That ‘spaceship’ that visited our solar system may have been just a crusty rock

ESO/M. Kornmesser
Not much of a spaceship. An artist’s impression of ‘Oumuamua.

The strange interstellar object that passed close to Earth this fall appears to be a naturally formed, icy object covered with a dry crust, an international team of scientists says.

Researchers who studied the object Oumuamua also said it resembled icy minor planets on the outskirts of our solar system, Queen’s University Belfast said in a statement.

Researchers from the university worked with other academics from elsewhere in the United Kingdom, as well as the United States, Canada, Taiwan, and Chile. Their results were published this week in studies in the journals Nature Astronomy and Astrophysical Journal Letters.


The new study comes after the excitement generated when Breakthrough Listen, an organization that is scanning the skies for alien life, said it would listen for radio transmissions coming from the object. As of last week, nothing had been detected.

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Queen’s University postdoctoral researcher Michele Bannister, whose team published its results in Astrophysical Journal Letters, said in the statement that researchers discovered that the object was a “planetesimal” or tiny planet, “with a well-baked crust that looks a lot like the tiniest worlds in the outer regions of our solar system.”

She said the object “has a greyish/red surface and is highly elongated, probably about the size and shape of the Gherkin skyscraper in London.” The Gherkin is a bullet-shaped futuristic building in London that rises nearly 600 feet.

Her team measured how Oumuamua reflects light in both the visible and near-infrared spectrums, using the Gemini-North Telescope in Chile and the William Herschel Telescope in the Canary Islands — both among the most powerful telescopes in the world, Professor Alan Fitzsimmons, whose team’s research was published in Nature Astronomy, said in an e-mail.

“It’s fascinating that the first interstellar object discovered looks so much like a tiny world from our own home system,” Bannister said. “This suggests that the way our planets and asteroids formed has a lot of kinship to the systems around other stars.”


Fitzsimmons said the outer crust explained why the ice inside did not vaporize when the object passed near the sun and was heated to over 300 degrees Centigrade, 0r 572 degrees Fahrenheit.

The crust, “an insulating organic-rich layer,” was formed after the asteroid was exposed to cosmic rays for millions, or even billions. of years, the university said. Fitzsimmons said the carbon-based crust was baked by the rays, turning Oumuamua’s surface dark red.

Fitzsimmons said his team had found that “a half-metre thick coating of organic-rich material could have protected a water-ice-rich comet-like interior from vaporizing.

“In between the stars there is gas, that’s the interstellar medium. It’s very rarified . . . it’s a very dilute gas, but when you travel through it for a billion years, things can stick to the surface of an object,” said Avi Loeb, professor and chair of the Harvard University astronomy department. “Perhaps there is a coating that we’re not familiar with.”

Fitzsimmons’ team measured the way that Oumuamua reflects sunlight, the university said. Fitzsimmons used both the Very Large Telescope in Chile along with the William Herschel Telescope to observe Oumuamua.


Fitzsimmons’ research incorporated spectroscopy, a technique used to decipher the composition of an object’s surface, while Bannister’s team “measured the more general colour of the object” in order to precisely compare it to trans-Neptunian objects in our solar system, Fitzsimmons said in an e-mail.

Loeb said Oumuamua’s “unique” characteristics set it apart from any other comet-like objects that have yet been observed, despite some similarities with known TNOs.

“Both papers indicate similarities in things we know, but it’s still very unusual. The surprise is this coating that they find on it surface and its very elongated shape,” he said. “Perhpas a more natural name for it would have been an interstellar ice cube.”

The “interstellar object,” the first ever observed intruding in the orbits of our planets, was picked up by telescopes in October at the University of Hawaii’s Haleakalā Observatory, the National Aeronautics and Space Administration said. The object made headlines. It has now passed Earth on its way to who knows where. Scientists say other “interstellar” objects may have sailed by in the past, undetected.

Loeb pointed to the upcoming Large Synoptic Survey Telescope project, set to begin monitoring the stars in late 2023, as an opportunity to find more objects like Oumuamua.

“Now that we know they exist, we would expect to find many more,” he said.

Loeb also noted that objects like Oumuamua, with a frozen core protected by a hardened crust, could allow for living things to be brought safely through space.

“One can imagine — especially with an object that has water, ice on it — if there was life in the region where it came from, it could be carried from one star to another,” he said. “That’s one way of transferring objects, by these icy objects that passed through.”

“It could be that life on Earth was initiated by a process like that,” Loeb said.

The findings of the two papers released Monday make it “pretty clear” that Oumuamua itself is a natural object though, Fitzsimmons said. He still believed Breakthrough Listen’s work to monitor the interstellar obejct is a worthwhile effort though, as any never-before-seen cosmic object could turn out to be something special.

“We would have felt like chumps if this thing had been blasting out the latest track by the interstellar equivalent of Beyonce’ and no-one had listened,” Fitzimmons said in an email. “And who knows what the next object will be like?”