The Planck mass is a small unit of mass used in physics . It is derived using an equation involving Planck 's constant , the speed of light in a vacuum , and the gravitational constant . Thus , the planck mass is an attempt to absorb the gravitational coupling constant into the unit of mass ( and those of distance\/time as well ) , as the planck charge does for the fine-structure constant ; naturally it is impossible to truly set either of these dimensionless numbers to zero .