Planets smaller than Neptune are ubiquitous in the Galaxy and those around M stars constitute the bulk of warm and temperate worlds amenable for detailed atmospheric characterization. In this talk, we present a re-analysis of all the available data on small transiting planets around M dwarfs, refining their masses and radii (Luque & Pallé 2022, in press). Our precisely characterized sample reveals that this population is well described by only three discrete planet density populations, with bulk densities centered at 1.0, 0.5 and 0.24 relatively to Earth's. The first are rocky planets, the second are water worlds, and the third are puffy planets with Neptune-like densities. This density classification offers a much better insight to disentangle planet formation and evolution mechanisms, which are degenerate when using a radius-based classification. Our results are at odds with atmospheric mass-loss models aiming to explain the bimodal radius distribution and suggest that the gap separates dry from water worlds rather than rocky planets with or without H/He envelopes. Formation models including type I migration explain naturally the observations independently of the accretion mechanism: rocky planets form within the snow line, water worlds form beyond and later migrate inwards. These results are to be published in Science and are currently under embargo.