This interstellar rock has been confusing astronomers for years. When it was first spotted in 2017, they thought it was a comet. Then the European Southern Observatory announced it was an asteroid based on ESO measurements and observations — making it the first known asteroid from interstellar space. Now researchers have a new theory: it's a shard from a planet torn apart by its host star.
So what do we really know about the strange object with the confusing origin story?
Scientists named it 'Oumuamua. "Ou" means "reach out for," and "mua" means "first, in advance of" — reflecting the nature of the object as a "scout" or "messenger" from the past.
This is how it's pronounced:
When was it discovered?
The unique rock was first spotted in October 2017 when the Pan-STARRS 1 telescope in Hawaii picked up a faint point of light moving across the night sky. Measurements of the object's trajectory quickly made it obvious that it couldn't possibly be from our solar system; this was an interstellar object, a momentary visitor to our neighborhood that had likely been wandering around alone in space for billions of years.
Where did 'Oumuamua come from?
A team of scientists have since narrowed down the possibilities of where the cigar-shaped object comes from to four stars — red dwarf HIP 3757, sunlike star HD 292249 and two other unnamed stars. They based their findings on the information showing that 'Oumuamua behaved like a comet and combined it with data from ESA's Gaia mission showing the stars that would have had a close encounter with the comet.
There's even an unusual theory that 'Oumuamua is an alien spacecraft sent to investigate Earth, according to researchers from the Harvard Smithsonian Center for Astrophysics. "Considering an artificial origin, one possibility is that 'Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment," wrote the study's authors. It's worth noting that the offbeat theory, published in the Astrophysical Journal Letters, received a lot of push-back.
One thing we do know is that this peculiar object has been baffling scientists for years.
The planet shard theory
The newest theory, from study author Yun Zhang, a researcher at the Côte d’Azur Observatory in France, says the object we are seeing now emerged from an ancient traffic accident of sorts. She and her associates believe the object is a fragment created when an interstellar object got too close to its parent star and got torn apart as a result. They are not the first to pose the theory but they are the first to investigate it more thoroughly.
"A tidal encounter between a planet or small body and a star is a tug-of-war game between the gravitational pull of the star and the self-gravity of the flyby body," Zhang told The Guardian. The researchers say 'Oumuamua is a shard created by that encounter, a process called tidal disruption.
To reach that conclusion, Zhang explained to Smithsonian magazine, she and her colleagues retraced the object's path through space and time with a series of computer simulations until they finally found one that fit all the odd characteristics like the object's motion through space, its color and even its memorable shape. The shard theory, which they published in the journal Nature Astronomy, produced the best match.
The comet theory
A team of scientists came to the conclusion that 'Oumuamua must be a comet because the rock is accelerating — something comets do in space. "There was something else that was pushing ‘Oumuamua out from the sun, so it was moving faster than it should be just due to gravity alone," astronomer Alan Fitzsimmons told The Verge.
However, comets typically have a trail of gas from the sun melting ice on the surface that pushes them through space, and 'Oumuamua doesn't appear to have gas surrounding it. "The dust could have been stripped from the comet as it flew through space, or maybe astronomers just missed it. And gas is actually hard to detect," said astronomer Karen Meech. "You need a bright comet, or a really big telescope. And this was a very faint comet. So people tried, but the data were very noisy." Meech also notes that the comet may contain different materials than comets from our solar system, which could explain why there's no gas present.
Additionally, a 2019 study from a team of Yale and CalTech researchers explains that even though scientists didn't observe gas or a tail emitting from 'Oumuamua, it's still a comet.
"A volatile-rich gas-venting structure for 'Oumuamua provides the simplest explanation for its odd trajectory,” the team wrote in their paper. They determined this by creating a model version of 'Oumuamua that emitted a jet of vapor particles, and the model accelerated and moved just like 'Oumuamua does.
In 2017, researchers from Queen's University in Belfast (QUB), Ireland, noticed 'Oumuamua reflected sunlight and is similar to icy objects in space that have a dry crust, meaning there may be the presence of water.
"This is because 'Oumuamua has been exposed to cosmic rays for millions, or even billions, of years, creating an insulating organic-rich layer on its surface," Fitzsimmons said in a December 2017 statement. "We have also found that a half-meter thick coating of organic-rich material could have protected a water-ice-rich comet-like interior from vaporizing when the object was heated by the sun, even though it was heated to over 300 degrees centigrade."
The asteroid theory
It's because of 'Oumuamua's different physical properties that astronomers believed it was an asteroid.
In 2017, scientists observed 'Oumuamua using the ESO's Very Large Telescope and noticed it was dark reddish in color and appeared highly elongated like a cigar.
"These properties suggest that 'Oumuamua is dense, possibly rocky or with high metal content, lacks significant amounts of water or ice, and that its surface is now dark and reddened due to the effects of irradiation from cosmic rays over millions of years. It is estimated to be at least 400 metres long," the ESO said in a release. Researchers surmised that 'Oumuamua's dark, red color is indicative of a high metal content that has been irradiated from cosmic rays over the course of millions of years.
Asteroids are comprised of metals and rocky material, while comets are made up of gas, dust and water. Thus explaining why scientists previously believed 'Oumuamua was an asteroid.
A March 2018 study published in Monthly Notices of the Astronomical Society also revealed 'Oumuamua was more than likely from a binary star system — when two stars orbit around a common center. Researchers arrived at this hypothesis by determining that binary systems can eject rocky objects like 'Oumuamua into space. "They also concluded that it probably came from a system with a relatively hot, high-mass star since such a system would have a greater number of rocky objects closer in," the Royal Astronomical Society stated in its press release. Researchers also believe the asteroid probably was ejected sometime around the formation of the planets when it passed through our solar system.
Thankfully, this object isn't quite so talkative and hostile, and already looks to be rapidly departing the solar system.
"We had to act quickly," explained team member Olivier Hainaut about discovering the object's rapid escape trajectory. "'Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space."
What else makes it unique?
'Oumuamua's tumbling motion is unusual, too, as a team of QUB researchers reported in a letter published in Nature Astronomy in February 2018. Its chaotic movements point to a "violent past," they noted, possibly due to an ancient collision with another asteroid before it was thrown out of its own solar system.
"Our modeling of this body suggests the tumbling will last for many billions of years to hundreds of billions of years before internal stresses cause it to rotate normally again," QUB astrophysicist Wes Fraser said in a statement. "While we don't know the cause of the tumbling, we predict that it was most likely sent tumbling by an impact with another planetesimal in its system, before it was ejected into interstellar space."
Scientists are racing to learn as much as they can about 'Oumuamua before it leaves us forever. It's expected to pass above Jupiter's orbit in May 2018, followed by Saturn in January 2019 and then Neptune in 2022. Still, its brief visit might leave us with more questions than answers.
Although it's the first interstellar space rock ever observed, astronomers estimate that we have at least one such alien visitor per year. So after 'Oumuamua leaves us, we'll just have to keep looking for more.
"We are continuing to observe this unique object," said Hainaut, "and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones!"
Editor's note: This file has been updated with new information since it was originally published in November 2017.