Meet ‘Oumuamua, the Interstellar Cigar

Rapid-response observations by major observatories shows that the first-known interstellar visitor is 10 times longer than it is wide.

In Arthur C. Clarke's 1973 science-fiction novel Rendezvous with Rama, Earthlings discover and then investigate an interstellar "asteroid" that turns out to be a huge alien spaceship shaped like a long cylinder.

Life, it seems, sometimes imitates art.

In the days after the discovery of ‘Oumuamua, the first (real) interstellar object ever known to pass through our solar system, astronomers worldwide went on the astronomical equivalent of "Red Alert" and pointed their biggest guns at the unexpected interloper. "We dropped everything," explains Laura Ferrarese, who coordinated observations by the Gemini South observatory in Chile.

The result was a torrent of observations — and speculations — that have already found their way into many analyses posted online.

Interstellar object `Oumuamua

Artist's concept of interstellar asteroid 1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017. The aspect ratio of up to 10:1 is unlike that of any object seen in our own solar system.
ESO / M. Kornmesser

Early looks at these data show that 1I/2017 U1, as it's now known officially (with "I" denoting "interstellar"), has a reddish color not unlike the surfaces of comets and other distant, primitive objects in the solar system. Yet not even the most powerful telescopes revealed any hint of cometary activity — even though this body passed well inside Mercury's orbit at about 0.25 astronomical unit from the Sun.

While the mystery object's spectrum seems reasonable, its shape borders on bizarre. According to rapid-response observations pooled from five large telescopes in Hawai‘i and Chile, the apparent brightness of 1I/2017 U1 varies periodically and shows about 2½ magnitudes of range. Of the roughly 750,000 asteroids now known, only five display light curves with swings of at least 2.2 magnitudes — and none as high as 2½. As Karen Meech (University of Hawai‘i and others explain in a Nature article posted online on November 20th, this wide swing implies that the object has an extremely elongated shape - perhaps 10 times longer than it is wide.

If ‘Oumuamua has a very dark surface that reflects only 4% of sunlight, then its average diameter must be a bit more than 100 meters (330 feet) across. But that's a rather meaningless mean given the extreme light curve. Instead, note Meech and her colleagues, the true shape must be 800 m (0.5 mile) long and only 80 m across — more like a gigantic spindle or cigar than a ball of rock. As the team notes, "It raises the question of why the first known [interstellar object] is so unusual."

`Oumuamua light curve

This plot shows how the interstellar asteroid `Oumuamua varied in brightness by a factor of 10 during three days in October 2017. The colored 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 a cigar-shaped 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.
ESO / K. Meech et al.

The combined light curve shows that ‘Oumuamua rotates every 7.8 hours. Assuming that the spin axis goes through its shortest dimension (the most likely situation), this interstellar visitor must be made of rocky or metallic compounds with a fair amount of tensile strength to keep from flying apart.

So where did this thing come from? Conceivably, it could have come from our solar system — you can imagine that it might have been circling the Sun somewhere in the no man's land of the inner Oort cloud when it had a chance close encounter with some massive unseen planet that abruptly flung it sunward. But no one is exploring this scenario seriously.

Instead, this interstellar cigar seems to have entered our solar system from the direction of the constellation Lyra at about 26 km (16 miles) per second. This velocity and incoming direction, dynamicists note, matches the mean motion of stars in the Sun's neighborhood almost exactly. We might never determine which star ‘Oumuamua escaped from. It could be a renegade from a young system just a few million years old — or perhaps it's been floating freely among the stars for billions of years. Fittingly, ‘Oumuamua loosely means "a messenger that reaches out from the distant past."

Meech and her colleagues point out one other oddity about 1I/2017 U1. In their infancy, the young planets of our solar system tossed untold trillions of objects out to the distant Oort cloud and beyond. And while a handful of asteroidal bodies do lie at the solar system's outer fringe, ice-rich comets dominate overwhelmingly — at estimated ratios of 200:1 to 10,000:1. If this proportion holds true for the castoffs of other stars, then by any reasonable odds ‘Oumuamua should have been a comet.

