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'Oumuamua — the first object known to originate outside our solar system

Discovery / Explanations of Oumuamua    References

The debate over the origins and molecular structure of 'Oumuamua continued today with an announcement in The Astrophysical Journal Letters that despite earlier promising claims, the interstellar object is not made of molecular hydrogen ice after all. 1) 2)


Figure 1: An artist's rendering of 'Oumuamua, a visitor from outside the solar system (image credit: The international Gemini Observatory/NOIRLab/NSF/AURA artwork by J. Pollard)

The earlier study, published by Seligman & Laughlin in 2020—after observations by the Spitzer Space Telescope set tight limits on the outgassing of carbon-based molecules—suggested that if 'Oumuamua were a hydrogen iceberg, then the pure hydrogen gas that gives it its rocket-like push would have escaped detection. But scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Korea Astronomy and Space Science Institute (KASI) were curious whether a hydrogen-based object could actually have made the journey from interstellar space to our solar system.

"The proposal by Seligman and Laughlin appeared promising because it might explain the extreme elongated shape of ‘Oumuamua as well as the non-gravitational acceleration. However, their theory is based on an assumption that H2 ice could form in dense molecular clouds. If this is true, H2 ice objects could be abundant in the universe, and thus would have far-reaching implications. H2 ice was also proposed to explain dark matter, a mystery of modern astrophysics," said Dr. Thiem Hoang, senior researcher in the theoretical astrophysics group at KASI and lead author on the paper. "We wanted to not only test the assumptions in the theory but also the dark matter proposition." Dr. Avi Loeb, Frank B. Baird Professor of Science at Harvard and co-author on the paper, added, "We were suspicious that hydrogen icebergs could not survive the journey—which is likely to take hundreds of millions of years—because they evaporate too quickly, and as to whether they could form in molecular clouds."

Traveling at a blistering speed of 196,000 mph in 2017, 'Oumuamua was first classified as an asteroid, and when it later sped up, was found to have properties more akin to comets. But the 0.2 km radius interstellar object didn’t fit that category, either, and its point of origin has remained a mystery. Researchers focused on the giant molecular cloud (GMC) W51—one of the closest GMCs to Earth at just 17,000 light years away—as a potential point of origin for 'Oumuamua, but hypothesize that it simply could not have made the journey intact. "The most likely place to make hydrogen icebergs is in the densest environments of the interstellar medium. These are giant molecular clouds,” said Loeb, confirming that these environments are both too far away and are not conducive to the development of hydrogen icebergs.

An accepted astrophysical origin for solid objects is growth by sticky collisions of dust, but in the case of a hydrogen iceberg, this theory could not hold together. "An accepted route to form a km-sized object is first to form grains of micron-size, then such grains grow by sticky collisions," said Hoang. "However, in regions with high gas density, collisional heating by gas collisions can rapidly sublimate the hydrogen mantle on the grains, preventing them from growing further."

Although the study explored destruction of H2 ice by multiple mechanisms including interstellar radiation, cosmic rays, and interstellar gas, sublimation due to heating by starlight has the most destructive effect, and according to Loeb, "Thermal sublimation by collisional heating in GMCs could destroy molecular hydrogen icebergs of ‘Oumuamua-size before their escape into the interstellar medium." This conclusion precludes the theory that ‘Oumuamua journeyed to our solar system from a GMC, and further precludes the proposition of primordial snowballs as dark matter. Evaporative cooling in these situations does not reduce the role of thermal sublimation by starlight in the destruction of H2 ice objects.

Oumuamua first gained notoriety in 2017 when it was discovered screaming through space by observers at Haleakalā Observatory, and has since been the subject of ongoing studies. "This object is mysterious and difficult to understand because it exhibits peculiar properties we have never seen from comets and asteroids in our solar system," said Hoang.

