  The reflected spectra of extrasolar giant planets are primarily influenced by
Rayleigh scattering, molecular absorption, and atmospheric condensates. We
present model geometric albedo and phase integral spectra and Bond albedos for
planets and brown dwarfs with masses between 0.8 and 70 Jupiter masses.
Rayleigh scattering predominates in the blue while molecular absorption removes
most red and infrared photons. Thus cloud-free atmospheres, found on giant
planets with effective temperatures exceeding about 400 K, are quite dark in
reflected light beyond 0.6 microns. In cooler atmospheres first water clouds
and then other condensates provide a bright reflecting layer. Only planets with
cloudy atmospheres will be detectable in reflected light beyond 1 micron.
Thermal emission dominates the near-infrared for warm objects with clear
atmospheres. However the presence of other condensates, not considered here,
may brighten some planets in reflected near-infrared light and darken them in
the blue and UV. Bond albedos, the ratio of the total reflected to incident
power, are sensitive to the spectral type of the primary. Most incident photons
from early type stars will be Rayleigh scattered, while most incident photons
from late type stars will be absorbed. The Bond albedo of a given planet thus
may range from 0.4 to 0.05, depending on the primary type. Condensation of a
water cloud increases the Bond albedo of a given planet by up to a factor of
two. The spectra of cloudy planets are strongly influenced by poorly
constrained cloud microphysical properties, particularly particle size and
supersaturation. Both Bond and geometric albedos are comparatively less
sensitive to variations in planet mass and effective temperature.

Astrophysics