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 asses
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