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The ratio of the amount of stellar/solar radiation or other megascale light source reflected from an object (such as planet, moon, orbital or habitat) to the total amount it receives. The higher the albedo the brighter the object. A white, completely reflecting object has an albedo of 1.0; a black object with no reflectivity has an albedo of 0.0.

Astronomical albedos of planets and other non-luminous objects are dependent on the reflectivity of the various visible surfaces that reflect light back to the observer. A planet with a covering of ice or light-coloured clouds such as water or ammonia clouds will be much brighter than an ice-less rocky planet with a thin, dry atmosphere. However some clouds, such as those containing silicon or metals, are much darker, and cause a much lower albedo value.

Albedo and world type

Gas giant worlds can be roughly classified by their reflectivity characteristics, with hot epistellar jovians with their dark clouds having lower albedo values than the hydrojovians (which have bright water clouds) and eujovians (which also have clouds of light-coloured ammonia). The much colder cryojovians and the warm azurijovians have fewer clouds, so represent a middle case.

Rocky worlds with thin atmospheres become increasingly bright depending on the amount of surface ice visible from space; hot Hermian-type worlds are dark, Mars-like Areans often have a small amount of visible ice, and LithicGelidian Type worlds are brigher and colder still. Planets with thick atmospheres such as Cytherians and Titanians are often bright, although like gas giants, some warmer worlds have darker particulates or gases in their atmospheres.

Some of the most complex planetary albedo characteristics are displayed by Gaian type worlds, both natural garden worlds and those which have been artificially terraformed. The water clouds of such worlds are very bright, but the amount of cloud cover on such a world can vary considerably. Similarly the amount of liquid water on the surface varies considerably from world to world. Note that from space, a body of open water has a low albedo, and the seas of a world can look very dark; but water usually has a significant specular reflection, which can cause a bright spot or patch to appear on a watery world.

Ice also has a fairly high specular reflection but reflects light in all directions very well. However on some worlds ice may be overlain by wind-blown dust or even light-absorbing lifeforms. Snow (a thin covering of precipitated ice) may be very bright, but often becomes dirty or slushy and may even be darker than unaffected landscape.

Bare rock or soil on a Gaian world may have a relatively low albedo, while some kinds of sandy desert are somewhat brighter; the spectrographic values of reflected light from a Gaian world can tell a lot about the different land surface types. And where life is present, the results can be even more informative. Most forms of photosynthetic life are significantly darker than the surrounding landscape, and a world with very abundant life may have a much lower albedo than an equivalent lifeless world. Various kinds of xenobiota display different albedo characteristics, and on some worlds the local 'plant' life may appear nearly black to human baseline vision.

Geometric effects

Because of the different ways that light is reflected back towards the observer from different surfaces, sometimes reflectivity appears much greater when seen with the light source behind the observer. Specular reflection, true reflection, backscatter and the shadowing effect may cause a planet to appear much brighter when at opposition than when seen at other times. The Geometric albedo is the ratio of a body's apparent brightness (when seen from the direction of the light source) to the brightness of an idealised, flat, perfectly matt (Lambertian) body. This value can be greater than 1 because reflected light can be focused on the observer by geometric effects in the landscape.

Another measurement of albedo is the Bond albedo, which is the ratio between the total amount of reflected light in all directions and the incident light. Bond albedo is always between 0 and 1 in value.
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Development Notes
Text by M. Alan Kazlev, amended by Steve Bowers
Initially published on 31 December 2007.