Probing Kepler's hottest small planets via homogeneous search and analysis of optical secondary eclipses and phase variations

Abstract

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 ultrashort-period (USP) (P < 1 d) sub-Neptunes (R p < 4R ⊕ ), 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.

K2 and Spitzer phase curves of the rocky ultra-short-period planet K2-141 b hint at a tenuous rock vapor atmosphere

“…The spectrum agrees well with the originally published eclipse depth. Subsequent analysis suggested that the eclipse depth may be even higher (Sheets & Deming 2014;Singh et al 2021). The thermal inversion model agrees with these values to within 2.4 and 3.6σ, respectively.…”

Near infrared and optical emission of WASP-5 b

“…Obviously, separation of A b and is not possible by using the occultation alone. However, by measuring the phase curve, one may attempt to derive the night side emissivity that depends solely on (Singh et al 2021). Because of the lack of very high quality data required by this method, we opted to parameterize the derived geometric albedo depending on the extreme limits of and omitting the negligible temperature decrease due to the expectedly small Bond albedo (e.g., Mallonn et al 2019).…”