Editing Protoplanetary disk (section)

==Debris disks==
Gas-poor disks of circumstellar dust have been found around many nearby stars—most of which have ages in the range of ~10 million years (e.g. [[Beta Pictoris]], [[51 Ophiuchi]]) to billions of years (e.g. [[Tau Ceti]]). These systems are usually referred to as "[[debris disks]]". Given the older ages of these stars, and the short lifetimes of micrometer-sized dust grains around stars due to [[Poynting Robertson drag]], collisions, and [[radiation pressure]] (typically hundreds to thousands of years), it is thought that this dust is from the collisions of planetesimals (e.g. [[asteroids]], [[comets]]). Hence the [[debris disks]] around these examples (e.g. [[Vega]], [[Alphecca]], [[Fomalhaut]], etc.) are not "protoplanetary", but represent a later stage of disk evolution where extrasolar analogs of the [[asteroid belt]] and [[Kuiper belt]] are home to dust-generating collisions between planetesimals.