Proplyd

A proplyd, a syllabic abbreviation of an ionized protoplanetary disk, is an externally illuminated photoevaporating disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to Earth. In the Orion Nebula the proplyds observed are usually one of two types. Some proplyds glow around luminous stars, in cases where the disk is found close to the star, glowing from the star's luminosity. Other proplyds are found at a greater distance from the host star and instead show up as dark silhouettes due to the self-obscuration of cooler dust and gases from the disk itself. Some proplyds show signs of movement from solar irradiance shock waves pushing the proplyds. The Orion Nebula is approximately 1,500 light-years from the Sun with very active star formation. The Orion Nebula and the Sun are in the same spiral arm of the Milky way Galaxy.

A proplyd may form new planets and planetesimal systems. Current models show that the metallicity of the star and proplyd along with the correct planetary system temperature and distance from the star are keys to planet and planetesimal formation. To date, the solar system, with 8 planets, 5 dwarf planets and 5 planetesimal systems, is the largest planetary system found. Most proplyds develop into a system with no planetesimal systems, or into one very large planetesimal system.

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