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Last week, something unusual was detected moving through the Solar System, a small object which didn't seem to behave like any known comets or asteroids. In fact, its behaviour suggested that it originated from outside of our Solar System. So what was this mystery interloper? While not 100 percent identified yet, it seems to be an interstellar asteroid or some similar rocky body.
A/2017 U1 was first discovered on Oct. 19 by the University of Hawaii's Pan-STARRS 1 telescope on Haleakala, Hawaii. At first, astronomers thought it was probably a comet, but photometric analysis by telescopes showed that it lacked a coma, even as it was getting closer to the Sun. This suggested it was rocky and with little or no ice, and the observations also showed that the surface is quite red in color, similar to some rocky objects in our own Kuiper Belt.
The object, now called A/2017 U1, was less than a quarter-mile (400 meters) in diameter and moving remarkably quickly. It was also moving on an extreme trajectory, perpendicular to the orbits of the other planets, "above" the ecliptic. This, plus the speed, about 15.8 miles (25.5 kilometers) per second, indicated that it must have originated outside of the Solar System and was passing through when it was discovered.
"This is the most extreme orbit I have ever seen," said Davide Farnocchia, a scientist at NASA's Center for Near-Earth Object Studies (CNEOS) at the agency's Jet Propulsion Laboratory in Pasadena, California. "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."
According to Rob Weryk, a postdoctoral researcher at the University of Hawaii Institute for Astronomy (IfA), "Its motion could not be explained using either a normal solar system asteroid or comet orbit. This object came from outside our Solar System."
The object crossed within Mercury's orbit and made its closest approach to the Sun on Sept. 9. The Sun's gravity caused it to make a "hairpin" turn, passing beneath Earth's orbit on Oct. 19 at a distance of about 15 million miles (24 million kilometers). Now, it is leaving the Solar System again, travelling at 27 miles per second (44 kilometers per second).
Animation of A/2017 U1
Such objects were theorized to exist, moving between stars, but this would be the first that one was ever actually seen. That shouldn't be too surprising, since there is already evidence for "rogue exoplanets," planets which have been flung out of their solar systems and now wander through in isolation.
"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 the IfA.
"We have been waiting for this day for decades," said CNEOS 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."
Of course, the idea of a such an odd object suddenly appearing in the Solar System brings up other more interesting possibilities, such as an alien probe on a reconnaissance mission. In this case, however, based on analysis so far, it appears to instead be a small natural rocky or metallic body, similar to an asteroid. As astronomer Phil Plait had noted, the trajectory the object was on would actually be ideal for such a mission, but he is also pretty sure this one is natural.
A Bracewell probe is one such idea, an autonomous robotic interstellar space probe with a high level of artificial intelligence. The science fiction classic Rendezvous with Rama posited the possibility of an alien probe being mistaken for an asteroid.
In 1991, another odd object, 1991 VG, was discovered and for a while seemed, tentatively, like it might be an alien interstellar probe, but it also turned out to be an asteroid.
Now that A/2017 U1 is leaving the Solar System (unless it turns around of course!), there is limited time to keep observing it, unfortunately. But astronomers are busy trying to get all the data they can from this mystery visitor before it disappears for good.