page_title: Interstellar object
section_title: Overview
hierarchical_section_title: Interstellar object / Overview
caption_attribution_description: Comet 96P/Machholz from HI-2 camera of STEREO-A spacecraft
context_page_description: An interstellar object is an astronomical object that is located in interstellar space and is not gravitationally bound to a star. This term can also be applied to an object that is on an interstellar trajectory but is temporarily passing close to a star, such as certain asteroids and comets. In the latter case, the object may be called an interstellar interloper.
  The first interstellar object to be discovered in the Solar System was 1I/ʻOumuamua in 2017. The second was 2I/Borisov in 2019. They both possess significant hyperbolic excess velocity, indicating they did not originate in the Solar System.
context_section_description: 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.
  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."
  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.
  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.
  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 and hyperbolic excess velocity of more than a few km/s, proving that it is not gravitationally bound to the Sun. In contrast, gravitationally bound objects follow elliptic orbits around the Sun. (There are a few objects whose orbits are so close to parabolic that their gravitationally bound status is unclear.)
  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.
  On 27 November 2018, Loeb and his undergraduate student at Harvard University, Amir Siraj, 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. They identified centaur candidates, such as 2017 SV₁₃ and 2018 TL₆, as captured interstellar objects that could be visited by dedicated missions. The authors pointed out that future sky surveys, such as with LSST, should find many candidates.
  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.Harvard astronomers suggest that matter—and potentially dormant spores—can be exchanged across vast distances. The detection of ʻOumuamua crossing the inner Solar 
caption: Comet Machholz 1 (96P/Machholz) as viewed by STEREO-A (April 2007)
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