An interstellar asteroid might have just been spotted for the first time

The interloper could carry information about the makeup of alien planet systems

A/2017 U1's trajectory

INTERSTELLAR INTERLOPER?  The unusual trajectory of an asteroid, called A/2017 U1, suggests it came from outside the solar system and is now on its way out again.


Astronomers may have just spotted the first asteroid caught visiting the solar system from another star.

The Pan-STARRS 1 telescope in Hawaii discovered the object, initially dubbed A/2017 U1 and later named ‘Oumuamua, on October 18. More observations from other telescopes around the world suggest the object’s trajectory is at an unusually steep angle to the plane on which all the planets lie, and it does not orbit the sun. A/2017 U1’s slingshot route suggests it is a recent visitor to the solar system — and is now on its way out again. The discovery was announced in a bulletin published October 25 by the International Astronomical Union’s Minor Planet Center.  

All asteroids previously seen come from within the solar system and circle the sun. Even comets, which come from a distant reservoir of icy rocks in the solar system called the Oort cloud and can have highly titled orbits, still orbit the sun.

Astronomers first pegged the object as a comet thanks to its elongated path, but additional telescope observations October 25 indicate it’s more likely that A/2017 U1 is an asteroid. Those observations revealed that the object looked like a single, sharp point of light, suggesting it is not a comet, which would have an extended icy halo. The asteroid, which is probably no more than 400 meters across, zoomed into the solar system at 25.5 kilometers per second and is now fleeing at 44 km/s.

The new data also supported the wacky trajectory, suggesting the object truly is a visitor from beyond. “It’s now looking very promising,” says planetary scientist Michele Bannister of Queen’s University Belfast in Northern Ireland, although she would still like to get more data to be sure. Astronomers are already planning to measure the colors in the asteroid’s reflected light to figure out what it’s made of, a clue to its origins.

VISITOR FROM BEYOND The dot in the center of this image is A/2017 U1, which may be the first asteroid caught visiting from another solar system. The image was taken with the William Herschel Telescope in La Palma, Spain. Alan Fitzsimmons/Queen’s University Belfast/Isaac Newton Group La Palma

Editor’s note: This story was updated November 9, 2017, with new information about the asteroid’s name and how fast it’s traveling on its way out of the solar system.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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