ESA is studying the possibility to merge two of the next scientific missions of its HORIZON 2000 Programme, the fourth cornerstone mission, the far IR and sub-millimeter telescope (FIRST) and the third medium-sized mission M3, Planck. FIRST is a multi-user observatory, which targets the IR and sub-millimeter part of the electromagnetic spectrum, in the wavelength range from 85 micrometers to 600 micrometers . The Planck mission is a survey mission dedicated to mapping the temperature anisotropies of the cosmic background radiation. The merger of the mission is presently being studied in view of the programmatic constraints on both missions, the fact that they use a similar orbit, the partial parallel development of both missions and the potential cost savings. The cryogenic system of FIRST is based on a Superfluid Helium Dewar at 1.65K with a design lifetime of more than 4.5 years. The very low temperature, required in the bolometer instrument will be obtained from a dedicated 3He-sorption cooler. The cryogenic system of Planck uses a sequence of passive radiator, H2 Joule-Thomson Sorption cooler, JT mechanical cooler, and dilution refrigerator.