The GTC exoplanet transit spectroscopy survey IX. Detection of haze, Na, K, and Li in the super-Neptune WASP-127b

Exoplanets with relatively clear atmospheres are prime targets for detailed studies of chemical compositions and abundances in their atmospheres. Alkali metals have long been suggested to exhibit broad wings due to pressure broadening, but most of the alkali detections only show very narrow absorption cores, probably because of the presence of clouds. We report the strong detection of the pressure-broadened spectral profiles of Na, K, and Li absorption in the atmosphere of the super-Neptune WASP-127b, at 4.1$\sigma$, 5.0$\sigma$, and 3.4$\sigma$, respectively. We performed a spectral retrieval modeling on the high-quality optical transmission spectrum newly acquired with the 10.4 m Gran Telescopio Canarias (GTC), in combination with the re-analyzed optical transmission spectrum obtained with the 2.5 m Nordic Optical Telescope (NOT). By assuming a patchy cloudy model, we retrieved the abundances of Na, K, and Li, which are super-solar at 3.7$\sigma$ for K and 5.1$\sigma$ for Li (and only 1.8$\sigma$ for Na). We constrained the presence of haze coverage to be around 52%. We also found a hint of water absorption, but cannot constrain it with the global retrieval owing to larger uncertainties in the probed wavelengths. WASP-127b will be extremely valuable for atmospheric characterization in the era of James Webb Space Telescope

K2-137 b: an Earth-sized planet in a 4.3-hour orbit around an M-dwarf

We report the discovery from K2 of a transiting terrestrial planet in an ultra-short-period orbit around an M3-dwarf. K2-137 b completes an orbit in only 4.3 hours, the second-shortest orbital period of any known planet, just 4 minutes longer than that of KOI 1843.03, which also orbits an M-dwarf. Using a combination of archival images, AO imaging, RV measurements, and light curve modelling, we show that no plausible eclipsing binary scenario can explain the K2 light curve, and thus confirm the planetary nature of the system. The planet, whose radius we determine to be 0.89 +/- 0.09 Earth radii, and which must have a iron mass fraction greater than 0.45, orbits a star of mass 0.463 +/- 0.052 Msol and radius 0.442 +/- 0.044 Rsol.Comment: 12 pages, 9 figures, accepted for publication in MNRA

WASP-86b and WASP-102b: super-dense versus bloated planets

We report the discovery of two transiting planetary systems: a super dense, sub-Jupiter mass planet WASP-86b (Mpl = 0.82 ± 0.06 MJ; Rpl = 0.63 ± 0.01 RJ), and a bloated, Saturn-like planet WASP-102b (Mpl = 0.62 ± 0.04 MJ; Rpl = 1.27 ± 0.03 RJ). They orbit their host star every ∼5.03, and ∼2.71 days, respectively. The planet hosting WASP-86 is a F7 star (Teff = 6330±110 K, [Fe/H] = +0.23 ± 0.14 dex, and age ∼0.8–1 Gyr); WASP-102 is a G0 star (Teff = 5940±140 K, [Fe/H] = −0.09± 0.19 dex, and age ∼1 Gyr). These two systems highlight the diversity of planetary radii over similar masses for giant planets with masses between Saturn and Jupiter. WASP-102b shows a larger than model-predicted radius, indicating that the planet is receiving a strong incident flux which contributes to the inflation of its radius. On the other hand, with a density of ρpl = 3.24± 0.3 ρJ, WASP-86b is the densest gas giant planet among planets with masses in the range 0.05 Mpl J. With a stellar mass of 1.34 M⊙ and [Fe/H]= +0.23 dex, WASP-86 could host additional massive and dense planets given that its protoplanetary disc is expected to also have been enriched with heavy elements. In order to match WASP-86b’s density, an extrapolation of theoretical models predicts a planet composition of more than 80% in heavy elements (whether confined in a core or mixed in the envelope). This fraction corresponds to a core mass of approximately 210M⊕ for WASP-86b’s mass of Mpl∼260 M⊕. Only planets with masses larger than about 2 MJ have larger densities than that of WASP-86b, making it exceptional in its mass range

Multicolour photometry for exoplanet candidate validation

Context. The TESS and PLATO missions are expected to find vast numbers of new transiting planet candidates. However, only a fraction of these candidates will be legitimate planets, and the candidate validation will require a significant amount of follow-up resources. Radial velocity follow-up can be carried out only for the most promising candidates around bright, slowly rotating, stars. Thus, before devoting RV resources to candidates, they need to be vetted using cheaper methods, and, in the cases for which an RV confirmation is not feasible, the candidate's true nature needs to be determined based on these alternative methods alone. Aims. We study the applicability of multicolour transit photometry in the validation of transiting planet candidates when the candidate signal arises from a real astrophysical source. We seek to answer how securely can we estimate the true uncontaminated star-planet radius ratio when the light curve may contain contamination from unresolved light sources inside the photometry aperture when combining multicolour transit observations with a physics-based contamination model. Methods. The study is based on simulations and ground-based transit observations. The analyses are carried out with a contamination model integrated into the PyTransit v2 transit modelling package, and the observations are carried out with the MuSCAT2 multicolour imager installed in the 1.5 m TCS in the Teide Observatory. Results. We show that multicolour transit photometry can be used to estimate the amount of flux contamination and the true radius ratio. Combining the true radius ratio with an estimate for the stellar radius yields the true absolute radius of the transiting object, which is a valuable quantity in statistical candidate validation, and enough in itself to validate a candidate whose radius falls below the theoretical lower limit for a brown dwarf.Comment: Accepted to A&

