Abstract Optically thick atmospheres provide conditions for life on free-floating planets (FFPs). These atmospheres may ensure conservation of energy released by radio-nuclides and the long-term thermal stability of a necessary life-supporting liquid solvent (e.g. water) on the surface. Not all atmospheric gases allow “reasonable” large opacities. The list of candidate atmospheric gases considered in this paper consists of nitrogen, carbon dioxide, methane and ethane. We show that bodies with water oceans are possible in interstellar space. This may happen on FFPs of (significantly) smaller or larger mass than the Earth. Generally, in case of FFP smaller in size than the Earth, the atmosphere exhibits a mixing layer near the surface and a radiative layer at higher altitudes while the atmosphere of FFPs larger in size than Earth does not exhibit a mixing layer. The prescribed thermodynamic state of water on the surface has some influence on the features of the atmosphere. The atmosphere mass of a life-hosting FFP of Earth size is two or three orders of magnitude larger that the mass of Earth atmosphere. For FFPs larger than the Earth and specific values of surface pressure and temperature, there are conditions for condensation (in the ethane atmosphere) or condensation and deposition (in the carbon dioxide atmosphere). Some arguments induce the conclusion than the associated surface pressures and temperatures should be treated with caution as appropriate life conditions.