Abstract
We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34d orbit. NGTS-4b has a mass M=$20.6\pm3.0$M_E and radius R=$3.18\pm0.26$R_E, which places it well within the so-called "Neptunian Desert". The mean density of the planet ($3.45\pm0.95$g/cm^3) is consistent with a composition of 100% H$_2$O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of $0.13\pm0.02$%, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey.
Highlights
Exoplanet population statistics from the Kepler mission reveals a scarcity of short-period Neptune-sized planets (Szabo & Kiss 2011; Mazeh, Holczer & Faigler 2016; Fulton & Petigura 2018)
We present the discovery of a new sub-Neptune-sized (R = 3.18 ± 0.26 R⊕) planet transiting a K-dwarf in a P = 1.337 34 d orbit from the Generation Transit Survey (NGTS) survey
The mass and radius of Next Generation Transit Survey (NGTS)-4b as measured in this work are consistent with a composition of 100 per cent H2O, this is likely to be unphysical given the proximity to the host star, and it is more likely to consist of a rocky core with a water and/or gaseous envelope
Summary
Exoplanet population statistics from the Kepler mission reveals a scarcity of short-period Neptune-sized planets (Szabo & Kiss 2011; Mazeh, Holczer & Faigler 2016; Fulton & Petigura 2018). They showed that while X-ray/EUV photoevaporation of subNeptunes can explain the low-mass/small radius boundary, the highmass/large radius boundary better corresponds to the tidal disruption barrier for gas giants undergoing high-eccentricity migration Their findings were consistent with the observed triangular shape of the desert, since photoevaporation is more prolific at shorter orbital periods, likewise more massive gas giants can tidally circularize closer to their stellar hosts. Due to their shallow transits, Neptune-sized planets (≈4 R⊕) have largely eluded wide-field ground-based transit surveys such as WASP (Pollacco et al 2006), HATNet (Bakos et al 2004), HATSouth (Bakos et al 2013), and KELT (Pepper et al 2007, 2012).
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