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 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?
A new study led by Alan Fitzsimmons of Queen’s University Belfast carefully analyzed the wavelengths of light in observations of ‘Oumuamua by two telescopes in late October. The study's analysis confirms that there’s no sign of even a faint tail, but the wavelengths that correspond to rocky minerals are also missing. So while it doesn’t have a comet’s tail, it’s also hard to argue that ‘Oumuamua is a rocky asteroid.
It looks like ‘Oumuamua has some patchy variations that help mark a seven- to eight-hour rotation. But overall, it’s somewhat reddish in color, which may be the key to the puzzle. All this is similar to some carbon-rich asteroids and the (much larger) distant, icy balls like Pluto. Simple organic (but not biological) carbon compounds react with cosmic rays to produce reddish surface coatings. And ‘Oumuamua should have had plenty of exposure to cosmic rays on its long journey—which might even have started in a star system older than ours.
If ‘Oumuamua is covered in this red stuff—and if some of its surface ice was already lost during a close encounter with its home star or even heating from nearby supernovae—its ice could be safely ensconced in an insulating organic shell. The researchers calculate that the heat of the Sun would only have penetrated about 40 centimeters below ‘Oumuamua’s surface. Any ice deeper than that wouldn’t sublimate to create a lovely comet tail.
The researchers also note an obvious objection to this—the comets from the Oort Cloud in the other reaches of our Solar System have had a pretty similar life history, yet they often produce tails when their orbits bring them close to the Sun. But, researchers say, we have seen tailless objects from the Oort Cloud, and ‘Oumuamua’s small size means it could easily have lost its surface ice at some point. It’s a plausible explanation.
It could be that ‘Oumuamua isn’t as weird as it seems. If that’s the case, a new telescope currently under construction in Chile might be able to spot a number of home-grown objects just like it in our own Solar System, even after ‘Oumuamua has left the building.
Nature Astronomy, 2017. DOI: 10.1038/s41550-017-0361-4 (About DOIs).
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