Description: When a cosmic gas cloud collapses to form a star, that nascent star is surrounded by a swirling disk of gas and dust. This is a protoplanetary disk, where planets form: Ice-covered dust particles will stick to each other, forming slightly larger clumps, which will continue to grow. There are still open questions about how the next stages happen: What, for instance, is the role of turbulent gas motion in bringing those clumps closer together? Eventually, so-called planetesimals more than about a kilometer in size are formed. Some of those are pulled together by their own gravity to form larger planets, others remain behind as the first asteroids. Some protoplanets manage to draw large amounts of gas towards them to become gas giants. Some other protoplanets in cold regions far from the central star will accrete large amounts of frozen material along with gas, becoming ice giants. Others, with less gas, become terrestrial planets.

Related Terms:

• Disk
• Dust
• Gas Giant
• Ice Giant
• Planet
• Protostar
• Terrestrial Planet

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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