Interstellar object
Updated: Wikipedia source
An interstellar object is an astronomical object in interstellar space, not gravitationally bound to a star. The term is used for objects including some asteroids, some comets, and rogue planets, but not stars or stellar remnants. The interstellar objects were once bound to a host star and have become unbound since. Different processes can cause planets and smaller objects (planetesimals) to become unbound from their host star. This term is also applied to an object that is on an interstellar trajectory but is temporarily passing close to a star, such as some asteroids and comets (that is, exoasteroids and exocomets). In this case the object may be called an interstellar interloper. Objects observed within the solar system are identified as interstellar interlopers due to possessing significant hyperbolic excess velocity, indicating they did not originate in the solar system. The first interstellar objects discovered were rogue planets, ejected from their original stellar system (e.g., OTS 44 or Cha 110913−773444), though they are difficult to distinguish from sub-brown dwarfs, planet-mass objects that formed in interstellar space as stars do. As of 2025, three interstellar objects have been discovered traveling through the Solar System: 1I/ʻOumuamua in 2017, 2I/Borisov in 2019, and 3I/ATLAS in 2025. The prefix "1I" identifies the object as the first confirmed interstellar interloper, "2I" as the second, and so on. There has been speculation that interstellar interlopers observed in the Solar System are extraterrestrial spacecraft, but there is currently no evidence for such claims to be plausible.