I'm trying to find a reasonable estimate of the density of Gliese 581 d (not c). According to Wiki, d is ~1.96x the radius of Earth and ~7.7x the mass. This makes for a density very near Earth's (2^3=8). Yet they then go on to list its density as 730kg/m^3 or 0.73g/cm^3 - almost as rarefied as Saturn. Can anyone point me at some more authoritative numbers? I know we're only estimating, but clearly one of those three numbers r,d or m is spurious.
They all could be right due to a few things. When you calculate the density via M/V you are basically assuming a constant density throughout the entire planet, but usually that is a decent for a very rough approximation. I notice the article you mention is lacking citations on both the radius and the density, so it is possible those are flawed as well.