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We examine the spectra and infrared colors of the cool methane-dominatedatmospheres at Teff < 1400 K expected for young gas giant planets. We couplethese spectral calculations to an updated version of the Marley et al. (2007)giant planet thermal evolution models that include formation by coreaccretion-gas capture. These relatively cool "young Jupiters" can be 1-6magnitudes fainter than predicted by standard cooling tracks that include atraditional initial condition, which may provide a diagnostic of formation. Ifcorrect, this would make true Jupiter-like planets much more difficult todetect at young ages than previously thought. Since Jupiter and Saturn are ofdistinctly super-solar composition, we examine emitted spectra for modelplanets at both solar metallicity and a metallicity of 5 times solar. Thesemetal-enhanced young Jupiters have lower pressure photospheres than field browndwarfs of the same effective temperatures arising from both lower surfacegravities and enhanced atmospheric opacity. We highlight several diagnosticsfor enhanced metallicity. A stronger CO absorption band at 4.5 $\mu$m for thewarmest objects is predicted. At all temperatures, enhanced flux in $K$ band isexpected due to reduced collisional induced absorption by H$_2$. This leads tocorrespondingly redder near infrared colors, which are redder than solarmetallicity models with the same surface gravity by up to 0.7 in $J-K$ and 1.5in $H-K$. Molecular absorption band depths increase as well, most significantlyfor the coolest objects. We also qualitatively assess the changes to emittedspectra due to nonequilibrium chemistry.

Synthetic Spectra and Colors of Young Giant Planet Atmospheres: Effects of Initial Conditions and Atmospheric Metallicity