And finally, consider that, statistically, interstellar objects (ISOs) shouldn't be rare. "Our estimates suggest that there is always about one ISO of about 250 m diameter (assuming 4% albedo) within 1 a.u. of the Sun, that is, interior to Earth's orbit," the team writes. These objects might not be rare after all — we simply need to be alert for the next one to pass our way.

For more details on the contributions of the observatories that contributed to the Nature article, check out these press releases from the University of Hawai'i, European Southern Observatory, Gemini Observatory, and NASA.

13 thoughts on “Meet ‘Oumuamua, the Interstellar Cigar

    1. Kelly BeattyKelly Beatty Post author

      Robert… yours is a perfectly reasonable explanation. however, the observatories took multi-wavelength data that shows a uniform color regardless of brightness, so the “spotted body” isn’t really viable. also, take a look at the light curve posted in my story. the spiky minima aren’t consistent with a spot’s rotation.

    1. PalomarJack

      Not so much Sci-Fi buffs, I am one, and also crime dramas, etc., purely for entertainment like it’s supposed to be. Not even close to to the camp fire ghost/ufo/hollow earth stories found on Coast to Coast AM. I know what you mean, “but I’m just sayin’…”.
      Cheers.

  1. bwanabwana

    “this body passed well inside Mercury’s orbit at about 0.25 astronomical unit from the Sun”

    Just dropping in for recharge of the ol’ nuclear engines before continuing its voyage through the universe…

  2. Djlatour

    high tensile strength, fairly uniform color, spinning on the short axis….I would compute the g at the outer ends and get an idea of the aliens’ home planet gravity.

  3. David Smith

    Looks like a chip off the ol’ planet (moon?).
    I can only speculate on the collision that launched this ejecta on its journey out of its parent system. It has no correlation to the globular shapes of naturally formed objects (including objects resembling peanuts resulting from collisions). I’m relaxed that it missed everything on its way into our system (or did it? We have no “BMEWS” alerts until it’s too late to throw a rock at it). Let’s just hope it misses everything on its way back out. Or, did it transition the system above or below the ecliptic?

  4. HaneystationHaneystation

    The shape of this object makes me think of a recent article about sending a similar shape object, though much smaller scale, to our nearest star. Keeping in mind how this object almost slipped through undetected and such mission we are considering should send two within a year of each other. That way if like us a possible message in the bottled slipped by the second one just might just be read. Of course at this point we have no way of knowing if this was just a shard or fragment of some distant world or again a message in a bottle.

  5. PalomarJack

    I suspect there is, but is there a report on not just the light curve, but mass and spectragram? If there was, it would put to bed most of the conspiracy theories found on Coast to Coast AM that it’s artificial. For example, can a rocky artifact hold together with that spin? What is it made of? We would not have to visit to find out. For example, if the spectragam says an object’s mostly metallic and mass calculations say it can hold together under a particular spin due to low mass for its size there is a fair chance it’s a manufactured artifact. Hardly inhabited, it’s too small, unless they were the size of spider monkeys. Most likely a discarded piece of technology, like a Saturn V booster stage. Funny thing is, it’s kind of similar to that size and dimensions. That’s what gave me the “discarded technology” idea. Not likely, but still a tiny, very tiny chance. I accept anything because it’s all interesting, imagine the stories it could tell about a billion year trip with no end in sight. Maybe we’ll visit or even capture the next one.

    Or maybe the folks running the computer simulation we live in just wanted to give us a cheap thrill. = P

  6. PalomarJack

    “Earthlings”? Really? Kind of campy, don’t you think. I just “back washed” in my literary mouth. Please, keep that term in canceled 1950s sci-fi TV shows where it belongs, forever forgotten.

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.

COMMENT