For a spherical body, the drag force is in a direction opposite to the particle velocity and its magnitude f_D can generally be expressed in the form:

where r_p and u are the particle radius and the relative velocity, respectively; and C_D is the non-dimensional drag coefficient.   In general, C_D is a function  of any two of three non-dimensional quantities: the Mach number M(=u/c, c being the sound velocity), the Knudsen number K (=l/r_p, l being the mean free path of gas molecules) and the Reynolds number (=2*\rho*u*r_p/\mu, \mu being the viscosity[=m*c_m/3*\sigma, m being the molecular mass of the gas molecules, c_m the mean thermal velocity{=\sqrt{8*KT/\pi*m}, and \sigma the collision cross section(=2*10-15 cm2 for molecular hydrogen).

In the cases of interest for us, M<<1 and K<<1, the gas drag forces depends essentially on the Reynolds number.   For R_e> 10³, then CD is nearly a constant, being in a narrow range between 0.4 and 0.5.  Hence, we can put:

If R_e<10 then f_D is given by the Stokes formula:

In the following figure we report a plot of the Reynolds number  (=2*\rho*u*r_p/\mu), as a function of the gas density, for a nebula made of molecular hydrogen.  The black line represent the limit for which the gas-drag approximation is valid.   The computation was made for the four jovian planets (from left to right), assuming a relative velocity of 0.005*v_K (keplerian velocity).