Gravitationally Unbound: Interstellar Interloper ʻOumuamua

So I'm pretty late to write about this and you may have already heard about it by now, but in case you haven't, on October 19th the Pan-STARRS telescope in Hawaii discovered the first known object in our solar system that's unquestionably of extrasolar origin. The tiny interstellar wanderer, now known as 1I/2017 ʻOumuamua, was caught forty days after its closest approach to the Sun, and it was quickly realized that its speed—both on approach to and away from the Sun—was far too high for it to be gravitationally bound to the solar system.

This was pretty exciting, as this is the first time we've discovered an asteroid in our solar system that definitely came from outside it. There's been an intermittent stream of papers on ʻOumuamua over the weeks that followed its discovery, speculating on its possible origin, trying to figure out whether it's a comet or an asteroid (the latter seems most likely, given its complete lack of visible coma even after passing within the orbit of Mercury), even calculations on how far the Earth−Moon system would have moved if it were a macroscopic chunk of nuclear-density dark matter! (About ten meters/thirty-three feet was the conclusion, so don't worry about it.)

One of weirdest things about ʻOumuamua so far is its shape. Now, like all known asteroids it's much too small to make out its shape directly (even passing less than a fifth of the distance to the Sun from Earth). However, we can measure how its brightness changes over time (called its light-curve) which can tell us both how fast it rotates and how elongated it is…and that's the strange part. According to the large difference between the maxima and minima of its light-curve, ʻOumuamua must have a length ratio of something like 1:6 or even 1:10. Think cigar-shaped and you've got an idea of what it must look like. This is, needless to say, really, really odd, as nothing we know of in our solar system has a shape approaching anywhere near that ratio. Whatever it's made of must be holding itself together with more than just gravity.

Finally, if you're familiar with my blog, you've probably already guessed that the name ʻOumuamua is of Hawaiian origin. (And if you did, good job!) ʻOu means “to reach for,” while mua has several meanings but is generally associated with the idea of being first, foremost, leader, or senior. “I mua!” means “Forward!”, and ʻoumuamua (with some of that Hawaiian reduplication I've talked about) means scout (in a military sense), one sent on ahead or before. Which is the intended meaning of the name in this case, as ʻOumuamua will likely be only the first of more interstellar objects that we'll detect in the future.

Interesting times we live in! A hui hou!