Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere

Abstract: We present new visible and infrared observations of the hot Jupiter Kepler-7b to determine its atmospheric properties. Our analysis allows us to 1) refine Kepler-7b's relatively large geometric albedo of Ag=0.35+-0.02, 2) place upper limits on Kepler-7b thermal emission that remains undetected in both Spitzer bandpasses and 3) report a westward shift in the Kepler optical phase curve. We argue that Kepler-7b's visible flux cannot be due to thermal emission or Rayleigh scattering from H2 molecules. We therefore conclude that high altitude, optically reflective clouds located west from the substellar point are present in its atmosphere. We find that a silicate-based cloud composition is a possible candidate. Kepler-7b exhibits several properties that may make it particularly amenable to cloud formation in its upper atmosphere. These include a hot deep atmosphere that avoids a cloud cold trap, very low surface gravity to suppress cloud sedimentation, and a planetary equilibrium temperature in a range that allows for silicate clouds to potentially form in the visible atmosphere probed by Kepler. Our analysis does not only present evidence of optically thick clouds on Kepler-7b but also yields the first map of clouds in an exoplanet atmosphere.

• The paper finds that Kepler-7b’s optical phase curve is offset from the substellar point, indicating non-uniform, reflective cloud coverage.
• The study employed combined optical phase-curve and infrared occultation data to robustly characterize the exoplanet’s atmospheric albedo and cloud distribution.
• The results reveal that the absence of strong infrared thermal emission supports the inference that clouds, rather than thermal radiation, dominate the observed visible flux.

Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere

The paper "Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere" presents an analysis of observational data obtained from the exoplanet Kepler-7b, particularly focusing on its atmospheric properties and cloud structure. The authors utilize data from visible and infrared observations to investigate Kepler-7b's atmosphere, employing telescopes like Kepler and Spitzer for empirical data collection. This study offers a comprehensive look at the planet’s atmospheric dynamics by analyzing the geometric albedo, thermal emission, and phase curve characteristics.

The study finds that Kepler-7b exhibits a relatively large geometric albedo of $A_g = 0.35 \pm 0.02$, and a westward shift in the Kepler optical phase curve. There is a strong argument that the visible flux from Kepler-7b cannot be attributed to thermal emission or Rayleigh scattering from $H_2$ molecules. Instead, the observations suggest the presence of high-altitude, optically reflective clouds, potentially composed of silicates, located westward from the substellar point. These clouds seem to contribute significantly to the albedo as they reflect a considerable portion of stellar light.

The authors employ a combination of optical phase-curve and infrared occultation data to substantiate their findings on the presence of clouds. In the infrared spectrum, particularly in the Spitzer 3.6 and 4.5-$\mu m$ channels, the data do not reveal significant thermal emissions from Kepler-7b. This aligns with the hypothesis that the visible light curve is more attributable to reflected light from the cloud layers rather than thermal emissions.

Furthermore, the phase curve analysis reveals Kepler-7b’s visible flux shows an asymmetric pattern with a noticeable phase shift away from the substellar point. The peak brightness occurs at a longitude significantly offset from the planet's hottest point. Such a deviation suggests dynamic weather patterns, with reflective clouds potentially being transported by atmospheric winds to positions offset from the expected thermal equilibrium.

The theoretical implications of this research are substantial, suggesting that atmospheric cloud dynamics in hot Jupiters might not only be influenced by thermal structures but also by complex cloud physics, including cloud formation, maintenance, and sedimentation processes. Practically, these findings underscore the critical role of clouds in shaping observed albedo and phase curve data, which is crucial for the characterization of exoplanet atmospheres.

The study’s approach to modeling the atmospheric phenomena observed on Kepler-7b lays groundwork for further investigations on similar exoplanets. As techniques for direct observation improve, particularly with missions aiming for higher resolution and multi-wavelength data, this foundational research will inform both observational strategies and atmospheric models. Future studies may explore cloud composition and distribution, atmospheric dynamics, and how these influence weather patterns on similar bodies, potentially through the use of polarimetry and narrow-band optical observations.

In conclusion, this analysis of Kepler-7b provides significant insights into the presence and role of inhomogeneous clouds in exoplanetary atmospheres. This research contributes to a broader understanding of atmospheric physics in exoplanets, emphasizing the link between observed optical properties and underlying atmospheric processes. As observational technologies advance, such insights will progressively enhance our capacity to characterize and understand the complexity of exoplanetary environments.