While the nature of the interstellar traveler is currently an unsolved mystery, Loeb suggests it won’t remain so for much longer, especially if it’s not alone. "If 'Oumuamua is a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect roughly one 'Oumuamua-like object per month. We will all wait with anticipation to see what it will find."

Discovery and some explanations of the object 'Oumuamua

• April 13, 2020: A close encounter with another star may have torn the cigar-shaped interstellar visitor ‘Oumuamua from its parent body, flinging it toward our solar system. 3)

- One of the weirdest objects ever discovered in our solar system — the alien space rock ‘Oumuamua — is still sparking debates more than two years after its discovery. And, most recently, the conversation has shifted to whether the cosmic visitor could be a fragment ripped from a larger world.

- In 2017, the Pan-STARRS asteroid-hunting telescope in Hawaii spotted an object moving at a breathtaking pace of 54 miles (87 km/s) per second. But, unlike most comets and asteroids before it, ‘Oumuamua would only approach our Sun once before continuing its journey through space. This made it the first known interstellar object to have passed through our solar system.

- Scientists named the alien space rock ‘Oumuamua, which roughly translates as “messenger from afar arriving first” in Hawaiian. And though astronomers agree that ‘Oumuamua visited our solar system from another star, that’s about where the agreement ends.

- Is it an alien asteroid, an alien comet, or even an alien spaceship?

- Now, a pair of astronomers have used complex computer modeling to explain ‘Oumuamua’s combination of strange properties, finding the space rock may be a small shard ripped from a larger parent body.

- Their models suggest that when an object — anything from a comet to a super-Earth — passes too close to its star, intense tidal forces can cause heated fragments to slough off. These melted shards then refreeze, locking in their unique shapes.

- If these astronomers are right, then we should expect to spot plenty of oddly shaped worlds like ‘Oumuamua in the coming years.

- “We anticipate many more interstellar visitors with similar traits to ‘Oumuamua will be discovered by future observation,” says study author Yun Zhang from the National Astronomical Observatories of the Chinese Academy of Sciences.

- The new findings were published Monday in the journal Nature Astronomy.

'Oumuamua: Asteroid or comet?

- Astronomers expected the first interstellar object they’d discover would be an alien comet. This is because comets orbit in the outskirts of star systems, and stars pass relatively near each other somewhat frequently. So, when two stars fly by each other, the gravitational interaction can rip comets from their loosely tethered orbits, flinging them out into space.

- This ejection mechanism was further supported last year when astronomers discovered the interstellar comet 2I/Borisov, which looks reassuringly like the “dirty snowballs” found in our own solar system. It also exhibited a conventional cometary tail — the extended, fuzzy line of sublimated gas and ice that streams from comets as they approach the Sun.

- But ‘Oumuamua left astronomers with more confusing hints about its nature and history. Instead of growing an icy tail, this alien space rock looked relatively dry. It also appeared surprisingly dense and rocky.

- Its shape was even more perplexing. Most space rocks in our solar system are shaped something like a potato — or, in the case of Arrokoth, like two of them smashed together. But ‘Oumuamua is long and stretched out, more like a cigar.


Figure 2: This shot of the interstellar comet Borisov, along with some background galaxies, was captured in November 2019. You can see its faint tail glowing against the background stars (image credit: NSF’s National Optical-Infrared Astronomy Research Laboratory/NSF/AURA/Gemini Observatory)

Sand castles among the stars

- Zhang and study co-author Douglas Lin from the University of California, Santa Cruz, set out to explain ‘Oumuamua’s weird properties. They simulated what would happen as an object flew extremely close to its home star. Their models showed that the tidal forces could literally shred it into long fragments before chucking them out into space.

- The authors say that you can imagine planetary bodies as sand castles floating in space. Much like children on the beach molding castles from many grains of sand, even modest worlds are built from many smaller pieces that are held together by gravity.

