The first object to visit our solar system from interstellar space is currently making a clean getaway from our cosmic neighborhood. But according to astronomers watching its escape, the space rock’s surface is anything but clean.
After examining how the object reflects sunlight, scientists found that the strange rock, called ‘Oumuamua, is probably coated with a carbon-rich schmutz in a layer that’s more than a foot thick. The residue likely formed as interstellar radiation bombarded the object’s surface as it tumbled through the void.
The results, published on Monday in Nature Astronomy, could help explain why the object—likely an amalgam of rock, ice, and dust—didn’t spew out plumes of water vapor as it flew by the sun, even as its surface reached hundreds of degrees Fahrenheit.
“If you take comets out there at interstellar space, they’ll form these dark, carbon-rich objects, and that looks like what we saw,” says study coauthor Alan Fitzsimmons, an astronomer at Queen’s University Belfast. “We don’t have a very decisive picture of this thing; all we can say is our measurements are consistent with this idea.”
Discovered in October as it raced past Earth, ‘Oumuamua is one of a kind: The cigar-shaped object is up to a quarter mile long, tumbling end over end as it exits our solar system at more than 98,000 miles an hour. Astronomers long thought that other star systems flung off such planetary debris, since our own solar system does the same. Now, for the first time, astronomers have caught one in the act.
Originally, ‘Oumuamua was thought to be a comet, but University of Hawaii astronomer Karen Meech ruled out this tidy classification soon after its discovery, because ‘Oumuamua didn’t produce any plumes of gas or dust, which are the calling cards of comets.
“That was the first big surprise: We expected these things to be icy, and then this thing looked for all the world like an asteroid,” says Fitzsimmons.