Albedo

The albedo of any celestial body is the portion of incident light that that body reflects back into space so that an outside observer might perceive it.

The total brightness of an object is a function of its size, color, and albedo. Therefore, an astronomer can infer albedo from an object's radius, or vice-versa. But sometimes, as is the present case with Pluto's moons Nix and Hydra, neither diameter nor albedo are known independently of the other measurement, and so one cannot assign either quantity with certainty.

• Astronomy