- And when an outside force acting on a planetary grain grows stronger than the gravity binding it to the parent body, that smaller piece is yanked from the whole. This is what happens when an object ventures too close to a star. Astronomers previously saw the process unfold when comet Shoemaker-Levy 9 was ripped apart by Jupiter.

- ”The near and far parts of the planetary body are pulled apart from each other by the star’s tidal forces, forming an elongated band of ‘sand particles,’” Zhang says, extending the analogy. “At the same time, since the body is so close to the star, some of its surface melts and freezes after it flies away. This process glues the surface ‘sand particles’ together, and helps to form elongated fragments.”

- And according to the research, ‘Oumuamua could very well be one such elongated fragment.

- The researchers aren’t the first to propose an idea like this, however. Previous modeling work published by astronomer Sean Raymond of Laboratoire d’Astrophysique de Bordeaux in Bordeaux, France, had already used computer models to show that ‘Oumuamua could be a shredded chunk of planetesimal created during the formation of a fledgling planetary system.

- But not everyone is convinced by the argument.

The debate continuous

- Karen Meech, an astronomer at the University of Hawai‘i who has studied ‘Oumuamua but wasn’t involved in this research, says she still believes the space rock has to be a comet. Part of why she thinks that is because one piece of evidence hinting that the space rock is an asteroid could simply be a dearth of observations.

- She points out that ‘Oumuamua’s high-speed trip through the solar system was too quick for astronomers to get a detailed look at the object.

- “Most of the observations of ‘Oumuamua had to occur within a one-week period, and we simply did not have the ability to detect water,” she says. “That isn’t a lack of water. We just didn't have the sensitivity to see it.”

- And there’s another outstanding property these researchers haven’t explained yet, she says. ‘Oumuamua’s movement through our solar system implied it was being driven by more than just gravity — like a comet whose direction is slightly changed by the force of gases streaming off of it.

- ”I think it absolutely is a comet,” Meech says. “The non-gravitational motion that our group detected was super strong, and there’s no other way to maintain it except for outgassing. There’s no other explanation.”

- Because of ‘Oumuamua’s incredible speed, even the fastest spaceship humanity has ever built, NASA’s Parker Solar Probe, couldn’t catch it. As quickly as it came, the alien interloper was gone, never to be seen again.

- “We will never know for sure where ‘Oumuamua came from or where it was created, and I’ve been very impressed with the models and the way people have tried to explain it,” Meech says. “I don’t think it proves anything.”

- However, future observations still could help resolve the argument. If Zhang and Lin are right, interstellar asteroids may be even more common than interstellar comets, the authors say. These alien asteroids would be relatively small and lack the comas of comets, which would make them much harder to detect while they’re passing through our solar system. However, new cutting-edge telescopes, like the Vera C. Rubin Observatory being built in Chile, could spot them.

- ”'Oumuamua is just the tip of the iceberg,” Lin said in a media release. “We anticipate many more interstellar visitors with similar traits will be discovered by future observation.”


Figure 3: The Pan-STARRS Observatories watch for asteroids, supernovas and other transient objects from the island of Maui (photo credit: Rob Ratkowski/Institute for Astronomy)

• July 1, 2019: On October 19, 2017, astronomers discovered the first known interstellar object to visit our solar system. First spotted by the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) telescope located at the University of Hawaii's Haleakala Observatory, the object defied easy description, simultaneously displaying characteristics of both a comet and an asteroid. 4)

- Astronomers formally named the object 1I/2017 U1 and appended the common name 'Oumuamua, which roughly translates to "scout" in Hawaiian. Researchers from around the world raced to collect as much data as possible before 'Oumuamua traveled beyond the reach of Earth's telescopes. In all, they had only a few weeks to observe the strange visitor.

