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As of August 2016, there are around 3,400 confirmed exoplanets in 2,617 systems, with 590 systems having multiple planets. The composition of an exoplanet is determined by its protoplanetary disk, and its size is limited by the boundaries of 1.7 and 3.9 Earth radii. Exoplanets below 1.7 Earth radii are primarily composed of non-volatiles, such as rock, while those between 1.7 and 3.9 Earth radii are gas dwarfs with a mix of non-volatiles and hydrogen/helium atmospheres. Above 3.9 Earth radii, exoplanets are primarily composed of hydrogen and helium, and are considered gas giants. The metallicity of a stellar system determines the types of planets that can form, with high metallicity systems having a greater chance of forming Earth-like planets, super-Earths, and mini-Earths. Examples of exoplanets include ice planets like OGLE-2005-BLG-390Lb, Chthonian planets like COROT 7b, ocean planets like Gliese 1214 b, and carbon planets with carbon-based mineral matrices.