*Update, 20 November, 12:50 p.m.: During the brief visit by this interstellar visitor over the past month, many of the world’s most powerful telescopes swung to take a look. What they saw, reported today in Nature, was both familiar and extraordinary. The asteroid, now dubbed ‘Oumuamua (scout or messenger in the Hawaiian language), is dark red in color, similar to objects from the far reaches of our own solar system. The authors conclude it’s most likely a lump of metal-rich rock without much water or ice that has been reddened by millions of years of bombardment by cosmic rays as it crossed interstellar space. The surprise was that the light coming from it pulsed in brightness by a factor of 10 every 7.3 hours, suggesting both that it is spinning rapidly and is 10 times longer than it is wide—more elongated than anything known among our planets. They estimate that its mean radius is about 100 meters and its length is about 800 meters. Researchers believe that such visitors may zip through our neighborhood about once a year, but they’re usually too small to see. They’ll be watching extra keenly now that they’ve bagged their first one. Astronomers continue to monitor ‘Oumuamua as it fades into the distance, trying to figure out where it might have come from and where it will go next.
Here's our original story, published on 27 October: Around the world, telescopes are swiveling to welcome, and then wave farewell to, a new guest to the solar system: a fast-moving asteroid, or potentially a comet. It could be the first interstellar object to visit the solar system that has been detected and observed by astronomers, NASA announced yesterday.
Discovered on 19 October at the University of Hawaii’s Pan-STARRS 1 telescope on Haleakalā, the object, temporarily dubbed “A/2017 U1,” is 400 meters in diameter and moving quickly. It was first detected by Rob Weryk, an astronomer at the University of Hawaii (UH) in Honolulu, and confirmed by the European Space Agency’s telescope on Tenerife in the Canary Islands.
The object’s incoming motion—25.5 kilometers per second—was so extreme that astronomers believe it is not the kind of asteroid or comet typically seen inside the solar system.
“This is the most extreme orbit I have ever seen,” said Davide Farnocchia, a trajectory expert at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, in a release. “It is going extremely fast and on such a trajectory that we can say with confidence that this object is on its way out of the solar system and not coming back.”
Based on its current trajectory, the visitor came from the constellation Lyra and approached our solar system from “above,” perpendicular to the plane that most planets orbit the sun. (NASA has prepared this nice visualization of the object’s path.) On 9 September, it made its closest approach to the sun, with gravity then tugging it on a route “under” the solar system. On 14 October, it made its closest pass by Earth, at 60 times the distance to the moon. It is now looping back above the planetary plane and, traveling at 44 kilometers per second, is shooting toward the constellation Pegasus.
It’s no surprise that such a space rock, or comet, exists—scientists expect such grist to be wobbling around the galaxy, the ejected remnants of planetary formation. More observations are needed, and coming, to confirm its origins. Ultimately, the visitor will need a name, and rules do not yet exist for naming such extra–solar system guests.
“We have long suspected that these objects should exist, because during the process of planet formation a lot of material should be ejected from planetary systems. What’s most surprising is that we’ve never seen interstellar objects pass through before,” said Karen Meech, an astronomer at UH’s Institute for Astronomy in Honolulu specializing in small bodies and their connection to solar system formation.
Because this is the first object of its type ever discovered, rules for naming this type of object will need to be established by the International Astronomical Union.
“We have been waiting for this day for decades,” said JPL’s Center for Near-Earth Object Studies Manager Paul Chodas. “It’s long been theorized that such objects exist—asteroids or comets moving around between the stars and occasionally passing through our solar system—but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it.”