- Early reports of 'Oumuamua's odd characteristics led some to speculate that the object could be an alien spacecraft, sent from a distant civilization to examine our star system. But a new analysis co-led by Matthew Knight, an associate research scientist in the University of Maryland Department of Astronomy, strongly suggests that 'Oumuamua has a purely natural origin. The research team reported their findings in the July 1, 2019, issue of the journal Nature Astronomy. 5)


Figure 4: This artist's impression shows the first interstellar object discovered in the Solar System, 'Oumuamua. Observations made with the NASA/ESA Hubble Space Telescope, CFHT, and others, show that the object is moving faster than predicted while leaving the Solar System. The inset shows a color composite produced by combining 192 images obtained through three visible and two near-infrared filters totaling 1.6 hours of integration on October 27, 2017, at the Gemini South telescope (image credit: ESA/Hubble, NASA, ESO/M. Kornmesser, Gemini Observatory/AURA/NSF)

- "We have never seen anything like 'Oumuamua in our solar system. It's really a mystery still," Knight said. "But our preference is to stick with analogs we know, unless or until we find something unique. The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it."

- As Knight and his colleagues summarized in their study, 'Oumuamua is red in color, similar to many small objects observed in our solar system. But that's where the familiarity ends.

- 'Oumuamua likely has an elongated, cigarlike shape and an odd spin pattern—much like a soda bottle laying on the ground, spinning on its side. According to Knight, its motion through our solar system is particularly puzzling. While it appeared to accelerate along its trajectory—a typical feature of comets—astronomers could find no evidence of the gaseous emissions that typically create this acceleration.

- "The motion of 'Oumuamua didn't simply follow gravity along a parabolic orbit as we would expect from an asteroid," Knight said. "But visually, it hasn't ever displayed any of the cometlike characteristics we'd expect. There is no discernable coma—the cloud of ice, dust and gas that surrounds active comets—nor a dust tail or gas jets."

- Knight worked with Alan Fitzsimmons, an astronomer at Queen's University Belfast in Northern Ireland, to assemble a team of 14 astronomers hailing from the U.S. and Europe. The International Space Science Institute in Bern, Switzerland, served as a virtual home base for the collaboration.

- "We put together a strong team of experts in various different areas of work on 'Oumuamua. This cross-pollination led to the first comprehensive analysis and the best big-picture summary to date of what we know about the object," Knight explained. "We tend to assume that the physical processes we observe here, close to home, are universal. And we haven't yet seen anything like 'Oumuamua in our solar system. This thing is weird and admittedly hard to explain, but that doesn't exclude other natural phenomena that could explain it."

- The new research paper is primarily an analysis of existing data, including a December 2017 study of 'Oumuamua's shape and spin pattern co-authored by Knight and a team of UMD (University of Maryland) astronomers. This paper, published in The Astrophysical Journal Letters, relied on data from the Discovery Channel Telescope (DCT) at the Lowell Observatory in Arizona. UMD is a scientific partner of the DCT, along with Boston University, the University of Toledo and Northern Arizona University.

- Knight, Fitzsimmons and their colleagues considered a number of mechanisms by which 'Oumuamua could have escaped from its home system. For example, the object could have been ejected by a gas giant planet orbiting another star. According to theory, Jupiter may have created the Oort cloud — a massive shell of small objects at the outer edge of our solar system — in this way. Some of those objects may have slipped past the influence of the sun's gravity to become interstellar travelers themselves.

- The research team suspects that 'Oumuamua could be the first of many interstellar visitors. Knight is looking forward to data from the Large Synoptic Survey Telescope (LSST), which is scheduled to be operational in 2022.

- "In the next 10 years, we expect to begin seeing more objects like 'Oumuamua. The LSST will be leaps and bounds beyond any other survey we have in terms of capability to find small interstellar visitors," Knight said. "We may start seeing a new object every year. That's when we'll start to know whether 'Oumuamua is weird, or common. If we find 10-20 of these things and 'Oumuamua still looks unusual, we'll have to reexamine our explanations."

