are used. @1999NewA....4..443B give a comprehensive review for the foreground components relevant for the [[*Planck*]{}]{} mission. As foreground components we include thermal emission from dust in the Galactic plane (Sect. \[foreground\_dust\]), Galactic synchrotron (Sect. \[foreground\_synchro\]) and free-free emission (Sect. \[foreground\_freefree\]), line emission from rotational transitions of carbon monoxide molecules in giant molecular clouds (Sect. \[foreground\_co\]), sub-millimetric emission from planets (Sect. \[foreground\_planets\]) and from minor bodies of the Solar system (Sect. \[foreground\_asteroids\]). Foreground components omitted at this stage are discussed in Sect. \[foreground\_omitted\].

In this work, no attempt is made at modelling the interactions between various foreground components because of poorly known parameters such as the spatial arrangement along the line-of-sight of the emitting and absorbing components. Exemplarily, the reader is referred to @2003ApJS..146..407F, where the absorption of Galactic free-free emission by dust is discussed.

Galactic dust emission {#foreground_dust}
----------------------

At frequencies above $\sim$ 100 GHz, the thermal emission from dust in the disk of the Milky Way is the most prominent feature in the microwave sky. Considerable effort has been undertaken to model the thermal emission from Galactic dust [@1997AAS...191.8704S; @1998ApJ...500..525S; @1999ApJ...524..867F; @2000ApJ...544...81F]. The thermal dust emission is restricted to low Galactic latitudes and the thin disk is easily discernible.

