Illustration of a planet formed outside of the soot line in a young stars protoplanetary, or planet-forming, disc. The soot line is the distance from a star where it is cool enough for carbon to be a solid. Planets formed between the discs soot line and ice line will be silicate and soot-rich. These planets are known as sooty worlds. The atmosphere of sooty worlds is methane (CH4) rich. This leads to the formation of hazes as sunlight interacts with the methane. Carbon is a fundamental building block of life and therefore carbon-rich planets such as sooty worlds could harbour life.