Three Small Planets Transiting a Hyades Star

We present the discovery of three small planets transiting K2-136 (LP 358 348, EPIC 247589423), a late K dwarf in the Hyades. The planets have orbital periods of $7.9757 \pm 0.0011$, $17.30681^{+0.00034}_{-0.00036}$, and $25.5715^{+0.0038}_{-0.0040}$ days, and radii of $1.05 \pm 0.16$, $3.14 \pm 0.36$, and $1.55^{+0.24}_{-0.21}$ $R_\oplus$, respectively. With an age of 600-800 Myr, these planets are some of the smallest and youngest transiting planets known. Due to the relatively bright (J=9.1) host star, the planets are compelling targets for future characterization via radial velocity mass measurements and transmission spectroscopy. As the first known star with multiple transiting planets in a cluster, the system should be helpful for testing theories of planet formation and migration.Comment: Accepted to The Astronomical Journa

Exoplanets around Low-mass Stars Unveiled by K2

We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the K2 mission. Based on light-curve analysis, adaptive-optics imaging, and optical spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during K2 campaigns 5-10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96-33 days. For one of the planets (K2-151b) we present ground-based transit photometry, allowing us to refine the ephemerides. Combining our K2 M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius $R_p$ on stellar insolation and metallicity [Fe/H]. We confirm that for periods $P\lesssim 2$ days, planets with a radius $R_p\gtrsim 2\,R_\oplus$ are less common than planets with a radius between 1-2$\,R_\oplus$. We also see a hint of the "radius valley" between 1.5 and 2$\,R_\oplus$ that has been seen for close-in planets around FGK stars. These features in the radius/period distribution could be attributed to photoevaporation of planetary envelopes by high-energy photons from the host star, as they have for FGK stars. For the M dwarfs, though, the features are not as well defined, and we cannot rule out other explanations such as atmospheric loss from internal planetary heat sources, or truncation of the protoplanetary disk. There also appears to be a relation between planet size and metallicity: those few planets larger than about 3 $R_\oplus$ are found around the most metal-rich M dwarfs.Comment: 29 pages, 21 figures, 6 tables, Accepted in Astronomical Journa

The K2-ESPRINT Project II: Spectroscopic follow-up of three exoplanet systems from Campaign 1 of K2

We report on Doppler observations of three transiting planet candidates that were detected during Campaign 1 of the K2 mission. The Doppler observations were conducted with FIES, HARPS-N, and HARPS. We measure the mass of EPIC 201546283b, and provide constraints and upper limits for EPIC 201295312b and EPIC 201577035b. EPIC 201546283b is a warm Neptune orbiting its host star in 6.77 days and has a radius of 4.45_(-0.33)^(+0.33)R_⊕ and a mass of 29.1_(-7.4)^(+7.5)M_⊕, which leads to a mean density of 1.80_(-0.55)^(+0.70) cm^(-3). EPIC 201295312b is smaller than Neptune with an orbital period of 5.66 days, a radius of 2.75_(-0.22^)(0.24)R_⊕, and we constrain the mass to be below 12 M_⊕ at 95% confidence. We also find a long-term trend indicative of another body in the system. EPIC 201577035b, which was previously confirmed as the planet K2-10b, is smaller than Neptune, orbiting its host star in 19.3 days, with a radius of 3.84_(-0.34)^(+0.35)R_⊕. We determine its mass to be 27_(-16)^(+17)M_⊕, with a 95% confidence upper limit at 57M_⊕, and a mean density of 2.6_(-1.6)^(+2.1)g cm^(-3). These measurements join the relatively small collection of planets smaller than Neptune with measurements or constraints of the mean density. Our code for performing K2 photometry and detecting planetary transits is now publicly available

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

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 0.31−0.03+0.04 MJ, 1.13−0.03+0.03 RJ, 0.22−0.02+0.03 ρJ and 1290−10+20 K, and 0.39−0.02+0.02 MJ, 1.55−0.08+0.10 RJ, 0.11−0.02+0.02 ρJ and 1700−40+40 K, respectively. Their host stars are early G type stars (with mag V ~ 13) and their orbital periods are 4.53 and 3.33 days, respectively. WASP-156b is a super-Neptune orbiting a K type star (mag V = 11.6). It has a mass of 0.128−0.009+0.010 MJ, a radius of 0.51−0.02+0.02 RJ, a density of 1.0−0.1+0.1 ρJ, an equilibrium temperature of 970−20+30 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 a recently published model. WASP-156b, being one of the few well characterized 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 behavior 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