1I/Oumuamua is the first confirmed interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered by Canadian physicist Robert Weryk using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii, on 19 October 2017, approximately 40 days after it passed its closest point to the Sun on 9 September. When it was first observed, it was about 33 million km (21 million mi; 0.22 AU) from Earth (about 85 times as far away as the Moon) and already heading away from the Sun.
Oumuamua is a small object estimated to be between long, with its width and thickness both estimated between . It has a red color, like objects in the outer Solar System. Despite its close approach to the Sun, it showed no signs of having a coma, the usual nebula around comets formed when they pass near the Sun. Further, it exhibited nongravitational acceleration, potentially due to outgassing or a push from solar radiation pressure. It has a rotation rate similar to the Solar System's asteroids, but many valid models permit it to be unusually more elongated than all but a few other natural bodies observed in the solar system. This feature raised speculation about its origin. Its light curve, assuming little systematic error, presents its motion as "tumbling" rather than "spinning", and moving sufficiently fast relative to the Sun that it is likely of extrasolar origin. Extrapolated and without further deceleration, its path cannot be captured into a solar orbit, so it will eventually leave the Solar System and continue into interstellar space. Its planetary system of origin and age are unknown.
Oumuamua is remarkable for its extrasolar origin, high obliqueness, and observed acceleration without an apparent coma. By July 2019, most astronomers concluded that it was a natural object, but its precise characterization is contentious given the limited time window for observation. While an unconsolidated object (rubble pile) would require Oumuamua to be of a density similar to rocky asteroids, a small amount of internal strength similar to icy comets would allow it to have a relatively low density. Proposed explanations of its origin include the remnant of a disintegrated rogue comet, or a piece of an exoplanet rich in nitrogen ice, similar to Pluto. On 22 March 2023, astronomers proposed the observed acceleration was "due to the release of entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body", consistent with Oumuamua being an interstellar comet, "originating as a planetesimal relic broadly similar to solar system comets".
Name
As the first known object of its type, Oumuamua presented a unique case for the International Astronomical Union, which assigns designations for astronomical objects. Originally classified as comet C/2017 U1, it was later reclassified as asteroid A/2017 U1 due to the absence of a coma. Once it was unambiguously identified as coming from outside the Solar System, a new designation was created: I, for Interstellar object. As the first object so identified, Oumuamua was designated 1I, with rules for the eligibility of objects for I-numbers and the names to be assigned to these interstellar objects yet to be codified. The object may be called 1I; 1I/2017 U1; 1I/Oumuamua; or 1I/2017 U1 (Oumuamua).
The name comes (), and reflects the way the object is like a scout or messenger sent from the distant past to reach out to humanity. It roughly translates to 'first distant messenger'. The first character (not a diacritic) is a Hawaiian okina, not an apostrophe, and is pronounced as a glottal stop; the Pan-STARRS team chose the name in consultation with Kaiu Kimura and Larry Kimura of the University of Hawaii at Hilo.
Before the official name was decided, Rama was suggested, the name given to an alien spacecraft discovered under similar circumstances in the 1973 science fiction novel Rendezvous with Rama by Arthur C. Clarke.
Observations Observations and conclusions concerning Oumuamua's trajectory were primarily obtained with data from the Pan-STARRS1 Telescope, part of the Spaceguard Survey, and the Canada–France–Hawaii Telescope (CFHT), and its composition and shape from the Very Large Telescope and the Gemini South telescope in Chile, and the Keck II telescope in Hawaii. These were collected by Karen J. Meech, Robert Weryk, and their colleagues and published in Nature on 20 November 2017. After the announcement, the space-based telescopes Hubble and Spitzer joined in the observations.
Oumuamua is small and not very luminous. It was not seen in STEREO HI-1A observations near its perihelion on 9 September 2017, limiting its brightness to approximately 13.5 mag. By the end of October, it had already faded to about apparent magnitude 23, and in mid-December 2017, it was too faint and fast-moving to be studied by even the largest ground-based telescopes.
Oumuamua was compared to the fictional alien spacecraft Rama due to its interstellar origin. Adding to the coincidence, both the real and the fictional objects are unusually elongated. Oumuamua has a reddish hue and unsteady brightness, which are typical of asteroids.
The SETI Institute's radio telescope, the Allen Telescope Array, examined Oumuamua, but detected no unusual radio emissions. More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were performed; the data were searched for narrowband signals and none were found. Given the close proximity to this interstellar object, limits were placed to putative transmitters with the extremely low effective isotropically radiated power of 0.08 watts.
