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.)
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.
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."