NGTS-21b: an inflated Super-Jupiter orbiting a metal-poor K dwarf
Wheatley, Peter J.; Bayliss, Daniel; Worters, Hannah L.; Moyano, Maximiliano; Günther, Maximilian N.; Eigmüller, Philipp; Henderson, Beth A.; Casewell, Sarah L.; Gill, Samuel; Goad, Michael R.; Alves, Douglas R.; Burleigh, Matthew R.; Gillen, Edward; Jenkins, James S.; Kendall, Alicia; Lendl, Monika; Osborn, Ares; Udry, Stéphane; Vines, Jose I.; West, Richard G.; Smith, Alexis M. S.; Bryant, Edward M.; Watson, Christopher A.; Bouchy, François; Nielsen, Louise D.; Anderson, D. R.; Acton, Jack S.; Sefako, Ramotholo R.; Breytenbach, Hannes; Costes, Jean C.; Tilbrook, Rosanne H.; Thomas, Jessymol K.
Chile, Germany, United Kingdom, Switzerland, South Africa, Netherlands, France
Abstract
We report the discovery of NGTS-21b , a massive hot Jupiter orbiting a low-mass star as part of the Next Generation Transit Survey (NGTS). The planet has a mass and radius of 2.36 ± 0.21 MJ and 1.33 ± 0.03 RJ, and an orbital period of 1.543 d. The host is a K3V (Teff = 4660 ± 41 K) metal-poor ([Fe/H] = -0.26 ± 0.07 dex) dwarf star with a mass and radius of 0.72 ± 0.04 M⊙ and 0.86 ± 0.04R⊙. Its age and rotation period of $10.02^{+3.29}_{-7.30}$ Gyr and 17.88 ± 0.08 d, respectively, are in accordance with the observed moderately low-stellar activity level. When comparing NGTS-21b with currently known transiting hot Jupiters with similar equilibrium temperatures, it is found to have one of the largest measured radii despite its large mass. Inflation-free planetary structure models suggest the planet's atmosphere is inflated by $\sim \! 21{{\ \rm per\ cent}}$, while inflationary models predict a radius consistent with observations, thus pointing to stellar irradiation as the probable origin of NGTS-21b's radius inflation. Additionally, NGTS-21b's bulk density (1.25 ± 0.15 g cm-3) is also amongst the largest within the population of metal-poor giant hosts ([Fe/H] < 0.0), helping to reveal a falling upper boundary in metallicity-planet density parameter space that is in concordance with core accretion formation models. The discovery of rare planetary systems such as NGTS-21 greatly contributes towards better constraints being placed on the formation and evolution mechanisms of massive planets orbiting low-mass stars.