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 $2.36\pm 0.21$ M${}_{\mathrm{J}}$, and $1.33\pm 0.03$ R${}_{\mathrm{J}}$, and an orbital period of 1.543 days. The host is a K3V ($T_{\mathrm{eff}}=4660\pm 41$, K) metal-poor ($[Fe/H]=-0.26\pm 0.07$, dex) dwarf star with a mass and radius of $0.72\pm 0.04$, M${}_{\odot }$,and $0.86\pm 0.04$, R${}_{\odot }$. Its age and rotation period of $10.02_{-7.30}^{+3.29}$, Gyr and $17.88\pm 0.08$, 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 ($1.25\pm 0.15$, 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.

12 pages, 13 figures, accepted for publication in MNRAS

References in corpus (26)

• The Transiting Exoplanet Survey Satellite
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• DHP Framework: Digital Health Passports Using Blockchain -- Use case on international tourism during the COVID-19 pandemic
• astroquery: An Astronomical Web-Querying Package in Python
• KEPLER's First Rocky Planet: Kepler-10b
• A new code for automatic determination of equivalent widths: Automatic Routine for line Equivalent widths in stellar Spectra (ARES)
• A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass
• The Mass of KOI-94d and a Relation for Planet Radius, Mass, and Incident Flux
• Kepler constraints on planets near hot Jupiters
• Precise Radial Velocities of Giant Stars VII. Occurrence Rate of Giant Extrasolar Planets as a Function of Mass and Metallicity
• Inferring probabilistic stellar rotation periods using Gaussian processes
• A fast hybrid algorithm for exoplanetary transit searches
• Long Term Evolution of Close Planets Including the Effects of Secular Interactions
• The Orbit of WASP-12b is Decaying
• ARIADNE: Measuring accurate and precise stellar parameters through SED fitting
• Hot Jupiters: Origins, Structure, Atmospheres
• HAT-P-65b and HAT-P-66b: Two Transiting Inflated Hot Jupiters and Observational Evidence for the Re-Inflation of Close-In Giant Planets
• NGTS clusters survey. I. Rotation in the young benchmark open cluster Blanco 1
• Investigating the Planet-Metallicity Correlation for Hot Jupiters
• A transit timing variation observed for the long-period extremely low density exoplanet HIP 41378f
• The Next Generation Transit Survey - Prototyping Phase
• NGTS-12b: A sub-Saturn mass transiting exoplanet in a 7.53 day orbit
• NGTS 15b, 16b, 17b and 18b: four hot Jupiters from the Next Generation Transit Survey
• Periodic stellar variability from almost a million NGTS light curves
• NGTS-8b and NGTS-9b: two non-inflated hot-Jupiters

Cited by in corpus (2)

• Planet Hunters NGTS: New Planet Candidates from a Citizen Science Search of the Next Generation Transit Survey Public Data
• TOI-333b: A Neptune Desert planet around a F7V star