Model Visible and Near-infrared Spectra of Extrasolar Giant Planets
M. Marley, C. Gelino, D. Leeber, D. Stephens (NMSU), J. Lunine, A. Burrows, W. Hubbard (U. Ariz.), T. Guillot (U. Reading), R. Freedman (NASA/Ames), D. Saumon (Vanderbilt U.)
The reflected spectra of extrasolar giant planets are
remarkably sensitive to the presence or absence of clouds.
There will not be abundant condensates in atmospheres with
effective temperatures between about 400 and 1600 K (see
abstract by Lunine et al.). Since red photons must
penetrate more deeply into the atmosphere before
Rayleigh scattering than blue photons, such photons are
far more likely to be absorbed before scattering
in a cloud-free atmosphere. Thus planets in this effective
temperature range will
be substantially darker in reflected light beyond about
than cooler or warmer planets with clouds (which
are far more grey). In
the near-infrared the difference in reflected flux can be several orders
of magnitude. Hence proposed direct
detection methods which hinge upon a Jupiter-like albedo
in the red or near-infrared may implicitly be screening out a
large phase space of possible planetary companions.
A complicating effect is the substantial super-Planckian
emission in the near-infrared Z, J, H, and K bands of
extrasolar giant planets. This emission, from
deeper, hotter layers of the atmosphere, arises (as does Jupiter's
emission) from the lack of substantial
, 
, 
, or 
opacity
in portions of those bands. Depending
on the individual object, thermal emission in these bands will begin to
dominate reflected light at effective temperatures as low as 400 to 500 K.
Thus the near-infrared spectra of objects
such as 70 Vir b or HD 114762 b which lie in this temperature range
will be a complex mixture of reflected starlight and thermal emission.
We will present model spectra for a selection of extrasolar giant planets with and without clouds. Effects of variation in cloud particle size, composition, and optical depth will be discussed.