 Given its known distance from the sun, this meant that the higher Pluto's albedo was assumed to be, the smaller it would have to be.Weinbaum assumed that Pluto's surface was as dark as coal, and that its diameter was greater than Earth's, with a correspondingly greater gravity.Weinbaum also assumed that Pluto was airless and had a twenty-hour period of rotation and a surface temperature of 10 kelvins.