mountain profile Institute for Astronomy University of Hawaii

Earth's First Known Interstellar Visitor Unmasked

20 November 2017


Dr. Karen Meech
UH Institute for Astronomy
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Dr. Richard Wainscoat
UH Institute for Astronomy
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Dr. Olivier Hainaut
ESO, Garching, Germany
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Dr. Roy Gal
Media Contact
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Artist's impression of insterstellar object
'Oumuamua (artist's impression)

This artist's impression shows the first detected interstellar asteroid: `Oumuamua. This unique object was discovered on October 19, 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from Gemini, ESO's Very Large Telescope in Chile, CFHT, UKIRT, and other observatories around the world show that it was on a path which must have been travelling through interstellar space for millions of years before its chance encounter with our star system. `Oumuamua seems to be a dark red highly-elongated metallic or rocky object, about 400 meters long, and is unlike anything normally found in the Solar System.

Credit: ESO/M. Kornmesser
High Res JPG / High Res TIFF

In October, astronomers at the University of Hawaii's Institute for Astronomy (IfA) made a stunning discovery with the Pan-STARRS1 telescope - the first interstellar object seen passing through our Solar System. Now, an international team lead by Karen Meech (ifA) has made detailed measurements of the visitor's properties. "This thing is very strange," said Karen Meech.

Originally denoted A/2017 U1 (with the A for asteroid), the body is now the first to receive an I (for interstellar) designation from the International Astronomical Union, which created the new category after the discovery. In addition, it has been officially given the name `Oumuamua. The name, which was chosen in consultation with Hawaiian language experts Ka`iu Kimura and Larry Kimura, reflects the way this object is like a scout or messenger sent from the distant past to reach out to us (`ou means "reach out for", and mua, with the second mua placing emphasis, means "first, in advance of"). The object's full official name is 1I/2017 U1 (`Oumuamua), and can also be correctly referred to as 1I, 1I/2017 U1, and 1I/`Oumuamua .

`Oumuamua is rapidly fading as it heads out of the Solar System and recedes from both the Sun and the Earth, so getting new observations as fast as possible was crucial. The IfA team - including those who discovered 1I - was already prepared to rapidly follow up solar system discoveries from Pan-STARRS, which is operated by the IfA and funded by NASA. "We were able to rapidly develop a follow-up strategy on a very short timescale. It is exciting to think that the brief visit by `Oumuamua gave us the opportunity to do the first characterization of a sample from another solar system," says Meech. As a result, they are the first to publish their results, appearing in the November 20th online issue of the journal Nature.

The team gathered data from telescopes around the world, including the Canada-France-Hawaii Telescope (CFHT), the United Kingdom Infrared Telescope (UKIRT) and the Keck Telescope on Maunakea, the Gemini South telescope, and the European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile. Marshalling all of these resources yielded a wealth of data that revealed `Oumuamua's unusual nature.

"We had to act quickly," explains team member Olivier Hainaut from ESO in Garching, Germany. "`Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space. This felt very much like the beginning of the famous science fiction story, Rendezvous with Rama."

"Needless to say, we dropped everything so we could quickly point the Gemini telescopes at this object immediately after its discovery," said Gemini Director Laura Ferrarese, who coordinated the Gemini South observations for Meech's group.

"The CFHT data was absolutely critical for understanding the light curve, for our initial understanding of the orbit, and determining that this object was more like an asteroid and not a comet," noted IfA's Richard Wainscoat.

"What we found was a rapidly rotating object, at least the size of a football field, that changed in brightness quite dramatically," according to Meech. "This change in brightness hints that `Oumuamua could be more than 10 times longer than it is wide - something which has never been seen in our own Solar System," according to Meech.

"An axis ratio like that is truly extraordinary - we have never seen anything in the solar system that is this elongated", says Lance Benner, a specialist in radar imaging of near-Earth and main-belt asteroids at the Jet Propulsion Laboratory in California.

`Oumuamua does have some similarities to small objects in the outer Solar System, especially the distant worlds of the Kuiper Belt - a region of rocky, frigid worlds far beyond Neptune. "While study of `Oumuamua's colors shows that this body shares characteristics with both Kuiper Belt objects and organic-rich comets and trojan asteroids," said Meech, "its hyperbolic orbit says it comes from far beyond."

"We are continuing to observe this unique object," added Hainaut, "and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones!"

