ʻOumuamua

The first messenger from distant stars

 

1I/ʻOumuamua, imaged with the 4.2 meter William Herschel Telescope in the Canary Islands on 28 October 2017, is seen as a point of light in the centre of the image. Background stars appear linear because the telescope is tracking the moving object.
Credit: Alan Fitzsimmons (Astrophysics Research Centre, Queen's University Belfast), Isaac Newton Group

1I/ʻOumuamua (formally designated 1I/2017 U1), the first known interstellar object detected passing through the Solar System, was discovered by Canadian astronomer Robert Weryk (b. 1981) on October 19, 2017 at Haleakalā Observatory, Hawaii, where the Pan-STARRS1 telescope system detected an object moving rapidly west at 6.2 degrees per day. A search of images from the previous nights found the object had also been imaged on October 18. Additional images acquired with the Canada-France-Hawaii Telescope (CFHT) on October 22 confirmed that this object is unique, with the highest known hyperbolic eccentricity. The astronomical community was immediately notified and many observatories rushed to observe it. Unfortunately, it was already on its way out of the solar system, having passed closest to the Sun on September 9, 2017. Because of its rapid motion, there was only a short interval during which observations were possible. Within a week the brightness had dropped by a factor of 10 and within a month by a factor of 100. The object was last seen with the Hubble Space Telescope (HST) on January 2, 2019.

   As the first known object of its type, ʻOumuamua 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, it was designated 1I. Given its discovery and follow-up observations from multiple Hawai‘i observatories, 1I/2017 U1 has been named ‘Oumuamua, which in Hawaiian reflects the way this object is like a scout or messenger sent from the distant past to reach out to us.

 

[a] Pan-STARRS1 discovery image of ‘Oumuamua on 2017 October 19. ‘Oumuamua is the faint trail centered in the circle. Red regions are masked pixels. [b] CFHT image obtained on October 22 showing no hint of coma. [c] Deep image combining Gemini and VLT g and r-band data. The black dots mark the flux in individual pixels. The red dots show the average flux in annuli at each radius.
Source: Karen J. Meech, Robert Weryk et al. A brief visit from a red and extremely elongated interstellar asteroid. Nature. 2017 Dec 21; 552(7685): 378–381

   ʻOumuamua entered the Solar System from north of the plane of the ecliptic, roughly from the direction of Vega in the constellation Lyra. It had a hyperbolic excess velocity (velocity at infinity) of 26.33 km/s – its speed relative to the Sun when in interstellar space. Based on observations spanning 80 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed until 2I/Borisov was discovered in August 2019. The pull of the Sun's gravity caused it to speed up until it reached its maximum speed of 87.71 km/s as it passed south of the ecliptic on September 6, where the Sun's gravity bent its orbit in a sharp turn northward at its closest approach (perihelion) on September 9 at a distance of 0.256 au. ʻOumuamua had its closest approach to Earth at 0.162 au (24.2 million km) on October 14. When it was first observed, it was already about 33 million km (0.22 au) from Earth and heading away from the Sun.

  ‘Oumuamua passed the distance of Jupiter’s orbit in early May 2018 and Saturn’s orbit January 2019. It reached a distance corresponding to Uranus’ orbit in August 2020 and the distance of Neptune's orbit in late June 2024. In late 2025 ‘Oumuamua will reach the outer edge of the Kuiper Belt, and then the heliopause — the edge of the Solar System — in November 2038. It will continue to slow down until it reaches a speed of 26.33 kilometres per second relative to the Sun, the same speed it had before its approach to the Solar System.

 

1I/’Oumuamua’s trajectory as it entered the inner solar system (dashed line indicates the section that lies below the ecliptic plane). The open circles show the position of the planets at the time of 1I/’Oumuamua’s discovery. Based on a figure by Matthew Twombly for Jewitt & Moro-Mart´ın (2020)

   The average brightness measured in visible wavelengths during the week after its discovery gave H_V = 22.4. Studies based on visible/near-infrared observations lead to an effective radius in the range of 55–130 m. Spitzer Space Telescope observations in the infrared on November 21–22 did not detect ‘Oumuamua. Relatively few minor bodies this small have been as well characterized physically, which hampers aspects of direct comparison of ‘Oumuamua with similar objects from the Solar System. ‘Oumuamua is red, similar to many Solar System small bodies, e.g., comets, D-type asteroids, some Jupiter Trojans, and the more neutral trans-Neptunian objects. While the color is consistent with organic-rich surfaces, it is also consistent with iron-rich minerals, and with space weathered surfaces. 

