A few weeks ago, we reported on a small object visiting from beyond our solar system. Now astronomers have scrutinized data from this object, which has been given the name `Oumuamua, and which must have traveled through space for millions of years before its chance encounter with our star system. The conclusion is that it’s a dark, reddish, highly-elongated rocky or high-metal-content object. And, indeed, it is the first known asteroid from interstellar space. These new results were published today (November 20, 2017) in the peer-reviewed journal Nature.
Some astronomers thought the object was a comet when the Pan-STARRS 1 telescope in Hawai`i first picked it up on October 19, as a faint point of light moving across the sky. Others thought it looked like a typical fast-moving small asteroid. As they tracked its motion through space, astronomers began to be able to calculate its orbit, showing beyond any doubt that this body did not originate from inside our solar system, like all other asteroids or comets ever observed.
Instead, this object was doubtless from interstellar space.
Observations revealed no signs of cometary activity after it passed closest to the sun in September 2017. It has now been reclassified as an interstellar asteroid – the first ever observed – and named 1I/2017 U1 (`Oumuamua). A statement from the Institute for Astronomy (IfA) at the University of Hawaii described the intricacies of naming this object:
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.
But all of that – name, designations, characterizations of the object – came later. First, astronomers had to observe it and try to understand just what this speedy visitor to our solar system might be. And they had to do it quickly. By the time earthly telescopes first noticed it, `Oumuamua had already passed its closest point to the sun, and was heading back into interstellar space. An international team lead by astronomer Karen Meech of IfA observed the object. They 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. These observations led to detailed measurements of the visitor’s properties. Meech commented:
This thing is very strange.
What we found was a rapidly rotating object, at least the size of a football field, that changed in brightness quite dramatically. 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.
`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, its hyperbolic orbit says it comes from far beyond.
Meech also said the object has:
… a dark red color, similar to objects in the outer solar system, and [we] confirmed that it is completely inert, without the faintest hint of dust around it.
The astronomers said these properties suggest that `Oumuamua is dense, possibly rocky or with high metal content, lacks significant amounts of water or ice, and that its surface is now dark and reddened due to the effects of irradiation from cosmic rays over millions of years.
It is estimated to be at least 400 meters long.
At first – by looking backwards along the orbit that had been calculated for `Oumuamua – astronomers might have said the object had come from the approximate direction of the bright star Vega, in the northern constellation Lyra the Harp.
Things aren’t that simple, though, in our Milky Way galaxy, where everything is always moving. Although it’s travelilng about 60,000 miles/hour (95,000 km/hour), `Oumuamua has taken so long to journey to our solar system that Vega was not near that position when the asteroid was there (about 300,000 years ago). According to astronomers:
`Oumuamua may well have been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with the solar system.
In fact, astronomers were expecting to find an object like this one. They estimate that an interstellar asteroid similar to `Oumuamua passes through the inner solar system about once per year. We haven’t seen them before because they are so faint and hard to spot. But recent survey telescopes, such as Pan-STARRS, are powerful enough to discover them.
That’s why team member Olivier Hainaut of European Southern Observatory commented:
We are continuing to observe this unique object, 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!
Bottom line: Astronomers report on the first known interstellar asteroid, which swept nearest our sun in September, then sped away again. Astronomers have named this object `Oumuamua and say it is dark red and very elongated.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.