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Rock From Another Star System Is Unlike Anything Seen Before

The object, called ‘Oumuamua, is probably an asteroid that’s at least 10 times longer than it is wide.

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This artist’s impression shows the first interstellar asteroid: `Oumuamua. The object traveled through space for millions of years before its chance encounter with us. It seems to be a dark red, elongated metallic or rocky object about a quarter mile long. It is unlike anything normally found in the solar system.


Something strange sailed past Earth last month, and thanks to some quick work, astronomers managed to get their first good look at a visitor from interstellar space.

Now named ‘Oumuamua, Hawaiian for “a messenger from afar arriving first,” the object is the first known lump of rock and ice from another star system, which gives astronomers a chance to glimpse a scrap left over from an alien planet’s formation.

“This has been crazy-cool. For the asteroid community, this is as big as the gravitational-wave announcement,” NASA astronomer Joseph Masiero said when the object was discovered, referencing the recent detections of ripples in space-time that have been amazing astrophysicists.

And as researchers report today in Nature, the visitor lives up to its exotic origin: The object is at least 10 times longer than it is wide, resembling a giant cosmic pencil tumbling through the void.

“It’s extraordinarily elongated, which is extremely unusual—we don’t see anything like that in our solar system,” says study leader Karen Meech of the University of Hawaii’s Institute for Astronomy.

More to Come

Rob Weryk, a postdoctoral researcher at the University of Hawaii, first spotted the interstellar interloper on October 19, in an image made by the university’s Pan-STARRS 1 telescope.

When he followed up with European Space Agency astronomer Marco Micheli, the two realized that the object—provisionally called A/2017 U1—was outrunning the sun’s gravitational pull. By the time astronomers confirmed the object’s ballistic flight path, it was exiting the solar system at more than 98,000 miles an hour.

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NASA's Scientific Visualization Studio / Tom Bridgman

After observing the object on its outbound journey, scientists noted that ‘Oumuamua didn’t spawn a tail of gas or dust as it flew by the sun. That means the interstellar object is likely not a comet, and is instead probably an asteroid that formed in the inner part of another star system.

They also saw that its surface reflects more reddish light, like comets and some asteroids in our solar system. To account for this, Meech suggests that ‘Oumuamua is coated in a schmutz of organic materials, metallic iron, or minerals called pyroxenes. To the human eye, she says it’s probably a deep brown.

What’s more, the object dims and brightens by a factor of 10 every few hours. This cycle suggests that the asteroid is long and skinny and rotates every 7.34 hours. When ‘Oumuamua shines brightly, its long side faces us, more effectively reflecting sunlight. When it looks dim, we’re staring at one of its stubby ends.

The asteroid’s cycle also indicates that it’s anywhere from 590 feet to a quarter mile long but only up to 130 feet wide. That kind of narrowness is unprecedented. Only five known objects in our solar system are anywhere near as oblong, and they’re all squatter than ‘Oumuamua.

The object is already out of range for Earth-based telescopes, but astronomers are still following it with instruments in space. Meech says that the Spitzer Space Telescope, which looks at infrared light, is eyeing ‘Oumuamua this week, and her team will use the Hubble Space Telescope in January 2018 to track it further.

As this interstellar messenger soars out of the solar system, more are bound to take its place. Meech’s team says that at any given time, there’s at least one object the size of ‘Oumuamua within 93 million miles of the sun. The benefit is that know we have a better idea of what to look for to spot whether any have unusual origins.

“I think it bodes well for the future,” she says, “because there’s going to be more of these, and we’re going to be ready.”