Trajectory
Oumuamua appears to have come from roughly the direction of Vega in the constellation Lyra. Its inbound direction of motion is 6° from the solar apex (the direction of the Sun's movement relative to local stars), the most likely direction from which objects from outside the Solar System would come. On 26 October, two precovery observations from the Catalina Sky Survey were found dated 14 and 17 October. A two-week observation arc had verified a strongly hyperbolic trajectory. It has a hyperbolic excess velocity (velocity at infinity, ) of , its speed relative to the Sun when in interstellar space.
+ Oumuamua speed relative to the Sun Distance Date Velocity(km/s) 2300 AU 1606 26.41 1000 AU 1839 26.42 100 AU 2000 26.73 10 AU 2016 29.56 1 AU 9 August 2017 49.70Inbound 1 AU (passing Earth's orbit) Perihelion 9 September 2017 87.71 1 AU 10 October 2017 49.70 10 AU 2019 29.58 100 AU 2034 26.73 1000 AU 2195 26.44 2300 AU 2429 26.40Outbound 2300 AU in 2429
By mid-November, astronomers were certain that it was an interstellar object. Based on observations spanning 80 days, Oumuamua's orbital eccentricity is 1.20, the highest ever observed until 2I/Borisov was discovered in August 2019. An eccentricity exceeding 1.0 means an object exceeds the Sun's escape velocity, is not bound to the Solar System, and may escape to interstellar space. While an eccentricity slightly above 1.0 can be obtained by encounters with planets, as happened with the previous record holder, C/1980 E1, Oumuamua's eccentricity is so high that it could not have been obtained through an encounter with any of the planets in the Solar System. Even undiscovered planets in the Solar System cannot account for Oumuamua's trajectory or boost its speed to the observed value. For these reasons, it can only be of interstellar origin.
+ Inbound velocity at 200 AU from the Suncompared to Oort cloud objects Object Velocity(km/s) # of observations and obs arc 90377 Sedna 2.66 577 in 12512 days C/2010 X1 (Elenin) 2.96 2222 in 235 days C/1980 E1 (Bowell) 2.98 187 in 2514 days C/1997 P2 (Spacewatch) 2.99 94 in 49 days C/2012 S1 (ISON) 2.99 6514 in 784 days 1I/2017 U1 (Oumuamua) 26.55 207 in 80 days 2I/Borisov 32.43 3249 in 657 days
Oumuamua entered the Solar System from north of the plane of the ecliptic. The pull of the Sun's gravity caused it to speed up until it reached its maximum speed of as it passed south of the ecliptic on 6 September, where the Sun's gravity bent its orbit in a sharp turn northward at its closest approach (perihelion) on 9 September at a distance of from the Sun, i.e., about 17% closer than Mercury's closest approach to the Sun. It is now heading away from the Sun toward Pegasus, toward a vanishing point 66° from the direction of its approach.
On the outward leg of its journey through the Solar System, Oumuamua passed beyond the orbit of Earth on 14 October with a closest approach distance of approximately from Earth. On 16 October it moved back north of the ecliptic plane and passed beyond the orbit of Mars on 1 November. It passed beyond Jupiter's orbit in May 2018, beyond Saturn's orbit in January 2019, and beyond Neptune's in 2022. As it leaves the Solar System it will be approximately right ascension 23'51" and declination +24°42', in Pegasus. It will continue to slow down, approaching a speed of relative to the Sun, the same speed it had before its approach to the Solar System.
Non-gravitational acceleration On 27 June 2018, astronomers reported a non-gravitational acceleration to Oumuamua's trajectory, potentially consistent with a push from solar radiation pressure. The resulting change in velocity during the period when it was near its closest approach to the Sun summed to about 17 meters per second. Initial speculation as to the cause of this acceleration pointed to the comet-like outgassing, whereby volatile substances inside the object evaporate as the Sun heats its surface. Although no such tail of gasses was observed following the object, researchers estimated that enough outgassing may have increased the object's speed without the gases being detectable. A critical reassessment of the outgassing hypothesis argued that, instead of the observed stability of Oumuamua's spin, outgassing would have caused its spin to rapidly change due to its elongated shape, resulting in the object tearing apart.
Indications of origin Accounting for Vega's proper motion, it would have taken Oumuamua 600,000 years to reach the Solar System from Vega, although it was not in the same part of the sky at that time. Astronomers calculate that 100 years ago the object was from the Sun and traveling at 26.33 km/s with respect to the Sun. This interstellar speed is very close to the mean motion of material in the Milky Way in the neighborhood of the Sun, also known as the local standard of rest (LSR), and especially close to the mean motion of a relatively close group of red dwarf stars. This velocity profile also indicates an extrasolar origin, but appears to rule out the closest dozen stars. In fact, the closeness of Oumuamua's velocity to the local standard of rest might mean that it has circulated the Milky Way several times and thus may have originated from an entirely different part of the galaxy.