An interstellar object is a body other than a star or substellar object located in interstellar space, and not gravitationally bound to a star. They may include asteroids or comets (or exocomets). Besides known comets within the Solar System, or known 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 C/2017 U1—PANSTARRS, and A/2017 U1). The object has an eccentricity of about 1.195. It was initially named C/2017 U1 because it was assumed to be a comet, but 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".
With the first discovery of an interstellar object, the IAU is proposing 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, by a factor of 3–100. 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 region of the constellation Hercules because the Solar System is moving in that direction. 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.
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.
A dim object was discovered on October 19, 2017 by the Pan-STARRS telescope, at an apparent magnitude of 20. It has been designated as 1I/ʻOumuamua. The observations indicate that it is on a strongly hyperbolic orbit, in turn meaning that it is not gravitationally bound to the Solar System and likely to be an interstellar object.
It was found to be completely asteroidal in nature on October 25. The non-cometary nature of this object suggests an origin from the inner regions of whatever stellar system it came from, or that it had spent eons making perihelion passages close to the host star and has become an extinct comet. It has an eccentricity of 1.193, which is the highest eccentricity ever observed for any solar system object by a wide margin.
- ʻOumuamua, the first interstellar object detected passing through the Solar System
- List of Solar System objects by greatest aphelion
- Rogue planet
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