Mass-radius relation for all known planets with masses below 30 M_⊕ with overplotted bulk density contours. It is obvious that Kepler-87 c occupies a unique position on this parameter space as the lowest-density planet for its super-Earth mass range. Some planet names were shortened so that “K-X” stands for the planet Kepler-X, and similarly “C-X” and “H-X” stand for “CoRoT-X” and “HAT-P-X”. Solar system planets are designated with a letter with no error bars. We note Kepler-87 b is beyond the scope of this figure (see discussion in the main text).