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# What is the Albedo of an Exoplanet?

In general, albedo is a quantity that is a measure of the reflectivity of an illuminated object. In the particular context of exoplanets, two different types of albedo are commonly discussed and used. One is the bond albedo, and the other is the geometric albedo. It is the bond albedo that is used in estimating temperatures of exoplanets under certain simplistic assumptions of the balance of heating and cooling.

## Bond Albedo

The bond albedo does not depend on the wavelength of incident light or radiation because it is involves sums of energy over all wavelengths. It is the ratio of the total reflected energy to the total incident energy.

## Geometric Albedo

The geometric albedo depends on the wavelength of the incident light. At a given wavelength, it compares the straight-line (zero-phase) intensity of reflected light from a planet (as seen from the host star) to the intensity that would have been obtained if the planet reflected light equally in all directions. The geometric albedo is the ratio of the two intensities (one appropriate for the planet, one theoretical). In jargon, the theoretical object is a Lambertian sphere, which reflects light in a diffuse (isotropic) manner.

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File under: Exoplanet albedo; Different types of albedo; bond albedo; geometric albedo.