study of EGP spectral signatures and light curves, and their inversion to obtain planetary properties, rather complicated.

Nevertheless, the molecular mix described in §\[chemistry\] determines the emergent and reflected spectrum. Though H$_2$ is abundant, it has no permanent electric dipole moment, and, hence, a very low photon absorption cross section in the optical and infrared. Similarly, helium is all but transparent. The result is that gaseous water vapor, with its strong absorption features from 0.94 to $\sim$7 , can define much of an EGP’s spectrum. Because water resides in both the Earth’s and an EGP’s atmosphere, the water bands that bracket and determine the Earth’s photometric windows at $\sim$1.0 ($Z$), $\sim$1.25 ($J$), $\sim$1.65 ($H$), $\sim$2.2 ($K$), $\sim$3.45 ($L^{\prime}$), and $\sim$4-5 ($M$), through which ground-based infrared astronomy is possible, are exactly the same windows in an EGP or brown dwarf atmosphere through which emergent flux can pour. Thus, and fortuitously for brown dwarf observations, the emission peaks for SMOs coincide with the classic Terrestrial atmospheric bandpasses.

In lieu of measurements, theory fills the vacuum. Figure 2, taken from Burrows, Sudarsky, & Hubeny[@bur.hub], depicts “phase-averaged"[@Sudarsky00] planet/star flux ratios ($f$) from 0.5 to 30 for a 1-/5-Gyr EGP in a circular orbit at 