An interstellar object is a body other than a star or substar located in interstellar space, and not gravitationally bound to a star. They may include asteroids and comets (or exocomets). Besides known comets within the Solar System, or known exocomets (extrasolar comets), at present, an interstellar comet can only be detected if it passes through the Solar System, and could be distinguished from an Oort cloud comet by its strongly hyperbolic trajectory (indicating that it is not gravitationally bound to the Sun). Until 2017, the most eccentric known object, C/1980 E1, only had an eccentricity of 1.057, far less eccentric than an interstellar comet would be expected to be.
The first discovered and only known interstellar object to date is ʻOumuamua (previously called C/2017 U1 and A/2017 U1). The object has an eccentricity of about 1.20. It was initially named C/2017 U1 because it was assumed to be a comet, and was renamed to A/2017 U1 after no cometary activity was found. After its interstellar nature was confirmed, it was renamed to 1I/ʻOumuamua – '1' because it is the first such object to be discovered, 'I' for interstellar, and "‘Oumuamua" is a Hawaiian word meaning "a messenger from afar arriving first".
Recent research suggests that (514107) 2015 BZ509 may be a captured interstellar object from 4.5 billion years ago due it being in a co-orbital motion with Jupiter and its retrograde orbit around the Sun.
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.
1I/2017 U1 (ʻOumuamua)Edit
A dim object was discovered on October 19, 2017 by the Pan-STARRS telescope, at an apparent magnitude of 20. The observations showed that it follows a strongly hyperbolic trajectory around the Sun at a speed greater than the solar escape velocity, in turn meaning that it is not gravitationally bound to the Solar System and likely to be an interstellar object. On October 25, the object was found to be completely asteroidal in nature and as a consequence has been designated as 1I/2017 U1, the first of a new class 'I' of astronomical objects. 1I/2017 U1 was named ʻOumuamua.
The lack of cometary activity from ʻOumuamua suggests an origin from the inner regions of whatever stellar system it came from, losing all surface volatiles within the frost line, much like the rocky asteroids, extinct comets and damocloids we know from our Solar System. This is only a suggestion, as ʻOumuamua might very well have lost all surface volatiles to eons of cosmic radiation exposure in interstellar space, developing a thick crust layer after it was expelled from its parent system.
ʻOumuamua has an eccentricity of 1.199, which is the highest eccentricity ever observed for any object in the Solar System by a wide margin.
In September 2018, astronomers described several possible home star systems from which 'Oumuamua, that was detected passing through the Solar System in October 2017, may have begun its interstellar journey.
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