This column first ran in The Tablet in November 2017
In the month after its discovery we were able to train some of the largest telescopes in the world on the new visitor to our solar system (that I wrote about here), a small body whose orbit indicates that it is had come to us from outside our own solar system.
“We’ve been expecting for twenty years to find some body on an orbit like this,” explained Brett Gladman to me. He’s an astronomer at the University of British Columbia who specializes in the faint distant objects of our solar system. (I was visiting Vancouver in November 2017.)
He wasn’t surprised that its path was almost perpendicular to the plane where our sun’s planets orbit. There’s no reason why it couldn’t come from any direction, since it is not tied to our system. He wasn’t surprised that it was small, barely visible to our biggest telescopes. “In space, collisions mean that big things break into small things, so there will be more small things than big things. You’d expect the first bit we would see would be the smallest bit we could possibly see.” As our telescopes get better, we can expect to see more such small fragments.
“But I was certain it was going to be a comet,” Brett confessed to me. We know that, compared to rocky asteroids, there are about a hundred times more small bodies in the cold outer reaches of a star system that rich in ices, and the nearby passage of some giant planet in that system could well pull such a little guy into an orbit sent our way.
“So what I expected was that when this body got to about one AU” — the distance that Earth is from the Sun — “it would be heated up enough to evaporate all that ice, and we would see it suddenly grow a five times brighter.” Instead, this body emitted no visible gases at all, even as it fell to within a quarter of Earth’s distance to the Sun… closer even than the orbit of Mercury.
“Ninety-nine times out of a hundred, you’d expect a comet. That’s a ‘three-sigma’ result,” he said. The term “sigma” comes from probability theory to indicates how likely a particular event might be. Brett concluded, “I guess that shows that, to three sigma, I don’t know what I am talking about!”
But its gas-free composition was only the beginning of the surprises. Two teams of astronomers, led by Karen Meech (from Hawaii, but using telescopes in Chile) and Michelle Bannister (at Armagh, but using telescopes in Hawaii) managed to track how the brightness of the object changes in time. If the body is irregular in shape, like you would expect for a shard of a broken asteroid, then its rhythm of bright and dim tells you how fast it is spinning and (assuming it’s constant in colour) even its shape, turning brightest when we see it broadside and dimmest when we look down its ends. This result was completely unexpected: the change in brightness is so great, and it turns so dim so suddenly, that it must be shaped like a pencil: ten times longer than it is wide.
How big is it? We know how much light it reflects back to us, but not how dark its surface is. It could be big and dark, or small but shiny. If its surface is like bodies in our own solar system, it’s probably half a kilometer long and maybe 50 meters thick.
It does finally have a name. The discoverers have designated it ‘Oumuamua, Hawaiian for the first scout of a new land.
'Oumuamua was the subject of an entire session at the meeting of the American Astronomical Society's Division for Planetary Sciences in October, 2018. In order to fit its orbit, it turns out that there needs to be some non-gravitational forces acting on it such as would be expected from comet outgassing; so perhaps it really is a comet. On the other hand, as another researcher put it, it doesn't really exactly match anything that we have in our own solar system.