• November 14, 2017: On 19 October 2017, Dr. Robert Weryk, a postdoctoral fellow at the Institute for Astronomy, of the University Hawaii-Manoa, discovered a moving object using the Pan-STARRS telescope. At the time of discovery, the object was 0.2 AU (30,000,000 km) from Earth. The object was initially classified as a comet (C/2017 U1) and later as an asteroid (A/2017 U1). However, further observations indicated that because of its hyperbolic orbit and record-breakingly high eccentricity, the object was never gravitationally bound to the Solar System: a prototype of a new class of objects, an “interstellar asteroid”, had been discovered! 6)

- As the new object did not fit into any of the existing IAU designation schemes, it was necessary for a new one to be defined. The IAU Minor Planet Center, based at the Harvard-Smithsonian Center for Astrophysics, is responsible for identifying, designating and computing the orbit for minor planets, comets and outer irregular natural satellites of the major planets. The MPC proposed to the IAU Executive Committee that they adopt a designation scheme similar to the one used for comets and asteroids (characterized by the letters “C” and “A” respectively), using the letter “I” standing for “interstellar”. The IAU Executive Committee approved the proposal in less than 24 hours and the new object is now officially known as 1I/2017 U1.

- In addition to the technical designation, the MPC also exceptionally assigned the name 'Oumuamua to the new object, as proposed by the Pan-STARRS team. In Hawaiian 'Oumuamua means “a messenger from afar arriving first” that quite appropriately reflects the nature of the object and its discovery.

- Considering the growing interest in the observation and orbit determination of asteroids (see for example the International Asteroid Warning Network (IAWN) initiative, it is expected that the discovery of 1I/2017 U1 ('Oumuamua) will soon be joined by discoveries of more of such interlopers entering the inner Solar System from interstellar space. The scheme for their designation is ready, while the procedure for assigning them a name, similar to the one in use for minor planets, will soon be decided.


Figure 5: 1I/2017 U1 ('Oumuamua), previously known as C/2017 U1 (PAN-STARRS) and A/2017 U1, approaching from above, it was closest to the Sun on 9 September. Traveling at 44 km/s, the comet is headed away from the Earth and Sun on its way out of the Solar System (image credit: NASA/JPL-Caltech/IAU)

• November 1, 2017: Until very recently, all~750,000 known asteroids and comets originated in our own solar system. These small bodies are made of primordial material, and knowledge of their composition, size distribution, and orbital dynamics is essential for understanding the origin and evolution of our solar system. Many decades of asteroid and comet characterization have yielded formation scenarios that explain the mass distribution, chemical abundances and planetary configuration of today’s solar system, but it has remained a mystery how typical our solar system is. 7)

- On 19 October 2017, the Pan-STARRS1 survey telescope discovered asteroid A/2017 U1, the first object known to originate outside our solar system. Follow-up observations by other observers and subsequent analysis verified the extrasolar trajectory of A/2017 U1 and reveal the object to be asteroidal, with no hint of cometary activity despite an approach within 0.25 au of the Sun after multiple solitary orbits around the galaxy at temperatures that would preserve volatile ices for billions of years. Spectroscopic measurements show that the object’s surface is consistent with comets or organic-rich asteroid surfaces found in our own solar system.

- Light-curve observations of A/2017 U1 indicate that the object has an extreme oblong shape, with a 10:1 axis ratio and a mean radius of 102±4 m,and given its similar surface composition to solarsystem asteroids, suggests that such shapes are common during the formation epoch when asteroids are likely to be ejected from their solar systems.

- The discovery of A/2017 U1 suggests that previous estimates of the density of interstellar objects were pessimistically low. Imminent upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to produce more interstellar objects
in the upcoming years, creating opportunities to interrogate the mineralogical, elemental or isotopic composition of material from other solar systems.


