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Illustration of a planet (lower right) formed outside of the soot line in a young star's protoplanetary, or planet-forming, disc. Planets formed interior to the soot line will be silicate-rich, like the Earth. Planets formed between the soot line and ice line will be silicate and soot-rich. These planets are known as sooty worlds. Planets formed outside of the ice line will be water worlds. The soot line is the distance from a star where it is cool enough for carbon to be a solid. The atmosphere of sooty worlds (top right) 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. (KEYSTONE/SCIENCE PHOTO LIBRARY/NICOLLE R. FULLER)