NGTS-31b and NGTS-32b: two inflated hot Jupiters orbiting subgiant stars

Abstract

We present the discoveries of NGTS-31b(= TOI-2721), and NGTS-32b, two hot Jupiters from the Next Generation Transit Survey (NGTS) transiting slightly evolved stars. The orbital periods, radii, and masses are 4.16 and 3.31 d, 1.61 and 1.42 <inline-formula><tex-math id=TM0001 notation=LaTeX>$R_{J}$</tex-math></inline-formula>, and 1.12 and 0.57 <inline-formula><tex-math id=TM0002 notation=LaTeX>$M_{J}$</tex-math></inline-formula>, respectively. Both planets have an incident stellar flux significantly above the threshold where inflation occurs, with both planets showing signs of inflation. These planets have widely different equilibrium temperatures than other hot Jupiters of similar mass and radius, with NGTS-31b having a significantly lower temperature, and NGTS-32b being hotter. This dichotomy raises the question of how prevalent the roles of other inflation mechanisms are in the radius anomaly phenomena and will help further constrain different inflationary models.