Looking at the Solar System, there seem to be two basic types of planets.
The smaller planets, including Earth, are dense, lower mass, and composed of rock. The larger worlds—Jupiter and the other giants—are massive, made of compressed gas, and possess no surface to speak of. As we learn about exoplanets orbiting distant stars, those two basic categories seem to hold. However, as astronomers map the landscape of planets, they are discovering worlds that don’t fit what we once thought, and which suggest a richer galaxy of possibilities.
The most dramatic of these so far is a mega-Earth: a world about 17 times more massive than Earth. That’s approximately the same mass as the giant Neptune, yet this planet is denser than any other yet discovered, meaning it must be made of rock. That’s puzzling: based on the theory of planet formation, no rocky world should get that huge.
The same group of astronomers has also inferred a type of exoplanet that fits in between the rocky planets and the gas giants. These “gas dwarf” worlds seem to have rocky interiors, but thick atmospheres like Neptune’s. While this proposition is less head scratching than the mega-Earth, it fills in the gap between the largest rocky planets and the smallest gas giants. This type of planet may be fairly common, even though we have nothing in the Solar System like it.