Different planetary bulk compositions (Zeng & Seager 2008): 100% Fe (blue line), 50% Fe (orange line), Earth-like composition (green line), 50% water in mass (red line), and 100% water in mass (violet line), in comparison with the sample of low-mass close-in exoplanets around M dwarfs with masses and radius measurements (black dots) together with some simulated planets from different formation scenarios (the radius derived from the amount of water in mass calculated during the accretion process).