Mystery space object 'Oumuamua: What was the 'messenger from afar'?

An artist's impression of 'Oumuamua, spotted by the Pan-STARRS 1 telescope in Hawaii. Image supplied

All of the stars Matthew Knight saw through the giant telescope in Arizona were bright with persistent light. All of them but one, which appeared to be flashing, in a way - light that went in and out, dull and then bright, every hour. He assumed that there was something wrong with his data. But days earlier, the University of Maryland research scientist heard about a strange object in space, when it was discovered by the Pan-STARRS1 telescope in Hawaii.

What he was seeing was an interstellar object - the first we have ever been able to observe from Earth - tumble through our solar system, rotating and reflecting sunlight in pulses.

When it was first discovered, many theories about the object's origin emerged. One theory suggested that the object was from an alien civilisation, sailing into our solar system. Some researchers thought it was possible that the object was an alien solar sail, relying on the sun's light to push it through space.

"I don't want to completely say it's not aliens, because we didn't actually go to it and see it up close," Knight said. "But I think that's a very unlikely possibility."

For a week in late-October 2017, data was collected as the object sped through the solar system. Scientists concluded that the cigar-shape object, named 'Oumuamua, was natural. It did not originate from an alien civilization. In a paper published in the journal Nature Astronomy, 14 scientists, including Knight, wrote that they found "no compelling evidence to favour an alien explanation for 'Oumuamua," to the dismay of alien hunters everywhere.

Pronounced Oh-Moo-uh-Moo-uh, the Hawaiian word roughly translates to "messenger from afar." Co-author Karen Meech, an astronomer at the Institute for Astronomy at the University of Hawaii, asked Hawaii linguists to name the object after the first week of observations.

The object's path tipped scientists off to its origins, proving it was indeed interstellar - or, in other words, having come from outside our solar system. It was not taking the kind of path that one of our own solar system's objects would take.

"It didn't actually get captured here; it was just passing through," Knight said, referring to when a solar system's gravity draws an object into orbit. "So it whizzed by us, passed the sun and then it went back out."

Scientists noticed that 'Oumuamua appeared to unexpectedly accelerate. According to Knight, this could have been because the object was a comet, with ice that was vaporizing and giving it "a little bit of a kick" as it entered the solar system. But the scientists did not see a gas tail or directly detect any ice.

Instead, the scientists categorised it more generally as planetesimal, meaning that 'Oumuamua is likely "just a leftover remnant from another solar system's birth process," Meech said, like a giant boulder that at some point could have fused with other space rocks to form a planet but didn't. After that, scientists suspect, 'Oumuamua was ejected from its own solar system.

Now it's on a road trip through the galaxy.

"It was just traveling through space, kind of minding its own business, and at some point it got close enough to our solar system that then it started feeling the gravitational tug from our sun and then it got pulled through," Knight said.

Once in our solar system, 'Oumuamua sped and tumbled into visibility. Meech's team recorded data for a week, then, from November 2018 through the first week of January, the object was only faintly visible through the Hubble Space Telescope.

'Oumuamua's brightness changed when viewing the narrow side and then the long side. Knight compared it to looking at a bottle of soda.

"If you're seeing the length of it, it's a very wide cross section," Knight said. "But if you're looking at it down the cap, you're only seeing a narrow area."

Scientists don't know exactly how big 'Oumuamua is. They could only estimate because of how it reflected sunlight. Knight said "sizes from about (650 feet) to about (3,300 feet) would all be consistent with known asteroids and comets in our solar system."

For about the past 30 years, scientists predicted that objects from another star system could be discovered. Over the past 10 years, the technology to survey for such faint and fast objects improved. 'Oumuamua is the first, but likely not the last interstellar object to be observed. Over the next 10 years, scientists could likely see one every year.

In early July, 'Oumuamua was just beyond Saturn.

"Over the next hundreds of years it will be zooming out of our solar system," Knight said. "And then eventually it will just be back out in interstellar space between stars."