Data extracted from the Extrasolar Planets Encyclopaedia (http://exoplanet.eu) show the existence of planets that are more massive than same-size iron cores. After meticulous verification of the data, we have concluded that the mass of the smallest of these planets was actually unknown. However, three high-density planets, Kepler-52b, Kepler-52c, and Kepler-57b, which are between 30 and 100 times the Earth mass, have indeed density larger than asame-size iron planet. This observation triggered the present study that investigates under which conditions these planets could represent the naked cores of gas giants that would have lost their atmospheres during their migration towards the star. The bulk viscosity of the cores of giant planets can indeed be large enough to hold a very high density during geological timescales. This would make those planets a new kind of planets which, in return, would provide useful information on the interior structure of the gas giants.