A debris disk (American English), or debris disc (Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks are found around stars with mature planetary systems, including at least one debris disk in orbit around an evolved neutron star. Debris disks can also be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets. As of 2001, more than 900 candidate stars had been found to possess a debris disk. They are usually discovered by examining the star system in infrared light and looking for an excess of radiation beyond that emitted by the star. This excess is inferred to be radiation from the star that has been absorbed by the dust in the disk, then re-radiated away as infrared energy. Debris disks are often described as massive analogs to the debris in the Solar System. Most known debris disks have radii of 10–100 astronomical units (AU); they resemble the Kuiper belt in the Solar System, although the Kuiper belt does not have a high enough dust mass to be detected around even the nearest stars. Some debris disks contain a component of warmer dust located within 10 AU from the central star. This dust is sometimes called exozodiacal dust by analogy to zodiacal dust in the Solar System. Observation history In 1984 a debris disk was detected around the star Vega using the IRAS satellite. Initially this was believed to be a protoplanetary disk, but it is now known to be a debris disk due to the lack of gas in the disk and the age of the star. The first four debris disks discovered with IRAS are known as the "fabulous four": Vega, Beta Pictoris, Fomalhaut, and Epsilon Eridani. Subsequently, direct images of the Beta Pictoris disk showed irregularities in the dust, which were attributed to gravitational perturbations by an unseen exoplanet. That explanation was confirmed with the 2008 discovery of the exoplanet Beta Pictoris b. Other exoplanet-hosting stars, including the first discovered by direct imaging (HR 8799), are known to also host debris disks. The nearby star 55 Cancri, a system that is also known to contain five planets, also was reported to have a debris disk, but that detection could not be confirmed. Structures in the debris