First Interstellar Visitor 'Oumuamua Probably Isn't an Alien Spacecraft, Scientists Conclude

'Oumuamua—the mystery visitor from interstellar space—is unlikely to be a spacecraft from some extraterrestrial civilization, a team of scientists has said.

In 2017, researchers announced the very first discovery of a bizarre object from beyond our solar system. It was initially classified as a comet, but this was quickly dismissed as observations showed no evidence of cometary activity. It appeared to be rocky and metallic, showing properties of being both a comet and an asteroid. It was also found to have an orbit that indicated it came from interstellar space.

This mystery visitor quickly grabbed headlines when researchers like Harvard's Avi Loeb suggested it could be an alien spaceship.

This idea was based on the unusual characteristics of 'Oumuamua. It is cigar-shaped, measuring about 650 feet in length and 114 feet wide, and rotates once every seven hours. It was also found to be accelerating instead of slowing down—a discovery that was difficult to explain. This prompted Loeb and colleagues to say the object could be some sort of solar sail—a craft that uses radiation pressure from the Sun to push itself forward.

As Loeb explained to Universe Today: "We explain the excess acceleration of 'Oumuamua away from the Sun as the result of the force that the sunlight exerts on its surface. For this force to explain measured excess acceleration, the object needs to be extremely thin, of order a fraction of a millimeter in thickness but tens of meters in size. This makes the object lightweight for its surface area and allows it to act as a light-sail. Its origin could be either natural (in the interstellar medium or proto-planetary disks) or artificial (as a probe sent for a reconnaissance mission into the inner region of the solar system)."

Now, in a study published in Nature Astronomy, an international team of researchers has concluded 'Oumuamua has a "purely natural origin," and that a host of natural phenomena can explain it.

The team addresses the possibility that 'Oumuamua is a form of alien technology, saying that while the idea of a solar sail could explain "some aspects of the observations," it is not enough to address others. They said the argument that 'Oumuamua was sent towards Earth on purpose based on its "unusual kinematics" was "baseless," while the solar sail hypothesis does not work because of the dimensions. "We find no compelling evidence to favor an alien explanation for 'Oumuamua," they wrote.

Artist impression of 'Oumuamua. ESO/M. Kornmesser

However, scientists say there are still many other things they are unable to explain about 'Oumuamua, including its weird, elongated shape, its spin and its home system: "In spite of many attempts to trace the orbit of 'Oumuamua back to its home system or star cluster, no convincing candidate origin star systems or stellar associations have been identified," they said, adding that despite future space surveys we may never know exactly where it came from.

"We have never seen anything like 'Oumuamua in our solar system. It's really a mystery still," Matthew Knight, from the University of Maryland, who co-led the research, said in a statement. "But our preference is to stick with analogs we know, unless or until we find something unique. The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it."

The scientists believe 'Oumuamua may just be the first interstellar visitor and that over the coming years many more will be detected. "We may start seeing a new object every year," Knight said. "That's when we'll start to know whether 'Oumuamua is weird, or common. If we find 10 to 20 of these things and 'Oumuamua still looks unusual, we'll have to reexamine our explanations."