   ‘Oumuamua exhibited short-term brightness variation of over a factor of ten (>2.5 magnitudes) every 4 hours with a rotation period of about 8 hours. Of the minor planets in our Solar System with well-quantified light curves, there are only a handful of asteroids with brightness variations of this scale. This was interpreted as evidence of a morphology that was unusually elongated, with an axis ratio ranging from 3 to 10, or oblate. There are currently two possibilities that seem most likely: a pancake-shape with axis ratios of roughly 6:6:1, or a cigar-shape with axis ratios of ~8:1:1. The pancake shape is more likely because there is a higher chance of seeing a tumbling pancake edge-on compared with seeing a cigar’s shape tip-on, and ‘Oumuamua gets very dim more often than the cigar shape would suggest.

 

This very deep combined image shows the interstellar asteroid ‘Oumuamua at the centre of the picture. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving asteroid. This image was created by combining multiple images from ESO’s Very Large Telescope as well as the Gemini South Telescope. The object is marked with a blue circle and appears to be a point source, with no surrounding dust.
Credit: ESO/K. Meech et al.

   Upon close inspection of 1I/’Oumuamua’s outgoing orbit, it was discovered that the object was experiencing a non-gravitational acceleration, potentially due to outgassing or a push from solar radiation pressure. The data were fit with the addition of a radial acceleration varying as 1/r², where r is the heliocentric distance. This type of acceleration is usually interpreted as being due to an activity-driven cometary acceleration consistent with the decreasing energy with distance from the Sun. The mass loss needed to explain ‘Oumuamua’s observed non-gravitational acceleration is on the order of 1 kg s⁻¹. 

 

   However, despite its close approach to the Sun, Oumuamua showed no signs of having a coma. Sensitive searches for activity showed no evidence for micron-sized dust near ‘Oumuamua. However, the observations were not sensitive to the detection of millimeter-sized and larger dust. There was also no detection of any gas, including searches for CN, H₂O, CO and CO₂.

 

Artist’s view of the potential, pancake-like shape of ‘Oumuamua. Credit: William K. Hartmann

Artist’s impression of the potential, cigar shape of `Oumuamua. Credit: ESO/M. Kornmesser

   A number of processes have been invoked to explain ‘Oumuamua’s origins and peculiarities since its discovery. These models generally expect ‘Oumuamua or its parent body to have been born as a planetary building block – a planetesimal – in a gas-dominated protoplanetary disk around a young star, and ejected from its home system. It have been suggested that 1I/’Oumuamua’s acceleration was indeed due to outgassing from a new type of body made of molecular hydrogen ice, a cosmic hydrogen iceberg that originated in the starless coldest regions of a molecular cloud, or that 1I/’Oumuamua was made of N₂ ice, a fragment from an exo-Pluto surface. However, there are are theoretical and/or observational inconsistencies with existing models invoking the sublimation of pure H₂, N₂, and CO.

 

   In 2023, it was proposed the observed non-gravitational acceleration and spectrum of ʻOumuamua can be best explained by hydrogen outgassing from the water ice matrix. The buildup of the hydrogen in the water ice is expected to happen in the interstellar comets, due to low-temperature water ice radiolysis by cosmic ray particles in interstellar space. In spite of many attempts to trace the orbit of ‘Oumuamua back to its home system, or star cluster, no convincing candidate origin star systems or stellar associations have been identified.

   A small number of astronomers, led by Avi Loeb (b. 1962), suggested that ʻOumuamua could be a product of alien technology, like a lightsail, but there is no evidence in support of this hypothesis, and such a planar geometry is inconsistent with the shape and amplitude of ’Oumuamua’s light-curve. The SETI Institute's radio telescope, the Allen Telescope Array, and the Breakthrough Listen hardware and the Green Bank Telescope observed ʻOumuamua, but detected no unusual radio emissions.

 

 

References:

The ‘Oumuamua ISSI Team. The natural history of ‘Oumuamua. Nature Astronomy, Volume 3, pages 594–602 (2019). [full PDF]
Amaya Moro-Martín. Interstellar Planetesimals. Planetary Systems Now, pp. 333-379 (2023) [full PDF]
Jason T. Wright, Steven Desch, Sean Raymond. ‘Oumuamua: Natural or Artificial? Medium. July 18, 2023
NASA Science: ‘Oumuamua

 

© 2025, Andrew Mirecki