NGTS-21b: an inflated Super-Jupiter orbiting a metal-poor K dwarf. Issue 3 (7th October 2022)

• Record Type: Journal Article
• Authors: Alves, Douglas R; Jenkins, James S; Vines, Jose I; Nielsen, Louise D; Gill, Samuel; Acton, Jack S; Anderson, D R; Bayliss, Daniel; Bouchy, François; Breytenbach, Hannes; Bryant, Edward M; Burleigh, Matthew R; Casewell, Sarah L; Eigmüller, Philipp; Gillen, Edward; Goad, Michael R; Günther, Maximilian N; Henderson, Beth A; Kendall, Alicia; Lendl, Monika; Moyano, Maximiliano; Sefako, Ramotholo R; Smith, Alexis M S; Costes, Jean C; Tilbrook, Rosanne H; Thomas, Jessymol K; Udry, Stéphane; Watson, Christopher A; West, Richard G; Wheatley, Peter J; Worters, Hannah L; Osborn, Ares;
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 517:Issue 3(2022)
• Page Start: 4447
• Page End: 4457
• Publication Date: 2022-10-07
• Subjects: techniques: photometric -- techniques: radial velocities -- planets and satellites: detection -- planets and satellites: fundamental parameters -- planets and satellites: general -- stars: general; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac2884
• Languages: English
• ISSNs: 0035-8711

Abstract

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 M J and 1.33 ± 0.03 R J, and an orbital period of 1.543 d. The host is a K3V ( T eff = 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 ± 0.04 R ⊙ . 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.