Interstellar comet

An interstellar object is an astronomical object, other than a star or substar, that is located in interstellar space and is not gravitationally bound to a star. The term can also be applied to objects that are on an interstellar trajectory but are temporarily passing close to a star, such as certain asteroids and comets (including exocomets). An interstellar object is an astronomical object, other than a star or substar, that is located in interstellar space and is not gravitationally bound to a star. The term can also be applied to objects that are on an interstellar trajectory but are temporarily passing close to a star, such as certain asteroids and comets (including exocomets). Due to present observational difficulties, an interstellar object can usually only be detected if it passes through the Solar System, where it can be distinguished by its strongly hyperbolic trajectory, indicating that it is not gravitationally bound to the Sun. In contrast, gravitationally bound objects follow elliptic orbits around the Sun, like most asteroids, comets, and objects in the Oort cloud. 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 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 headed for interstellar space. The first discovered, and to date only known, interstellar object is ʻOumuamua (1I/ʻOumuamua, previously called C/2017 U1 and A/2017 U1). The object has an orbital eccentricity of about 1.20, indicating it did not originate from the Solar System. Recent research suggests that asteroid 514107 Kaʻepaokaʻawela may be a former interstellar object, captured some 4.5 billion years ago, as evidenced by its co-orbital motion with Jupiter and its retrograde orbit around the Sun. In addition, comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) has a non-negligible probability (0.726) of having an extrasolar provenance although an origin in the Oort cloud cannot be excluded. With the first discovery of an interstellar object, the IAU has proposed a new series of small-body designations for interstellar objects, 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. 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. If interstellar comets exist, they must occasionally pass through the inner Solar System. They would approach the Solar System 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 'Oumuamua, 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. 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 'Oumuamua, 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.' An interstellar comet can probably, on rare occasions, be captured into a heliocentric orbit while passing through the Solar System. Computer simulations show that Jupiter is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years. Comets Machholz 1 and Hyakutake C/1996 B2 are possible examples of such comets. They have atypical chemical makeups for comets in the Solar System.