Hi Delbert,

From what I understood from the methodologies the planet hunters used, they try to determine the mass of the planets by measuring the amount of wobble a star has (the bigger the wobble, the more massive the planet). Then some planets were characterized for a minimum size by the amount of light from the star that they block when transiting in front of them. This gives us a lower limit for the size of the planet (and hence volume).

And when you have both the mass figure from the radial velocity method and the volume figure from the transit method, you divide mass by volume and you get.... Density!

The epistellar giants that had these two measurements taken were shown to have low density, like that of Jupiter. That means they are gas giants. Rocky planets like earth have higher densities.

Hope this helps. :-)