So long, rock from an alien solar system. I mean that literally: So. Long.

Last month, an interstellar visitor was discovering shooting through our inner solar system.

(Just writing that down is still thrilling. Wow.)

At first thought to be a comet, then more likely an asteroid, it was moving so rapidly that after enough observations astronomers confirmed the Sun's gravity couldn't hold on to it: It was headed out into interstellar space.

But that works backwards, too: It was moving so rapidly it couldn't have started here in our solar system! It came from another star. Because of its unique status, a new designation was given to it: I/2017 U1 'Oumuamua. The I means it was interstellar (and the name is Hawaiian for "very first scout," and the "'" in front is like a glottal stop).

It was moving so rapidly that observations were difficult; it was screaming away from Earth and getting fainter by the minute. However, given the unprecedented nature of this object a lot of powerful telescopes were pointed at it, and, although it was too small to resolve as anything more than a dot, we still learned some interesting things about it.

Two different journal papers just came out about it (one in The Astrophysical Journal and the other in Nature) and they both have similar results: The object is extremely elongated. In the former paper they estimate it's about six times longer than it is wide, and in the latter it may be as much as ten times longer!

The results are fairly consistent with each other despite the different numbers. The brightness of 'Oumuamua went up and down with fair regularity over the time it was observed, and from experience we know this is due to it tumbling end over end, like a stick thrown overhand. As it spins we sometimes see it broadside, and sometimes more end-on. Since it shines by reflected sunlight, the more we see of it (like when it's broadside), the brighter it appears. The exact amount it gets brighter and dimmer determines the elongation, and different telescopes using different filters can easily get slightly different numbers. Plus, the object was very faint, so there's some uncertainty to the measurements.

The period of this cycle is about 7-8 hours, which isn't too surprising. Many asteroids are seen to rotate with a similar period. The amount of brightness change, and therefore its extremely elongated shape, are a surprise, though. Not many objects in our solar system are that elongated, so it's a little weird that the very first interstellar visitor we ever identify is so drawn out^*.

As a note, it's worth mentioning that it's possible it's not quite this extended. If its coloration is patchy, with darker and lighter spots, then that can cause a more extreme brightness variation as well. It's entirely possible this object is therefore not quite as elongated as all that. Still, it's almost certainly longer than it is wide by a good margin.

The “light curve” of ‘Oumuamua, its change in brightness over time. It fades periodically by a factor of nearly 10, most likely caused by its rotation. Credit: ESO/K. Meech et al.

Photo: ESO/K. Meech et al.

The reason this is interesting has to do with the composition of the object. Some asteroids are metal, and extremely tough. Some are rock, and more brittle. Others are "rubble piles," essentially shattered rock held in place by its own gravity and weak electrostatic forces. A metal asteroid can spin very rapidly without issue, but try that with a rubble pile and it'll fly apart. So the spin actually tells us something about what it's made of.

That also depends on its size, too. If it's extremely long, then a slower spin might still disrupt it. In this case, 'Oumuamua looks to be roughly 400 meters long, implying it may be a solid (monolithic) object.

It's also reddish in color. That's common for objects on the outer solar system, and given its colors it matches closest to a relatively uncommon kind of asteroid called "D-Type." We actually don't know much about them (though an observing campaign going on now may help alleviate that), so there's not much to add there.

Not long after it was discovered, astronomers were surprised when they took deep images of 'Oumuamua and it showed no fuzziness at all. If it were icy, like a comet, then when it got near the Sun some of that ice should have turned into a gas. No fuzziness means no gas, so it was reclassified as an asteroid.

But I wonder. It spent a long time in deep space, and was exposed to galactic cosmic rays, subatomic particles zipping through space. This could darken the surface by changing the chemical structure of the material there, but I wonder if that might also toughen it up as well. It could have ice below the surface, but it couldn't break free due to a hardened outer shell (we saw something like this with the comet 67/P Churyumov-Gerasimenkpo; though there were lots of cracks in the surface and ice there, much of the surface was harder than expected, which is why the lander Philae wasn't able to harpoon the surface and latch on).

The point? 'Oumuamua is pretty weird. Not I'm-not-saying-it's-aliens-but-it's-aliens weird, but weird nonetheless. We still have more questions than answers about it, and it’s likely that's the way things will stay, since this rock is on its way out.

But there may yet be hope. The fact that we saw this thing at all means there must be lots more passing through our solar system, objects that started off as comets or asteroids around alien stars and got ejected, perhaps by a close encounter with an exoplanet. How many, though?

I assumed there'd be quite a few when I wrote about this earlier, but one of the two recent papers did a bit of math to find out, and the answer still astonished me: They calculated there may be so many that, statistically speaking, there is always an interstellar object up to 250 meters in size somewhere in the volume of space defined by the size of Earth's orbit.

Whoa. That means there are a lot of these things just waiting to be found. They're small and dark, making them faint, but we're getting better at spotting such things. Sky surveys are improving, with bigger telescopes and more sky being watched. Up until now our tech wasn't up to spotting interstellar interlopers, but now we can!

So it's just a matter of time until we add more rocks to the category "I." That's really exciting. It's not too often we get to see something from an alien solar system up close… but that's about to change.

^* I've seen a lot of jokes on social media that this sounds like the dimensions of a spaceship. Rama or Battlestar Galactica, anyone? But think it through: This object is flipping end over end. A cylinder like Rama should be spinning around its long axis to provide centripetal acceleration to mimic gravity, and a ship like Galactica tumbling means it's long dead, out of power, and out of control. So a really long rock, while odd, is the least odd explanation!