We examine the spectra and infrared colors of the cool methane-dominated
atmospheres at Teff < 1400 K expected for young gas giant planets. We couple
these spectral calculations to an updated version of the Marley et al. (2007)
giant planet thermal evolution models that include formation by core
accretion-gas capture. These relatively cool "young Jupiters" can be 1-6
magnitudes fainter than predicted by standard cooling tracks that include a
traditional initial condition, which may provide a diagnostic of formation. If
correct, this would make true Jupiter-like planets much more difficult to
detect at young ages than previously thought. Since Jupiter and Saturn are of
distinctly super-solar composition, we examine emitted spectra for model
planets at both solar metallicity and a metallicity of 5 times solar. These
metal-enhanced young Jupiters have lower pressure photospheres than field brown
dwarfs of the same effective temperatures arising from both lower surface
gravities and enhanced atmospheric opacity. We highlight several diagnostics
for enhanced metallicity. A stronger CO absorption band at 4.5 μm for the
warmest objects is predicted. At all temperatures, enhanced flux in K band is
expected due to reduced collisional induced absorption by H2​. This leads to
correspondingly redder near infrared colors, which are redder than solar
metallicity models with the same surface gravity by up to 0.7 in J−K and 1.5
in H−K. Molecular absorption band depths increase as well, most significantly
for the coolest objects. We also qualitatively assess the changes to emitted
spectra due to nonequilibrium chemistry.Comment: Accepted to ApJ. Most figures in colo