NGTS-21b: an inflated Super-Jupiter orbiting a metal-poor K dwarf

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 days. 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 ± 0.04 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 $\sim 21\%$, 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/cm3) 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.

Planet Hunters NGTS: New Planet Candidates from a Citizen Science Search of the Next Generation Transit Survey Public Data

“…For context, we describe the internal vetting process that has been used in the discovery of 23 exoplanets to date by NGTS (Bayliss et al 2018;Günther et al 2018;Raynard et al 2018;Eigmüller et al 2019;Vines et al 2019;West et al 2019;Bryant et al 2020;Costes et al 2020;McCormac et al 2020;Grieves et al 2021;Smith et al 2021;Tilbrook et al 2021;Alves et al 2022;Jackson et al 2023;F. Bouchy et al 2024, in preparation).…”