Is ‘Oumuamua an Interstellar Pancake?

A new analysis of more than 800 telescopic observations suggests that our first known interstellar visitor could have the shape of a flattened disk.

It's been about five months since Robert Weryk, observing with the PanSTARRS 1 telescope atop Haleakala on Maui, discovered the first object (other than dust particles) known to have entered our solar system from interstellar space. But 1I/‘Oumuamua, as it came to be named, zipped away from Earth too quickly to give astronomers more than a few weeks to observe it. (‘Oumuamua is a combination of two Hawai'ian words that roughly mean "the first scout," and 1I indicates the first cataloged interstellar object.)

Portrayal of ‘Oumuamua (1I/2017 U1)

Artist's impression of ʻOumuamua.
ESO / M. Kornmesser

When I reported the early findings about this surprising interloper back in December, astronomers were fixated on its slightly reddish color and its tenfold swings in brightness (up to 2½ magnitudes). Assuming that it's highly elongated, 5 to 10 times longer than its width, most researchers imagined this object pinwheeling end over end with a spin axis through its shortest dimension.

But it's not that simple. The object's brightness swings couldn't be fit by a single period of rotation, and multiple teams quickly concluded that ‘Oumuamua must be tumbling in what's termed an "excited rotational state."

Now 18 observers led by small-body specialist Michael Belton (Belton Space Exploration Initiatives) have pooled 818 brightness estimates of ‘Oumuamua from nearly a dozen major instruments — including the Hubble Space Telescope — to try to unravel the details of the object's spin. Their conclusions appear in the April 1st Astrophysical Journal Letters.

"There is no doubt that 'Oumuamua is spinning in an excited state," say Belton and co-author Karen Meech (University of Hawai'i), with a pronounced wobble that takes 8.67 hours to complete. However, its main rotation might not be end-over-end, as had been assumed. "Our analysis shows that ‘Oumuamua could just as easily be in a high-energy state," they point out. In that case, the primary spin axis would be close to its long axis, perhaps rotating every 54.48 hours (the most likely period), while simultaneously precessing and nutating every 8.67 hours. Picture a wobbling, badly thrown football, and you get the idea.

Oumuamua painting Hartmann

Perhaps the interstellar object 1I/'Oumuamua is shaped more like a pancake than a cigar.
© William K. Hartmann

Moreover, this "long-axis mode" of spinning has eye-popping implications. Belton's team concludes that the shape of 'Oumuamua could be anything from "cigar-like" to something akin to a fat pancake. (Now picture a Frisbee carried off by a very strong gust of wind.) Astronomer-artist William Hartmann takes a stab at depicting the pancake scenario in the painting at right.

For the moment, either shape is equally likely. But Belton seems confident that, given more time to analyze all of the brightness measurements, a precise solution for the object's spin state is close at hand.

But Where Did ‘Oumuamua Come From?

Aside from puzzling out the object's shape and spin, many theorists are tackling the question of where ‘Oumuamua came from and how it got here. It came from the direction of the constellation Lyra, passing through at about 26 km (16 miles) per second. This incoming trajectory doesn't implicate any specific star as the source, but it's likely drifted through interstellar space for tens or hundreds of millions of years.

Early in our solar system's history, gravitational slingshots from the giant planets and the Sun cast countless objects (perhaps trillions of them) out into deep space, never to return. So ejection from another star system seems the most likely way that ‘Oumuamua escaped, but even then it's complicated.

For example, its overall spectrum and relatively rapid spin imply that ‘Oumuamua is rocky, perhaps even a single cohesive rocky fragment. This might suggest that it was ejected from the inner region of its host solar system. Yet Sean Raymond (University of Bordeaux, France) and others argue in Monthly Notices of the Royal Astronomical Society that too few asteroidal fragments would be ejected to make it statistically likely for one to reach here. Or perhaps it was ejected from a two-star system.

Cometary castoffs should be much more common, but ‘Oumuamua neither showed any hint of a coma or tail, nor was any icy material detected on its surface. And how would a comet end up with such a strange shape? Speculations to date have ranged from heating and reshaping by an aging, bloated star that drove off all the volatile compounds to a reassembled collection of gravitationally disrupted fragments.

