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Interstellar object
An interstellar object is an astronomical object in interstellar space, not gravitationally bound to a star. The term is used for objects including asteroids, 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[update], 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 insufficient evidence for such claims to be plausible.
With the first discovery of an interstellar object in the Solar System, the IAU has proposed a new series of small-body designations for interstellar interlopers, the I numbers, similar to the comet numbering system. The Minor Planet Center will assign the numbers. Provisional designations for interstellar objects will be handled using the C/ or A/ prefix (comet or asteroid), as appropriate.
Astronomers estimate that several interstellar objects of extrasolar origin (like ʻOumuamua) pass inside the orbit of Earth each year, and that 10,000 are passing inside the orbit of Neptune on any given day. Interstellar comets occasionally pass through the inner Solar System and approach with random velocities, mostly from the direction of the constellation Hercules because the Solar System is moving in that direction, called the solar apex. Until the discovery of ʻOumuamua, the fact that no comet with a speed greater than the Sun's escape velocity had been observed was used to place upper limits to their density in interstellar space. A paper by Torbett indicated that the density was no more than 1013 (10 trillion) comets per cubic parsec.[dubious – discuss] Other analyses, of data from LINEAR, set the upper limit at 4.5×10−4/AU3, or 1012 (1 trillion) comets per cubic parsec. A more recent estimate by David C. Jewitt and colleagues, following the detection of ʻOumuamua, predicts that "The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~1×104, each with a residence time of ~10 years."
Current models of Oort cloud formation predict that more comets are ejected into interstellar space than are retained in the Oort cloud, with estimates varying from 3 to 100 times as many. Other simulations suggest that 90–99% of comets are ejected. There is no reason to believe comets formed in other star systems would not be similarly scattered. Amir Siraj and Avi Loeb demonstrated that the Oort Cloud could have been formed from ejected planetesimals from other stars in the Sun's birth cluster. Both researchers proposed a search for ʻOumuamua-like objects which are trapped in the Solar System as a result of losing orbital energy through a close encounter with Jupiter.
It is possible for objects orbiting a star to be ejected due to interaction with a third massive body, thereby becoming interstellar objects. Such a process was initiated in the early 1980s when C/1980 E1, initially gravitationally bound to the Sun, passed near Jupiter and was accelerated sufficiently to reach escape velocity from the Solar System. This changed its orbit from elliptical to hyperbolic and made it the most eccentric known object at the time, with an eccentricity of 1.057. It is heading for interstellar space.
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Interstellar object
An interstellar object is an astronomical object in interstellar space, not gravitationally bound to a star. The term is used for objects including asteroids, 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[update], 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 insufficient evidence for such claims to be plausible.
With the first discovery of an interstellar object in the Solar System, the IAU has proposed a new series of small-body designations for interstellar interlopers, the I numbers, similar to the comet numbering system. The Minor Planet Center will assign the numbers. Provisional designations for interstellar objects will be handled using the C/ or A/ prefix (comet or asteroid), as appropriate.
Astronomers estimate that several interstellar objects of extrasolar origin (like ʻOumuamua) pass inside the orbit of Earth each year, and that 10,000 are passing inside the orbit of Neptune on any given day. Interstellar comets occasionally pass through the inner Solar System and approach with random velocities, mostly from the direction of the constellation Hercules because the Solar System is moving in that direction, called the solar apex. Until the discovery of ʻOumuamua, the fact that no comet with a speed greater than the Sun's escape velocity had been observed was used to place upper limits to their density in interstellar space. A paper by Torbett indicated that the density was no more than 1013 (10 trillion) comets per cubic parsec.[dubious – discuss] Other analyses, of data from LINEAR, set the upper limit at 4.5×10−4/AU3, or 1012 (1 trillion) comets per cubic parsec. A more recent estimate by David C. Jewitt and colleagues, following the detection of ʻOumuamua, predicts that "The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~1×104, each with a residence time of ~10 years."
Current models of Oort cloud formation predict that more comets are ejected into interstellar space than are retained in the Oort cloud, with estimates varying from 3 to 100 times as many. Other simulations suggest that 90–99% of comets are ejected. There is no reason to believe comets formed in other star systems would not be similarly scattered. Amir Siraj and Avi Loeb demonstrated that the Oort Cloud could have been formed from ejected planetesimals from other stars in the Sun's birth cluster. Both researchers proposed a search for ʻOumuamua-like objects which are trapped in the Solar System as a result of losing orbital energy through a close encounter with Jupiter.
It is possible for objects orbiting a star to be ejected due to interaction with a third massive body, thereby becoming interstellar objects. Such a process was initiated in the early 1980s when C/1980 E1, initially gravitationally bound to the Sun, passed near Jupiter and was accelerated sufficiently to reach escape velocity from the Solar System. This changed its orbit from elliptical to hyperbolic and made it the most eccentric known object at the time, with an eccentricity of 1.057. It is heading for interstellar space.