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 tex2html_wrap_inline11 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 tex2html_wrap_inline13 emission) from the lack of substantial tex2html_wrap_inline15 , tex2html_wrap_inline17 , tex2html_wrap_inline19 , or tex2html_wrap_inline21 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.