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-circle dot and 0.86 +/- 0.04R(circle dot). Its age and rotation period of 10.02(-7.30)(+3.29) 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 similar to 21 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 +/- 0.15g 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.