Exoplanets with low albedo
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Low Albedo Exoplanets: Observational Evidence and Key Examples
Many exoplanets, especially hot Jupiters and some rocky worlds, have been found to possess very low albedos, meaning they reflect only a small fraction of the light from their host stars. Observations of planets like HD 209458 b, WASP-43 b, and WASP-47 b show geometric albedos as low as 0.016–0.038, which is much darker than Jupiter in our own solar system and indicates that these exoplanets are among the least reflective known worlds Chubb2024Rowe2007Kane2020. Studies of Neptunian and terrestrial exoplanets observed by Kepler also reveal that these planets, on average, have low albedos, with upper limits of 0.23 for Neptunian and 0.42 for Terran exoplanets, suggesting a general trend toward dark surfaces or atmospheres among a wide range of exoplanet types .
Causes of Low Albedo: Atmospheric and Surface Properties
Iron-Bearing Clouds and Atmospheric Absorbers
The low albedos of hot Jupiters like HD 209458 b and WASP-43 b are explained by the presence of clouds containing iron-bearing species, which are highly effective at absorbing visible light. Without these Fe-bearing aerosols, models predict much higher albedos for these planets. The inclusion of strong optical absorbers such as TiO and VO can also lead to temperature inversions and further reduce reflectivity . Observational data for HD 209458 b confirm the absence of bright, reflective clouds, supporting the idea that dark, absorbing clouds dominate its atmosphere .
Lack of Reflective Clouds
Low albedo measurements rule out the presence of bright, reflective clouds in the atmospheres of several exoplanets. For example, the low reflectivity of HD 209458 b and WASP-47 b suggests that their atmospheres lack the types of clouds that would otherwise increase albedo, such as those made of bright minerals or water ice Rowe2007Kane2020. In contrast, models show that clouds made of iron or other dark materials keep the albedo low, while only high-altitude clouds with small, bright particles could significantly increase reflectivity .
Surface Composition of Rocky Exoplanets
For hot super-Earths and lava worlds, laboratory measurements and models indicate that both molten lava and quenched glass surfaces have inherently low albedos, with upper limits around 0.1. This means that even without an atmosphere, these planets would appear dark. High albedos observed in some cases are more likely due to atmospheric clouds rather than the surface itself . Observations of TOI-1468 b, a hot rocky exoplanet, show thermal emission consistent with a bare, low-albedo rock, further supporting this conclusion Vald'es2025Webber2015.
Statistical Trends and Broader Implications
A statistical analysis of 24 transiting exoplanets with measured secondary eclipses shows that low Bond albedos (less than 0.35) are common, confirming that most hot exoplanets are dark. This trend is independent of the presence of atmospheric temperature inversions and is especially pronounced among the hottest giant planets, which also tend to have less efficient heat redistribution . For solid, Earth-sized exoplanets, ensemble studies suggest that most are more similar to dark, rocky Mercury than to bright, cloud-covered Venus .
Mapping and Characterizing Low-Albedo Regions
Advanced mapping techniques using reflected light curves can identify both high- and low-albedo regions on exoplanets. For example, mapping of a cloudless Earth analog successfully identified the Pacific Ocean as a large, low-albedo region, demonstrating that similar methods could be used to study the surfaces and atmospheres of exoplanets in more detail in the future .
Conclusion
Exoplanets with low albedo are common across a range of planet types, from hot Jupiters to rocky lava worlds. Their darkness is primarily due to absorbing clouds (often containing iron), a lack of bright reflective clouds, or inherently dark surfaces. These findings help us understand the diversity of exoplanet atmospheres and surfaces, and guide future observations aimed at characterizing these distant worlds.
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The dark days are overcast: Iron-bearing clouds on HD 209458 b and WASP-43 b can explain low dayside albedos
Iron-bearing clouds in hot Jupiter exoplanets HD 209458 b and WASP-43 b explain low dayside albedos, indicating dark daysides, rather than a cloud-free atmosphere.
exocartographer: A Bayesian Framework for Mapping Exoplanets in Reflected Light
Exocartographer is a flexible Bayesian framework for solving the exocartography inverse problem, enabling the creation of low-resolution maps of exoplanets and constraints on their spin orientation.
EFFECT OF LONGITUDE-DEPENDENT CLOUD COVERAGE ON EXOPLANET VISIBLE WAVELENGTH REFLECTED-LIGHT PHASE CURVES
Small particle size and increasing cloud altitude significantly increase the albedo of exoplanets, with bright mineral clouds best fitting observed observations.
Mapping the Surface of Partially Cloudy Exoplanets is Hard
This study demonstrates that eight days of continuous synthetic observations can reconstruct a low resolution surface albedo map of an exoplanet without needing to make assumptions about cloud physics.
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