Matthew, Stassun, Keivan, Stockdale, Chris, Quinn, Samuel, Ziegler, Carl, Crossfield, Ian, Lissauer, Jack, Mann, Andrew, Matson, Rachel, Schlieder, Joshua, Zhou, George
• Other Authors: Midlands Ultracold Atom Research Centre, School of Physics and Astronomy Nottingham, University of Nottingham, UK (UON)-University of Nottingham, UK (UON), School of Physics and Astronomy Birmingham, University of Birmingham Birmingham, Joseph Louis LAGRANGE (LAGRANGE), Université Nice Sophia Antipolis (1965 - 2019) (UNS)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de la Côte d'Azur, Université Côte d'Azur (UniCA)-Université Côte d'Azur (UniCA)-Centre National de la Recherche Scientifique (CNRS), Department of Space Studies Boulder, Southwest Research Institute Boulder (SwRI), European Space Research and Technology Centre (ESTEC), Agence Spatiale Européenne = European Space Agency (ESA), INAF - Osservatorio Astronomico di Bologna (OABO), Istituto Nazionale di Astrofisica (INAF), DLR Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt Berlin (DLR), Università degli studi di Torino = University of Turin (UNITO), SETI Institute, National Optical Astronomy Observatory (NOAO), NASA Ames Research Center (ARC), Observatoire Astronomique de l'Université de Genève (ObsGE), Université de Genève = University of Geneva (UNIGE)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: planets and satellites: detection; planets and satellites: dynamical evolution and stabilit; planets and satellites: fundamental parameters; [SDU.ASTR.GA]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Galactic Astrophysics [astro-ph.GA]; Antarc*; Antarctica
• Online Access: https://hal.science/hal-04475618; https://hal.science/hal-04475618/document; https://hal.science/hal-04475618/file/2205.09046.pdf; https://doi.org/10.1093/mnras/stac1383

International audience We report on the discovery and characterisation of three planets orbiting the F8 star HD 28109, which sits comfortably in TESS's continuous viewing zone. The two outer planets have periods of 56.0067 ± 0.0003 days and 84.2597 +0.0010-0.0008 days, which implies a period ratio very close to that of the first-order 3:2 mean motion resonance, exciting transit timing variations (TTVs) of up to 60 mins. These two planets were first identified by TESS, and we identified a third planet in the TESS photometry with a period of 22.8911 ± 0.0004 days. We confirm the planetary nature of all three planetary candidates using ground-based photometry from Hazelwood, ASTEP and LCO, including a full detection of the ∼ 9 h transit of HD 28109 c from Antarctica. The radii of the three planets are R b = 2.199 +0.098-0.10 R ⊕ , R c = 4.23 ± 0.11 R ⊕ and R d = 3.25 ± 0.11 R ⊕ we characterise their masses using TTVs and precise radial velocities from ESPRESSO and HARPS, and find them to be M b = 18.5 +9.1-7.6 M ⊕ , M c = 7.9 +4.2-3.0 M ⊕ and M d = 5.7 +2.7-2.1 M ⊕ , making planet b a dense, massive planet while c and d are both under-dense. We also demonstrate that the two outer planets are ripe for atmospheric characterisation using transmission spectroscopy, especially given their position in the CVZ of JWST. The data obtained to date are consistent with resonant (librating) and non-resonant (circulating) solutions; additional observations will show whether the pair is actually locked in resonance or just near-resonant.