The planets in the Solar System belong to three groups:
Terrestrial — Mercury, Venus, Earth, and Mars — mostly rock and iron and between 0.4 and 1.0 Earth-radii.
Gas Giants — Jupiter and Saturn — mostly hydrogen and helium and about 10 Earth-radii.
Ice Giants — Uranus and Neptune — mostly rock and ice with a modest envelope of hydrogen/helium and about 4 Earth-radii.
Are these familiar planets representative of planets around other stars or does nature produce classes of planets not found in our Solar System?
Thanks to the discovery of extrasolar planets, we now know that nature creates planets that are wildly different than the Solar System planets. One of the most powerful aspects of exoplanet research is the ability to study types of planets not represented in our own Solar System.
Our Sun hosts no planets between Uranus and Saturn size (4.0–9.4 Earth-radii). However, we have found examples of these "sub-Saturns" around other stars. I am leading the Caltech Sub-Saturn Survey, a campaign to find sub-Saturns with K2 and to measure their masses using the Doppler technique (Petigura et al. 2017). Once we know a planet's mass and radius, we can constrain it's density and bulk composition.
Surprisingly, these sub-Saturns span a wide range of densities. Some are as dense as concrete (2 g/cc), while others have the density of styrofoam (0.07 g/cc). This diversity in density implies varying amounts of low-density hydrogen/helium in their envelopes. This may be the result of differences in where these planets formed in their protoplanetary disks and whether they suffered a massive collision/merger early on in their lifetimes.
Saturn (top) and Uranus (bottom) are 4.0 and 9.4 Earth-radii, respectively. The Solar System lacks any planets of intermediate sizes. Why?