In this talk, I review how volcanic degassing can be modeled from solubility laws in multicomponent silicate melts and how such models can be used to broadly capture the key role of planetary degassing on the redox bio-geo-dynamics. Applications of such degassing model are relevant to the early Earth as well as other planetary systems. After this introduction, I will here focus on two applications related to small planetary bodies: (i) the degassing from Jupiter's moon Io, that is a differentiated body, and (ii) the degassing of small undifferentiated primitive terrestrial bodies. In both cases, degassing triggers an important decrease in oxygen fugacity as it has been noted for volcanic system on Earth. However, because degassing in such systems occurs under very low pressure conditions (<0.01 bar), this phenomena is more extreme: degassing on Io produces conditions close to FMQ-4, that is, close to the domain of metal iron saturation, while degassing of carbonaceous material from un-differentiated bodies leads to much more reduced conditions, where iron exclusively exists in the metal form, like in enstatite chondrites.