In October, a group of wise women and men spotted a mysterious light in the sky. There has been much excited chatter since about what it might mean. A space rock has come travelling. And as December draws to a close, the unusual visitor is heading away again, its brief message to Earth seemingly delivered. Our interstellar guest is from another star system entirely, one it was bundled out of perhaps even billions of years ago. Long-predicted, this is the first confirmed visit of an object from so far away. We are probably its first company in some time. And already it has seen enough.
A bright light shone around it. Long-term exposure to cosmic rays has created an insulating organic-rich layer on its surface (A. Fitzsimmons et al. Nature Astron. http://doi.org/chks; 2017). This coating — more pink than silver — could have protected an ice-rich interior from being vaporized during its passage close to the Sun. And it could help to explain some of the initial confusion over the visitor’s true nature. Sky-watchers scanning for interstellar objects tend to be on the look-out for a comet. These are expected to produce a distinctive haze as their outer layers of ice sublimate, making them much easier to spot as they pass close to the Sun.
The absence of a tail saw the object reassigned instead as a rocky asteroid, which it could be. But its organic shield protects the unlikely possibility that it could be a comet after all — models suggest ice might be hidden underneath, undisturbed by the body’s flirtation with the Sun.
It could come from a planet a long way from here. If it is not a comet, a paper posted to the arXiv server this month speculates, it might be a fragment of a distant planet ripped apart by a process of gravitational vandalism known as tidal disruption (M. Ćuk preprint at https://arxiv.org/abs/1712.01823; 2017).
Despite the best listening efforts of telescopes on Earth, the object has remained silent. And to the disappointment of alien-hunters across the planet, there is no sign of technology. (It was always a long shot, but the unusual cigar-shape boosted hopes that it was built and not formed.) Nonetheless, astronomers have called it ‘Oumuamua, a Hawaiian term for scout. In Nature this week, its discoverers (who spotted it using the Pan-STARRS telescope on Hawaii’s Maui island) say the object seems to be a “messenger sent from the distant past to reach out to us”.
‘Oumuamua might not be talking but it could still be listening. At this time of year, it’s traditional for many radio stations across the United States to play the 1949 Hawaiian tune ‘Mele Kalikimaka’, which offers a greeting of love, peace, joy and compassion. As ‘Oumuamua speeds away there are worse impressions for it to take from Earth — even if, like most souvenirs, the significance is lost on many of the planet’s locals.