Ultra-short period (USP) planets are a class of exoplanets with periods shorter than one day. The origin of this sub-population of planets is still unclear, with different formation scenarios highly dependent on the composition of the USP planets. A better understanding of this class of exoplanets will, therefore, require an increase in the sample of such planets that have accurate and precise masses and radii, which also includes estimates of the level of irradiation and information about possible companions. Here we report a detailed characterization of a USP planet around the solar-type star HD 80653≡EP 251279430 using the K2 light curve and 108 precise radial velocities obtained with the HARPS-N spectrograph, installed on the Telescopio Nazionale Galileo. From the K2 C16 data, we found one super-Earth planet (R b = 1.613 ± 0.071 R ⊕ ) transiting the star on a short-period orbit (P b = 0.719573 ± 0.000021 d). From our radial velocity measurements, we constrained the mass of HD 80653 b to M b = 5.60 ± 0.43 M ⊕ . We also detected a clear long-term trend in the radial velocity data. We derived the fundamental stellar parameters and determined a radius of R ⋆ = 1.22 ± 0.01 R ⊙ and mass of M ⋆ = 1.18 ± 0.04 M ⊙ , suggesting that HD 80653 has an age of 2.7 ± 1.2 Gyr. The bulk density (ρ b = 7.4 ± 1.1 g cm −3 ) of the planet is consistent with an Earth-like composition of rock and iron with no thick atmosphere. Our analysis of the K2 photometry also suggests hints of a shallow secondary eclipse with a depth of 8.1±3.7 ppm. Flux variations along the orbital phase are consistent with zero. The most important contribution might come from the day-side thermal emission from the surface of the planet at T ∼ 3480 K.
Context. The analysis of exoplanetary atmospheres by means of high-resolution spectroscopy is an expanding research field which provides information on the chemical composition, thermal structure, atmospheric dynamics, and orbital velocity of exoplanets. Aims. In this work, we aim to detect the light reflected by the exoplanet 51 Peg b by employing optical high-resolution spectroscopy. Methods. To detect the light reflected by the planetary dayside, we used optical High Accuracy Radial velocity Planet Searcher and High Accuracy Radial velocity Planet Searcher for the Northern hemisphere spectra taken near the superior conjunction of the planet, when the flux contrast between the planet and the star is maximum. To search for the weak planetary signal, we cross-correlated the observed spectra with a high signal-to-noise ratio stellar spectrum. Results. We homogeneously analyze the available datasets and derive a 10−5 upper limit on the planet-to-star flux contrast in the optical. Conclusions. The upper limit on the planet-to-star flux contrast of 10−5 translates into a low albedo of the planetary atmosphere (Ag ≲ 0.05−0.15 for an assumed planetary radius in the range of 1.5−0.9 RJup, as estimated from the planet’s mass).
Context. High-precision photometry can lead to the detection of secondary eclipses and phase variations of highly irradiated planets. Aims. We performed a homogeneous search and analysis of optical occultations and phase variations of the most favorable ultra-short-period (USP) (P < 1 days) sub-Neptunes (Rp < 4 R⊕), observed by Kepler and K2, with the aim to better understand their nature. Methods. We first selected 16 Kepler and K2 USP sub-Neptunes based on the expected occultation signal. We filtered out stellar variability in the Kepler light curves, using a sliding linear fitting and, when required, a more sophisticated approach based on a Gaussian process regression. In the case of the detection of secondary eclipse or phase variation with a confidence level higher than 2σ, we simultaneously modeled the primary transit, secondary eclipse, and phase variations in a Bayesian framework, by using information from previous studies and knowledge of the Gaia parallaxes. We further derived constraints on the geometric albedo as a function of the planet’s brightness temperature. Results. We confirm the optical secondary eclipses for Kepler-10b (13σ), Kepler-78b (9.5σ), and K2-141b (6.9σ), with marginal evidence for K2-312b (2.2σ). We report new detections for K2-106b (3.3σ), K2-131b (3.2σ), Kepler-407b (3.0σ), and hints for K2-229b (2.5σ). For all targets, with the exception of K2-229b and K2-312b, we also find phase curve variations with a confidence level higher than 2σ. Conclusions. Two USP planets, namely Kepler-10b and Kepler-78b, show non-negligible nightside emission. This questions the scenario of magma-ocean worlds with inefficient heat redistribution to the nightside for both planets. Due to the youth of the Kepler-78 system and the small planetary orbital separation, the planet may still retain a collisional secondary atmosphere capable of conducting heat from the day to the nightside. Instead, the presence of an outgassing magma ocean on the dayside and the low high-energy irradiation of the old host star may have enabled Kepler-10b to build up and retain a recently formed collisional secondary atmosphere. The magma-world scenario may instead apply to K2-141b and K2-131b.
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