intensities of those lines is given by the ratio of their occupation numbers $\chi_J$: $$\label{eqn_occupation}
\chi_J = (2J+1)\cdot\exp\left(-\frac{Qhc}{k_\mathrm{B} T_\mathrm{CO}} J(J+1)\right)\mbox{,}$$ i.e. the relative line intensities $q_{J\leftrightarrow J+1}$ of two consecutive lines is given by: $$q_{J\leftrightarrow J+1} = \frac{\chi_{J+1}}{\chi_J} = 
\frac{2J+3}{2J+1}\cdot\exp\left(-\frac{2Qhc}{k_B T_\mathrm{CO}}\cdot(J+1)\right)$$ $\chi_J$ is detemined by a statistical weight $(2J+1)$ reflecting the degeneracy of angular momentum and a Boltzmann factor. For the determination of line intensities thermal equilibrium is assumed, common estimates for the temperature inside GMCs are $T_\mathrm{CO} = 10 - 30$ K. For the purpose of this work, we choose $T_\mathrm{CO} = 20$ K. From the brightness temperature $T_A$ one obtains the CO-flux $S_\mathrm{CO-line}(\nu)$ by means of the following equation: $$S_\mathrm{CO-line}(\nu) = 2\frac{\nu^2}{c^2}\cdot k_B T_A(\mathrm{K})\cdot p(\nu-\nu_{J\leftrightarrow J+1})\mbox{,}$$ where the line shape $p(\nu-\nu_{J\leftrightarrow J+1})$ is assumed to be small in comparison to [[*Planck*]{}’s ]{}frequency response windows such that its actual shape (for instance, a Voigt-profile) is irrelevant.

Planetary submillimetric emission {#foreground_planets}
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Planets produce infra-red and sub-millimetric radiation by absorbing sunlight and by re-emitting this thermal load imposed by the sun. The investigation of the thermal properties of Mars, Jupiter and Saturn has been the target of several space missions [@1997ApJ...488L.161G; @1986Icar...65..244G to name but a few]. For the description 