- On 19 October 2017, the Pan-STARRS1 telescope system detected an object moving rapidly west at 6.2 degrees per day (Figure 6 a). A search of images from the previous nights found the object had also been imaged on 18 October. Additional images acquired with the Canada-France-Hawaii Telescope (CFHT) on 22 October confirmed that this object is unique, with the highest known hyperbolic eccentricity of 1:188 ±0:016. Data obtained by our team and other researchers between 14-29 October refined its orbital eccentricity to a level of precision that confirms the hyperbolic nature at ~300 σ. Designated as A/2017 U1, this object is clearly from outside our solar system (Figure 7).


Figure 6: a: Discovery image of A/2017 U1 from Pan-STARRS1 on 19 October 2017. In this 45 second wP1-band sidereally-tracked image, the object is the trail centered in the circle. Red regions are masked pixels, mostly due to gaps between CCD cells. b: A/2017 U1 image obtained from CFHT on 22 October 2017, tracked at the object's rate of motion. This composite 180 second w-band image shows no hint of activity although the object was at a heliocentric distance of 1.22 au, just 43 days past perihelion at 0.25 au on 09 September 2017. c: Deep stacked image combining Gemini and VLT g and r-band data. The red line shows the average flux in the annulus at each radius and the blue line is the Moffat profile26 with a FWHM of 0.87'' (image credit: `Oumuamua Discovery Team)

The October 22 CFHT observations were tracked at the object's rate of motion and showed no evidence of cometary activity (Figure 6b) in excellent seeing (0.5''). A/2017 U1's point spread function was consistent with a stellar profile with no asymmetry and no coma, implying that it is asteroidal. Additional time-resolved sequences of images at multiple wavelengths on October 25-26 UT with the European Southern Observatory VLT (Very Large Telescope), and on October 26-27 UT with the Gemini South Telescope, further strengthened A/2017 U1's asteroidal identification (Figure 6c).


Figure 7: The path of A/2017 U1 through our solar system in comparison to the orbit of a typical Halley-type comet. The inset shows the inner solar system, with the solid line segment along A/2017 U1's trajectory indicating the short window during which it was bright enough to be detected by telescopes on Earth. The path is shown as a lighter shade when the object was below the ecliptic (image credit: Brooks Bays / SOEST Publication Services / UH Institute for Astronomy)

1) ”Scientists Determine 'Oumuamua Isn't Made From Molecular Hydrogen Ice After All,” Center for Astrophysics/Harvard&Smithsonian, 17 August 2020, Release No 2020-18, URL:

2) Thiem Hoang, Abraham Loeb, ”Destruction of molecular hydrogen ice and implications for 1I/2017 U1 (‘Oumuamua),” Draft version August 13, 2020, URL:

3) Eric Betz, ”Cosmic visitor ‘Oumuamua may have been ripped from an alien world,” Astronomy, 13 April 2020, URL:

4) ”'Oumuamua is not an alien spacecraft: study,” news, 1 July 2019, URL:

5) The ‘Oumuamua ISSI Team (Michele T. Bannister, Asmita Bhandare, Piotr A. Dybczyński, Alan Fitzsimmons, Aurélie Guilbert-Lepoutre, Robert Jedicke, Matthew M. Knight, Karen J. Meech, Andrew McNeill, Susanne Pfalzner, Sean N. Raymond, Colin Snodgrass, David E. Trilling & Quanzhi Ye), Nature Astronomy, Vol. 3, pp: 594-602, Published: 1 July 2019,

6) ”The IAU approves new type of designation for interstellar objects,” IAU (International Astronomical Union), 14 November 2017, URL:

7) Karen J. Meech, Robert Weryk, Marco Micheli , Jan T. Kleyna, Olivier Hainaut, Robert Jedicke,Richard J. Wainscoat, Kenneth C. Chambers,Jacqueline V. Keane, Andreea Petric, Larry Denneau, Eugene Magnier, Mark E. Huber, Heather Flewelling, Chris Waters, Eva Schunova-Lilly, and Serge Chastel,”Discovery and characterization of the first known insterstellar object,” ESO, Draft version November 1, 2017, Submitted to Nature, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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