We'll likely never know the real story, but astronomers have redoubled their efforts to spot other interstellar interlopers. Calculations by Aaron Do (University of Hawai'i) and two colleagues suggest interstellar escapees should average about one for every 5 cubic astronomical units of space. In other words, they conclude, "there are likely several of these objects in the inner solar system at any given time."

11 thoughts on “Is ‘Oumuamua an Interstellar Pancake?

  1. Robert-CaseyRobert-Casey

    Oh, maybe in the next 50 years, we’ll send a probe to look at Oumuamua up close. As we will know where to find it, and as it’s interstellar, it should be interesting to chase down.

  2. Russell SampsonRussell Sampson

    Excellent article with much food for thought. Some possible examples of flattened objects within our solar system would be Saturn’s unconventional moons; Pan and Atlas. If their flatten shapes were formed by the accretion of Saturnian ring material, could Oumuamua have been formed the same way and then ejected from a ringed exoplanet or a still-forming planet during its accretion phase (i.e primordial rings – like those hypothesized to have been around the post-impact Earth-Moon system)?

  3. Harry Powell

    I am baffled. I have a Physics degree, so I should be able to understand this, (though my degree was gained in 1962 and I worked on things other than Physics all my life) but I have seen mention of this object having multiple axes of rotation. If it is a rigid body, it has only one part, and it has an angular velocity. That can be represented by a vector omega (no insert character tool so no Greek letters, bold type etc.). If it had two axes, would not these sum to produce a single vector? It might look like an oddly thrown football but so what? The other thing that might cause an odd motion would be outgassing. Since it has clearly been travelling for a long time, outgassing must be due to heating as it went by the Sun. That might result in a CHANGING axis of rotation but surely not two axes at the same time, unless the other axis represents the couple acting to rotate the first axis (like G = Big Omega cross Little Omega).

    1. John Crawford

      If you picture a sphere, such as an earth globe, rotating around an axis like the earth’s poles, this axis could then rotate around, say, an axis perpendicular to its mid-point. The points where the poles are would then describe a circle, and a point on the surface of the sphere would describe a complex path, depending on the relative angular velocities of the two rotations, not a rotation around another combined axis. This circle could then rotate around another axis, etc. The resulting axial rotation and axis tumbling can occur at different, perhaps non-integer, multiples, resulting in chaotic motions of points on the surface of the body.

    2. Edward Schaefer

      Reading a bit between the lines, they are talking about components of rotation about the long and short axes. So there is only one value for the angular momentum of the object, but the axis about which the total spinning occurs is not along either the long axis or the short one. Remember that angular momenta are vectors that can be added. That is how the article can discuss one period of rotation about one axis and another period of rotation about another axis.

  4. Carla Rene

    Hi, Kelly,

    I’m pursuing double doctorates in Astrophysics and Applied Mathematics.

    While answering some questions on Quora about this very entity just two days ago (the hot-headed high-school student thought he would trip me up by raising this object as evidence that not everything in the universe rotates; sadly, I kid you not).

    In doing so, I found this article from Astronomy.com, written March 22, 2018, stating that it’s been determined that the object likely came from a binary star system. I have the link for you:

    http://www.astronomy.com/news/2018/03/interstellar-asteroid-oumuamua-came-from-binary-star-system

    Cheers.

  5. Joseph Gerver

    Angular momentum is conserved, so the angular momentum vector must always point in the same direction and have the same length. However, the angular momentum is equal to the angular velocity around the spin axis, times the moment of inertia around that axis. Unless the spinning body is a sphere, its moment of inertial will be different around different axes through the center of mass. That means the angular velocity need not remain constant, if the spin axis does not keep the same orientation relative to the geometric shape of the body. And Euler showed that the spin axis will NOT maintain the same orientation relative to the shape of the body, unless the spin axis coincides with either the axis of maximum moment of inertia (e.g. a spinning frisbee) or the axis of minimum moment of inertia (e.g. a well-thrown football). Look up “Chandler wobble”.

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