This interstellar rock has been confusing astronomers for years. First, they thought this interstellar rock was a comet. Then in 2017, the European Southern Observatory (ESO) announced it was an asteroid based on their measurements and observations — making it the first known asteroid from interstellar space. But now researchers are saying it's a comet. With all this back and forth, what exactly do we know about this strange object?
Scientists named it 'Oumuamua. ("Ou" means "reach out for," and "mua," with the emphasis on the second "mua," means "first, in advance of" — reflecting the nature of the object as a "scout" or "messenger" from the past.)
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.
Why call it a comet now?
A team of scientists came to the conclusion that 'Oumuamua must be a comet because the rock is actually 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.
It's because of 'Oumuamua's different physical properties that astronomers up until a few months ago believed it was an asteroid.
Why was it labeled an asteroid before?
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 meters 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 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."
Why was it originally called a comet?
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."
What else makes it unique?
'Oumuamua's tumbling motion is unusual, too, as a team of QUB researchers reported 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!"
And just in case you were wondering how to pronounce 'Oumuamua...
Editor's note: This file has been updated with new information since it was originally published in November 2017.