Abstract: Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii (R(sub ?)) range from low-density sub-Neptunes containing volatile elements to higher-density rocky planets with Earth-like or iron-rich (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process or different evolutionary paths that altered the planetary properties after …