The discovery of WASP-151b, WASP-153b, WASP-156b: Insights on giant planet migration and the upper boundary of the Neptunian desert

Abstract

To investigate the origin of the features discovered in the exoplanet population, the knowledge of exoplanets' mass and radius with a good precision ( 10 %) is essential. To achieve this purpose the discovery of transiting exoplanets around bright stars is of prime interest. In this paper, we report the discovery of three transiting exoplanets by the SuperWASP survey and the SOPHIE spectrograph with mass and radius determined with a precision better than 15 %. WASP-151b and WASP-153b are two hot Saturns with masses, radii, densities and equilibrium temperatures of 0.31 +0.04 −0.03 M J , 1.13 +0.03 −0.03 R J , 0.22 +0.03 −0.02 ρ J and 1, 290 +20 −10 K, and 0.39 +0.02 −0.02 M J , 1.55 +0.10 −0.08 R J , 0.11 +0.02 −0.02 ρ J and 1, 700 +40 −40 K, respectively. Their host stars are early G type stars (with magV ∼ 13) and their orbital periods are 4.53 and 3.33 days, respectively. WASP-156b is a Super-Neptune orbiting a K type star (magV = 11.6) . It has a mass of 0.128 +0.010 −0.009 M J , a radius of 0.51 +0.02 −0.02 R J , a density of 1.0 +0.1 −0.1 ρ J , an equilibrium temperature of 970 +30 −20 K and an orbital period of 3.83 days. The radius of WASP-151b appears to be only slightly inflated, while WASP-153b presents a significant radius anomaly compared to the model of Baraffe et al. (2008). WASP-156b, being one of the few well characterised Super-Neptunes, will help to constrain the still debated formation of Neptune size planets and the transition between gas and ice giants. The estimates of the age of these three stars confirms an already observed tendency for some stars to have gyrochronological ages significantly lower than their isochronal ages. We propose that high eccentricity migration could partially explain this behaviour for stars hosting a short period planet. Finally, these three planets also lie close to (WASP-151b and WASP-153b) or below (WASP-156b) the upper boundary of the Neptunian desert. Their characteristics support that the ultra-violet irradiation plays an important role in this depletion of planets observed in the exoplanet population.

The sub-Jupiter/Neptune desert of exoplanets: parameter dependent boundaries and implications on planet formation

“…Indeed, an overpopulation of planets around higher metallicity stars has been found at the lower boundary of the desert (Dong et al 2018;Petigura et al 2018), which can imply a metallicity-dependent photoevaporation (Owen & Murray-Clay 2018). Based on the positions of WASP-151b, WASP-153b, WASP-156b at the upper boundaries of the desert, Demangeon et al (2018) suggested that the ultraviolet irradiation plays an important role in this depletion of planets observed in the exoplanet population. However, Ionov et al (2018) concluded that the desert of short-period Neptunes could not be entirely explained by evaporation of planet atmosphere caused by the radiation from a host star.…”

New observations and transit solutions of the exoplanets HAT-P-54b and WASP-153b

“…WASP-153b was discovered very recently by Demangeon et al (2018). Its host star is early G0 type star (with magV∼13) and its orbital period is 3.33 days.…”

“…Its host star is early G0 type star (with magV∼13) and its orbital period is 3.33 days. Demangeon et al (2018) determined the parameters of WASP-153b ( Table 2) and found that Table 1. Parameters of HAT-P-54b: period P (days); epoch T 0 of transit center (HJD 2450000+); orbital semimajor axis a (AU); orbital inclination i (deg); stellar temperature T st (K); stellar mass M st (M ); stellar radius R st (R ); limb-darkening law (LDL); limb-darkening coefficients (LDC); impact parameter b = a cos i/R st ; planet mass M p (M J ); planet radius R p (R J ); planet density ρ p (ρ J ); Safronov number Θ.…”

“…The planet is at the border of the Neptunian desert (Szabo and Kiss 2011), delineated by lower and upper mass and radius boundaries of the short period Neptunian (Mazeh et al 2016). Demangeon et al (2018) Table 3 presents the log of our photometric observations at the Rozhen Observatory (target name, dates, exposures, number of frames, photometric precision). We used the 2-m RCC telescope with the CCD camera VersArray 1300B (1340 × 1300 pixels, 20 μm/pixel, diameter of field of 15 arcmin) and R filter.…”

K2-161b: a low-density super-Neptune on an eccentric orbit

“…Nonetheless, super Neptunes are among the least stud-ied type of transiting planets to date, due to the low number of detected systems around bright stars. The photometric precision of ground-based surveys is barely enough to discover planets slightly smaller than Jupiter (Bakos et al 2010;Demangeon et al 2018;Brahm et al 2018a), and hence most of what we know about these planets has come from statistical studies using ground-based radial velocity observations (e.g. Jenkins et al 2017;Udry et al 2017).…”