Interstellar visitor might be a comet covered in carbonaceous crud

Enlarge (credit: ESO/N. Kornmesser/Aurich Lawson)

When the object 1I/2017 U1 'Oumuamua was first picked up by telescopes in October, there was no question that it was an odd duck-and that's saying a lot considering that we recently explored a comet that looks like a duck. 'Oumuamua seems to be a momentary visitor from another star system, punching through the plane of our Solar System from "above" like it hadn't read our traffic signs. Oh, and it's shaped like a cigar.

The list doesn't end there. While it's incredible to identify something that isn't from our Solar System, it's not a shock that such wanderers exist. Models of star and planet formation show that the growth of gas giants from the rotating disk of rubble that makes up an infant star system could easily fling some objects out into interstellar space. And since gas giants form beyond the "snow line"-the distance from the star at which water can begin to exist as ice-most of these exiles should be comets, which are composed primarily of ice and dust rather than solid rock.

But while 'Oumuamua passed fairly close to the Sun, it showed no signs of the long tail that comets usually sport as warm sunlight turns ice to vapor. And that means there's no ice on its surface. So what is it?

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