Oumuamua: scientists finally decide on real identity of mysterious interstellar object

Despite lacking a distinctive 'tail', space rock from beyond our solar system is likely a comet, scientists conclude

After months of speculation about the identity of Oumuamua, or the first “alien” object from outside our solar system, scientists have come to a conclusion about what it is.

The space rock first identified by astronomers in November, has been variously described as an asteroid, a new variety of “interstellar object” and even an alien spacecraft.

But new tests have revealed it is in fact a comet, despite lacking some of the key characteristics that would normally allow experts to identify such an astronomical body.

“From theoretical models we were expecting interstellar objects to be mostly cometary, on the basis of the region of the planetary systems where they form,” said Dr Marco Micheli from the European Space Agency

“However, Oumuamua did not look as a comet in optical images we obtained, since it did not have a coma or a tail. This led to a reclassification of it as an asteroid.”

While asteroids are made up of rock and metals, comets form from ice, dust and rocky fragments and move through space in a different way.

Oumuamua lacked the tail that forms as comets melt and release gases and dust, meaning it was difficult to determine what exactly it was.

However, a combination of ground and space-based observations allowed Dr Micheli and his colleagues to reveal its true identity.

The path taken by the object through our solar system suggested that unlike asteroids, this object was driven by something other than the gravitational attraction of the sun, planets and large asteroids.

Instead, Oumuamua’s acceleration was actually directed largely away from the sun, suggesting it was not being powered by gravity.

“Our paper presents the detection of a weak non-gravitational acceleration acting on Oumuamua, which we could explain with a weak emission of material from the object, as comets typically do,” said Dr Micheli.

Essentially, as comets melt the gases they release power them through space, and this invisible propulsion system appeared to be powering the mysterious object.

“The intensity, direction and behaviour of the acceleration we detected are all explained assuming it has properties similar to comets in our own solar system,” said Dr Micheli.

The results of this analysis were published in the journal Nature.

As Oumuamua marks the first ever object identified from another planetary system it generated a huge amount of interest from the scientific community, but Dr Micheli says it is likely to be the first of many.

“Models predict that there are a lot of these objects in interstellar space, and a few of them are likely transiting inside our solar system at any given time,” he said. “Therefore, we expect that more of them will be found in the future, especially when new and more powerful survey telescopes will become operational.”