Astronomers estimate that an interstellar asteroid similar to `Oumuamua passes inside the orbit of Earth several times year, but they are faint and hard to spot, so they have been missed up until now. It is only recently that survey telescopes, such as Pan-STARRS, are powerful enough to have a chance to discover them. "Our successful follow-up observations are a model for the future - especially when the next major survey telescope, LSST, comes on line," added Meech.

This research is presented in a paper entitled "A brief visit from a red and extremely elongated interstellar asteroid", by K. Meech et al., appears in the journal Nature on November 20, 2017.

The discovery of the first interstellar object, `Oumuamua, was made possible by the worldwide effort presently being made to discover Near-Earth Objects. Much of the funding for this effort, including funding for operating the Pan-STARRS telescope via Grant No. NNX14AM74G, comes from the National Aeronautics and Space Administration's Near Earth Object Observations Program, which is part of the agency's Planetary Defense Coordination Office within NASA's Science Mission Directorate.

This animation shows the interstellar asteroid 1I/2017 (`Oumuamua). Observations with Gemini, ESO's Very Large Telescope, CFHT, and others have shown that this unique object is dark, reddish in colour and highly elongated.
This video is available in a variety of formats here.
Credit: ESO/M. Kornmesser
Gemini Observatory image of interstellar object
Gemini South telescope color composite image of `Oumuamua (center).

This composite was produced by combining 192 images obtained through three visible and two near-infrared filters totaling 1.6 hours of integration on October 27 at the Gemini South telescope. Processing removes the background stars. The field of view represents a patch of sky 5,000 km (~3,100 miles) on a side at the distance of `Oumuamua. Visible colors for the image were assigned to each filter as follows:
g (398-552 nm) = blue
r (562-692 nm) = green
i (706-850 nm) = yellow
z (830-925 nm) = orange
Y (970-1070 nm) = red
While assigning visible colors to filtered images is somewhat subjective, the resulting color of `Oumuamua in optical light is similar to the hue of some of the moons of outer planets in our Solar System, and possibly indicates a similar composition (a combination of minerals, carbon, iron, and organic compounds). Color composite produced by Travis Rector, University of Alaska Anchorage, using Gemini South GMOS data obtained and processed by Meech et. al.
Credit: Gemini Observatory/AURA/NSF
This animation (annotated) shows the path of the interstellar asteroid 1I/2017 (`Oumuamua) through the Solar System.
This video is available in a variety of formats here.
Credit: ESO/M. Kornmesser, L.Calcada. Music: Azul Cobalto
light curve of interstellar object
'Oumuamua's Varying Brightness

This plot shows how the interstellar asteroid `Oumuamua varied dramatically in brightness during three days in October 2017. The large range of brightness - about a factor of ten (2.5 magnitudes) - is due to the very elongated shape of this unique object, which rotates every 7.3 hours. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object's shape or surface albedo.
Credit: ESO/K. Meech et al.
High Res JPG / High Res TIFF

Companion Press Releases

Gemini Observatory

ESO (European Southern Observatory)



Discovery press release from Oct. 26, 2017

The team is composed of Karen J. Meech (Institute for Astronomy, Honolulu, Hawaii, USA [IfA]) Robert Weryk (IfA), Marco Micheli (ESA SSA-NEO Coordination Centre, Frascati, Italy; INAF-Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy), Jan T. Kleyna (IfA) Olivier Hainaut (ESO, Garching, Germany), Robert Jedicke (IfA) Richard J. Wainscoat (IfA) Kenneth C. Chambers (IfA) Jacqueline V. Keane (IfA), Andreea Petric (IfA), Larry Denneau (IfA), Eugene Magnier (IfA), Mark E. Huber (IfA), Heather Flewelling (IfA), Chris Waters (IfA), Eva Schunova-Lilly (IfA) and Serge Chastel (IfA).

More information about The PanSTARRS project can be found at:

The PanSTARRS project website

The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) is a wide-field survey observatory operated by the University of Hawai'i Institute for Astronomy. The Minor Planet Center is hosted by the Harvard-Smithsonian Center for Astrophysics and is a sub-node of the Planetary Data System Small Bodies Node at the University of Maryland ( ). JPL hosts the Center for Near-Earth Object Studies (CNEOS). All are projects of NASA's Near-Earth Object Observations Program, and elements of the agency's Planetary Defense Coordination Office within NASA's Science Mission Directorate.

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakalā and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.