Measure of a planet's reflectivity (6)

I believe the answer is:

albedo

(Other definitions for albedo that I've seen before include "Measure of reflectiveness, doable (anag.)" , "Proportion of astronomers" , "Proportion of light a planet reflects" , "Reflectivity of a surface" , "Reflecting power" .)

I've seen this clue in The Wall Street Journal.

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