We think of both comets and asteroids as belonging to our own solar system, our family of planets orbiting the sun, but NASA reported on October 26, 2017 that astronomers have been watching a small body – perhaps an asteroid, perhaps a comet – apparently from beyond our solar system, from somewhere in interstellar space. If so, it would be the first interstellar asteroid (or comet) to be observed and confirmed. The object is currently designated A/2017 U1, and it’s less than a quarter-mile (400 meters) in diameter. NASA said it is moving remarkably fast, some 15.8 miles (25.5 km) per second (similar to Earth’s own speed in orbit around the sun). Astronomers around the world are aiming earthly telescopes, and telescopes in space, in this unusual object’s direction. They’re trying to find out as much as they can about A/2017 U1, perhaps to determine its composition, and hopefully to confirm if indeed it is visiting us from somewhere else in our Milky Way galaxy, before it shoots away again … forever.
The University of Hawaii’s Pan-STARRS 1 telescope spotted A/2017 U1 on October 19, in the course of a nightly search for near-Earth objects. Rob Weryk, a postdoctoral researcher at the University of Hawaii Institute for Astronomy, was first to notice the object as a starry point, moving in front of the stars. He was the first to submit it to the IAU’s Minor Planet Center, which is the worldwide organization in charge of collecting observational data for minor planets and comets in our solar system.
From the shape of its orbit, it quickly became apparent this object wasn’t an ordinary member of our solar system. NASA said:
Weryk subsequently searched the Pan-STARRS image archive and found it also was in images taken the previous night, but was not initially identified by the moving object processing.
Weryk immediately realized this was an unusual object …
Weryk contacted IfA graduate Marco Micheli, who had the same realization using his own follow-up images taken at the European Space Agency’s telescope on Tenerife in the Canary Islands. But with the combined data, everything made sense … This object came from outside our solar system.
This is the most extreme orbit I have ever seen. 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.
A/2017 U1 came from the direction of the constellation Lyra the Harp. NASA explained:
The object approached our solar system from almost directly above the ecliptic, the approximate plane in space where the planets and most asteroids orbit the sun, so it did not have any close encounters with the eight major planets during its plunge toward the sun. On September 2, the small body crossed under the ecliptic plane just inside of Mercury’s orbit and then made its closest approach to the sun on September 9. Pulled by the sun’s gravity, the object made a hairpin turn under our solar system, passing under Earth’s orbit on October 14 at a distance of about 15 million miles (24 million km) — about 60 times the distance to the moon. It has now shot back up above the plane of the planets and, travelling at 27 miles per second (44 km per second) with respect to the sun, the object is speeding toward the constellation Pegasus.
So it exists, it’s moving fast, its orbit indicates an interstellar origin. That’s about all we know for now. However, there’s a context for understanding what this object might be. Karen Meech is an astronomer at the Institute for Astronomy at the University of Hawaii. She specializes in small bodies and their connection to solar system formation. She commented:
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.
The designation A/2017 U1 by the Minor Planet Center is temporary. Since this is the first object of its type ever discovered, rules for naming this type of object will likely be established by the International Astronomical Union. But not so fast. Astronomers, always cautious, don’t yet have enough data points along this object’s orbit to confirm it’s from outside our solar system. CNEOS Manager Paul Chodas said:
So far, everything indicates this is likely an interstellar object, but more data would help to confirm it.
Want to know how astronomers identify new asteroids and comets? Watch the video below.
Bottom line: Astronomers have been tracking a small body temporarily designated A/2017 U1. They believe it’s from outside our solar system, and, if so, it’s the first interstellar asteroid (or comet) ever discovered.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.